ASSIGNMENT AND SECURITY AGREEMENT

     THIS  ASSIGNMENT  AND SECURITY  AGREEMENT  (this  "Agreement")  is made and
entered into as of the 1st day of September,  1999, by and between Prime Medical
Operating, Inc., a Delaware corporation ("PMOI"), Prime/BDR Acquisition, L.L.C.,
a Delaware limited  liability  company ("Prime") (PMOI and Prime are referred to
herein,  jointly and severally,  as "Secured  Party") and David P. Bates III and
Jane A. Bates (collectively referred to as the "Debtor").
                                    RECITALS:

         A. Debtor and Secured Party have  executed and  delivered  that certain
Stock  Purchase   Agreement  dated  as  of  September  1,  1999  (the  "Purchase
Agreement"),  pursuant to which  Secured  Party  purchased  from Debtor  certain
shares of the $0.01 par value  common  stock of Horizon  Vision  Center,  Inc, a
Nevada corporation ("Horizon").

         B. Secured Party has requested  that Debtor pledge the  Collateral  (as
defined below) to secure certain obligations and liabilities that Debtor may now
or hereafter have to Secured Party, including, without limitation, any indemnity
obligations arising under the Purchase Agreement.

     C. Debtor desires to enter into this Agreement as a material  inducement to
Secured  Party's  purchase  of  Debtor's  shares of Horizon  under the  Purchase
Agreement.

                                   AGREEMENT:

         NOW, THEREFORE, in consideration of the foregoing and the covenants and
agreements hereinafter set forth, and other good and valuable consideration, the
receipt and sufficiency of which Debtor  acknowledges,  Debtor and Secured Party
agree as follows:

                                    ARTICLE I

                       COLLATERAL AND SECURED OBLIGATIONS

         1.1 Grant of Security Interest.  Debtor hereby assigns,  transfers, and
pledges to Secured  Party,  and Debtor hereby grants to Secured Party a security
interest   in,   the   following   described   collateral   (collectively,   the
"Collateral"):

                  (a)  Shares  of  Horizon.  From  and  after  the  date of this
Agreement,  (i) all shares of the common  stock of Horizon  owned or acquired in
any manner by Debtor or Secured Party,  (collectively,  the "Shares"),  (ii) any
replacements,  substitutions,  or exchanges of the certificates representing the
Shares, and (iii) any and all options,  rescission rights,  registration rights,
conversion rights, subscription rights, contractual or quasi-contractual rights,
warrants,  redemption rights,  redemption proceeds, calls, preemptive rights and
all other rights and benefits pertaining to the Shares;

                  (b)  Accounts.  All  accounts  and  rights  now  or  hereafter
attributable to any of the Collateral  described in (a) above, and all rights of
Debtor now or hereafter arising under any agreement pertaining to the Collateral
described  in  (a)  above,   including  without  limitation  all  distributions,
proceeds, fees, dividends,  preferences,  payments or other benefits of whatever
nature  which  Debtor is now or may  hereafter  become  entitled to receive with
respect to any Collateral described in (a) above;

                  (c) Additional  Property.  "Collateral" shall also include the
following  property  (collectively,  the  "Additional  Property")  which  Debtor
becomes  entitled  to receive or shall  receive  in  connection  with any of the
Collateral  described in this Section 1.1: (i) any stock certificate,  including
without  limitation,  any  certificate  representing  a  stock  dividend  or any
certificate in connection with any recapitalization,  reclassification,  merger,
consolidation,  conversion,  sale of assets, combination of shares, stock split,
reverse  stock split or  spin-off;  (ii) any option,  warrant,  subscription  or
right,  whether as an addition to or in  substitution  of any of the  Collateral
described in this Section 1.1; (iii) any dividends or  distributions of any kind
whatsoever,  whether  distributable in cash,  stock or other property;  (iv) any
interest,  premium or principal  payments;  and (v) any conversion or redemption
proceeds; and

                  (d) Proceeds.  All proceeds (cash and non-cash) arising out of
the sale, exchange, collection or other disposition of all or any portion of the
Collateral  described in (a), (b), or (c) above,  including  without  limitation
proceeds in the form of stock, accounts, chattel paper, instruments,  documents,
goods, inventory and equipment.

The  security  interest in the  Collateral  hereby  granted by Debtor to Secured
Party may sometimes be referred to in this Agreement as the "Security Interest".

         1.2 Obligations.  This Agreement and the Security Interest shall secure
full and punctual payment and performance of the following indebtedness,  duties
and obligations (collectively, the "Obligations"):

                  (a) All liabilities and obligations of Debtor to Secured Party
(including, without limitation, any principal, interest, fees and other amounts,
and any  indemnity  obligations)  under and pursuant to the Purchase  Agreement,
this Agreement and/or any other contract or agreement  between Secured Party and
Debtor or any affiliate of Debtor (collectively, "Other Agreements"; and

                  (b) (i)  all  indebtedness,  obligations  and  liabilities  of
Debtor and/or any affiliate of Debtor to Secured Party of any kind or character,
now existing or hereafter arising, whether direct, indirect, related, unrelated,
fixed,  contingent,  liquidated,  unliquidated,  joint,  several  or  joint  and
several,  arising from,  connected with, or related to the Purchase Agreement or
any Other Agreement, or any other document, agreement, or instrument executed in
connection  either of them,  (ii) all accrued but unpaid  interest on any of the
indebtedness  described in (i) above, (iii) all obligations of Debtor and/or any
affiliate  of  Debtor  to  Secured  Party  under  any  documents  or  agreements
evidencing,  securing,  governing  and/or  pertaining  to all or any part of the
indebtedness  described  in (i) and (ii)  above,  (iv) all  costs  and  expenses
incurred by Secured Party in connection  with the collection and  administration
of all or any part of the  indebtedness  and obligations  described in (i), (ii)
and (iii) above or the protection or preservation  of, or realization  upon, the
collateral  securing  all or any  part of  such  indebtedness  and  obligations,
including  without  limitation  all  attorneys'  fees,  and  (v)  all  renewals,
extensions, modifications and rearrangements of the indebtedness and obligations
described in (i), (ii), (iii) and (iv) above.

                  (c) All sums now or  hereafter  loaned or  advanced by Secured
Party to  Debtor,  or  expended  by Secured  Party for the  account of Debtor or
otherwise owing by Debtor to Secured Party, in respect of the  Obligations,  and
all other sums  expended or advanced  by Secured  Party  pursuant to any term or
provision of this Agreement or any Other Agreement (i) to collect and/or enforce
the Obligations or (ii) to maintain, protect and preserve the Collateral.

                                   ARTICLE II

       DEBTOR'S REPRESENTATIONS AND WARRANTIES WITH RESPECT TO COLLATERAL

       Debtor hereby represents and warrants to Secured Party as follows:

         2.1 Ownership of Collateral.  Debtor has good and  marketable  title to
the Collateral  free and clear of any liens,  security  interests,  shareholders
agreement, calls, charge, or encumbrance,  except for this Security Interest. No
financing  statement or other  instrument  similar in effect covering all or any
part of the  Collateral is on file in any recording  office,  except as may have
been filed in favor of Secured Party relating to this Agreement.

         2.2  Power  &  Authority.  Debtor  has the  lawful  right,  power,  and
authority  to grant the Security  Interest in the  Collateral.  This  Agreement,
together  with all filings and other  actions  necessary or desirable to perfect
and protect such security interest,  which have been duly taken,  create a valid
and perfected first priority  security  interest in the Collateral  securing the
payment and performance of the Obligations.

         2.3  No  Agreements.  The  Shares  are  not  subject  to any  right  of
redemption,  or any  call or put  options,  voting  trust,  proxy,  shareholders
agreement,  right of first  refusal,  or any other  document or agreement  which
would in any way impair or adversely affect this Security Interest or the rights
of Secured Party under this Agreement.

         2.4  Securities.  Any  certificates  evidencing  securities  pledged as
Collateral  are valid and  genuine  and have not been  altered.  All  securities
pledged as Collateral have been duly  authorized and validly  issued,  are fully
paid and  non-assessable,  and were not issued in  violation  of the  preemptive
rights of any party or of any agreement by which Debtor or the issuer thereof is
bound.  No  restrictions  or  conditions  exist with  respect to the transfer or
voting of any securities pledged as Collateral.

                                   ARTICLE III

                  DEBTOR'S OTHER REPRESENTATIONS AND WARRANTIES

         3.1 Solvency of Debtor.  As of the date hereof,  (i) Debtor is solvent;
(ii) the fair saleable  value of Debtor's  assets exceeds  Debtor's  liabilities
(both fixed and contingent);  (iii) Debtor has sufficient capital to satisfy all
of Debtor's  obligations  as they  become due;  (iv) no  receiver,  trustee,  or
custodian has been appointed for, or taken  possession of, all or  substantially
all of the assets of Debtor,  either in a  proceeding  brought by Debtor or in a
proceeding  brought against Debtor;  (v) Debtor is not the subject of a petition
for relief under the United  States  Bankruptcy  Code or any similar  federal or
state insolvency law, including without limitation a petition filed by Debtor or
a petition filed by a third party seeking relief against Debtor; and (vi) Debtor
has no  intention  of filing a  petition  for  relief  under the  United  States
Bankruptcy  Code or any similar  federal or state  insolvency law, or of seeking
any other form of  creditor  relief,  within  the  two-year  period  immediately
following the date of this Agreement.

         3.2  Authority and  Compliance.  Debtor has full power and authority to
enter into this Agreement. Debtor has full power and authority to enter into and
perform its obligations  under the Purchase  Agreement and each Other Agreement.
No further  consent or approval is  required as a condition  to the  validity of
this  Agreement,  the Purchase  Agreement or any Other  Agreement.  Debtor is in
compliance with all applicable laws, ordinances,  statutes, orders, regulations,
judgments, writs, or decrees of any governmental entity to which it is subject.

         3.3 Binding Agreement.  This Agreement, the Purchase Agreement and each
Other Agreement  constitute valid and legally binding  obligations of Debtor, in
accordance  with  their  terms,  subject  to  (a)  the  applicable   bankruptcy,
insolvency,  reorganization,  moratorium,  and similar laws affecting creditors'
rights  generally  and  (b)  restrictions  imposed  by any  court  of  competent
jurisdiction on the enforcement of non-competition and exclusive use restrictive
covenants imposed under the Purchase Agreement or any Other Agreement.

         3.4 Litigation.  There are no proceedings  pending or, to the knowledge
of Debtor,  threatened before any court or  administrative  agency which will or
may have a material adverse effect on the financial  condition of Debtor or upon
Debtor's ability to perform its obligations  under this Agreement,  the Purchase
Agreement or any Other Agreement.

         3.5 No Conflicting Agreements.  There are no provisions of any existing
agreement,  mortgage,  indenture or contract  binding on Debtor or affecting its
property,  which  would  conflict  with or in any  way  prevent  the  execution,
delivery, or carrying out of the terms of this Agreement, the Purchase Agreement
or any Other Agreement.

         3.6  Ownership  of  Assets.  Debtor  has  good  and  full  title to the
Collateral,  and the  Collateral  is owned  free and  clear of  liens,  charges,
claims, security interests, and other encumbrances.

     3.7 Taxes. Debtor has filed all tax returns required to be filed by Debtor.

                                   ARTICLE IV

                  DEBTOR'S COVENANTS WITH RESPECT TO COLLATERAL

         Debtor  covenants  and agrees  that from the date  hereof and until the
payment  and  performance  in  full  of the  Obligations  unless  Secured  Party
otherwise consents in writing:

         4.1  Delivery of  Instruments  and/or  Certificates.  Contemporaneously
herewith,   Debtor  covenants  and  agrees  to  deliver  to  Secured  Party  any
certificates,   documents,   or  instruments   representing  or  evidencing  the
Collateral,  with Debtor's  endorsement  thereon and/or  accompanied by property
instruments of transfer and assignment duly executed in blank with, if requested
by Secured Party,  signatures  guaranteed by a member or member  organization in
good standing of an authorized Securities Transfer Agents Medallion Program, all
in form and substance satisfactory to Secured Party.

         4.2  Further  Assurances.   Debtor  will   contemporaneously  with  the
execution  hereof  and from  time to time  thereafter  at its  expense  promptly
execute and deliver all further  instruments  and documents and take all further
action  necessary or  appropriate or that Secured Party may request in order (i)
to perfect and protect the security  interest created or purported to be created
hereby and the first priority of such security interest,  (ii) to enable Secured
Party to exercise  and enforce its rights and  remedies  hereunder in respect of
the  Collateral,  and (iii) to otherwise  effect the purposes of this Agreement,
including  without  limitation:  (A)  executing  and  filing  any  financing  or
continuation  statements,  or any  amendments  thereto;  (B)  obtaining  written
confirmation  from the issuer of any  securities  pledged as  Collateral  of the
pledge of such securities,  in form and substance satisfactory to Secured Party;
(C)  cooperating  with Secured Party in registering the pledge of any securities
pledged as Collateral with the issuer of such securities;  (D) delivering notice
of Secured Party's security interest in any securities  pledged as Collateral to
any securities or financial  intermediary,  clearing  corporation or other party
required by Secured Party, in form and substance  satisfactory to Secured Party;
and  (E)  obtaining  written  confirmation  of  the  pledge  of  any  securities
constituting Collateral from any securities or financial intermediary,  clearing
corporation  or other party  required by Secured  Party,  in form and  substance
satisfactory to Secured Party.

         4.3 Additional Property. All Additional Property, as defined in Section
1.1(c)  above,  received by Debtor shall be received in trust for the benefit of
Secured Party.  All Additional  Property and all  certificates  or other written
instruments or documents  evidencing and/or representing the Additional Property
that is  received  by Debtor,  together  with such  instruments  of  transfer as
Secured Party may request,  shall  immediately be delivered to or deposited with
Secured Party and held by Secured  Party as  Collateral  under the terms of this
Agreement.  If the  Additional  Property  received  by Debtor and  delivered  to
Secured  Party  pursuant  to this  Section  shall  be  shares  of stock or other
securities,  such shares of stock or other  securities shall be duly endorsed in
blank or  accompanied  by proper  instruments  of transfer and  assignment  duly
executed in blank with, if requested by Secured Party,  signatures guaranteed by
a member or member  organization  in good standing of an  authorized  Securities
Transfer Agents  Medallion  Program,  all in form and substance  satisfactory to
Secured  Party.  Secured  Party  shall  be  deemed  to  have  possession  of any
Collateral in transit to Secured Party or its agent.

         4.4  Sale,  Transfer,  Encumbrance.  Debtor  will not  sell,  transfer,
mortgage,  or otherwise  encumber any  Collateral or impair the value thereof in
any manner without  Secured  Party's prior written  consent,  including  without
limitation by purchase, lease, barter, trade, payment deferral, or the creation,
assumption  or guarantee of  indebtedness  or other  lending of credit.  Secured
Party's written consent to any sale,  mortgage,  transfer,  or encumbrance shall
not be construed to be a waiver of this  provision in respect to any  subsequent
proposed sale, mortgage, transfer, or encumbrance.

         4.5 Liens. Neither Debtor nor any person acting on Debtor's behalf has,
or shall have any right, power, or authority to and shall not create,  incur, or
permit to be placed or  imposed,  upon the  Collateral,  any lien of any type or
nature whatsoever, other than the liens in favor of Secured Party.

         4.6 Matters or  Occurrences  Affecting  Collateral  or this  Agreement.
Debtor will promptly  notify Secured Party of any and all matters or occurrences
that may have a material adverse effect on the status or value of the Collateral
or this Agreement,  including  without  limitation the occurrence of an Event of
Default,  or an event  which,  with giving of notice or lapse of time,  or both,
would constitute an Event of Default.

     4.7  Agreements  Pertaining to  Collateral.  Debtor will not enter into any
type of contract or agreement  pertaining to any of the Collateral or in any way
transfer any voting rights pertaining to the Collateral to any person or entity.

         4.8 Dilution of Ownership.  As to any securities pledged as Collateral,
Debtor  will not  consent to or approve of the  issuance  of (i) any  additional
shares  of  any  class  of  securities  of  such  issuer,  (ii)  any  instrument
convertible  voluntarily  by  the  holder  thereof  or  automatically  upon  the
occurrence or  non-occurrence  of any event or condition  into, or  exchangeable
for, any such  securities,  or (iii) any warrants,  options,  contracts or other
commitments  entitling any third party to purchase or otherwise acquire any such
securities.  Notwithstanding  the  foregoing  or any  other  provision  of  this
Agreement to the contrary,  Debtor (i) shall comply with his  obligations  under
the  Purchase  Agreement  and each Other  Document,  and (ii) may consent to any
issuance of shares of Horizon if such  issuance has been  approved by a majority
of the Board of Directors of Horizon.

         4.9  Restrictions  on  Securities.  Debtor  will  not  enter  into  any
agreement  creating,  or otherwise permit to exist, any restriction or condition
upon the transfer,  voting or control of any  securities  pledged as Collateral,
except as consented to in writing by Secured Party. As to any securities pledged
as collateral, Debtor will not consent to or approve of any stock split, reverse
stock  split,  stock  dividend,  reclassification,   or  other  similar  act  or
transaction  regarding  the  Shares  unless  consented  to in writing by Secured
Party.

                                    ARTICLE V

                         DEBTOR'S AFFIRMATIVE COVENANTS

         Until payment and performance of all Obligations,  Debtor covenants and
agrees that it shall promptly  advise Secured Party in writing of any litigation
filed  against  Debtor  and of any  condition,  event or act which  comes to its
attention  that  would or might  have a  material  adverse  effect  on  Debtor's
financial condition or on Debtor's ability to perform the Obligations.

                                   ARTICLE VI

                               NEGATIVE COVENANTS

         Until payment and performance of all Obligations,  Debtor covenants and
agrees that Debtor will not, without the prior written consent of Secured Party:

     6.1 Liens. Grant, suffer, or permit liens on, or security interests in, the
Collateral.

         6.2  Violate  Other  Covenants.  Violate  or fail to  comply  with  any
covenants  or  agreements  regarding  other  debt  which  will or would with the
passage  of time or upon  demand  cause the  maturity  of any  other  debt to be
accelerated.

                                   ARTICLE VII

                              DEFAULT AND REMEDIES

     7.1 Events of Default.  An Event of Default  (herein so called) shall exist
if any one or more of the following events shall occur:

                  (a) The  failure  of Debtor to pay any amount  required  to be
paid under the Purchase Agreement (including,  without limitation, the indemnity
provisions  contained  therein),  or any other  amount  which  Debtor may now or
hereafter  owe to Secured Party under any Other  Agreement or otherwise,  within
(15) calendar days after such amount is due;

     (b) The failure of Debtor to pay any  Obligation  within (15) calendar days
after such amount is due; and

                  (c)  Debtor's  breach of a covenant in this  Agreement  or any
other  failure to perform  its  obligations  under this  Agreement  or any Other
Agreement.

     7.2 Secured Party's Remedies. Upon the occurrence of an Event of Default:

                  (a) Secured Party may declare the Obligations in whole or part
immediately due and may enforce payment and performance of the same and exercise
any rights under the Texas UCC,  rights and remedies of Secured Party under this
Agreement, or otherwise.

                  (b) Secured  Party may, at Secured  Party's  option and at the
expense of Debtor,  either in Secured Party's own right or in the name of Debtor
and in the same manner and to the same extent that Debtor  might  reasonably  so
act  if  this  Agreement  had  not  been  made:  (i) do  all  things  requisite,
convenient,  or  necessary  to enforce the  performance  and  observance  of all
rights,  remedies and privileges of Debtor arising from the  Collateral,  or any
part thereof,  including without limitation compromising,  waiving, excusing, or
in any manner releasing or discharging any obligation of any party to or arising
from the Collateral;  (ii) take possession of the books, papers,  chattel paper,
documents of title, and accounts of Debtor,  wherever  located,  relating to the
Collateral; (iii) sue or otherwise collect and receive money attributable to the
Collateral;  and (iv) exercise any other lawfully  available powers or remedies,
and do all other things  which  Secured  Party deems  requisite,  convenient  or
necessary  or which the  Secured  Party  deems  proper to protect  the  Security
Interest.

                  (b) Secured Party may foreclose  this  Agreement in the manner
now or  hereafter  provided  or  permitted  by law and may upon such  reasonable
notification  prior thereto as may be required by applicable  law (Debtor hereby
agreeing  that ten  days'  notice is  commercially  reasonable),  sell,  assign,
transfer,  or otherwise  dispose of the Collateral at public or private sale, in
whole  or in  part,  and  Secured  Party  may,  in its own  name or as  Debtor's
attorney-in-fact  effectively  assign and transfer the  Collateral,  or any part
thereof,   absolutely,  and  execute  and  deliver  all  necessary  assignments,
conveyances,  bills of sale, and other  instruments with power to substitute one
or more persons or  corporations  with like power.  Any such  foreclosure  sale,
assignment,  transfer,  or other  disposition  shall, to the extent permitted by
law,  be a  perpetual  bar,  both at law and in equity,  against  Debtor and all
persons and corporations  lawfully  claiming by or through or under Debtor.  Any
such foreclosure sale may be adjourned from time to time. Upon any sale, Secured
Party may bid for and purchase the  Collateral,  or any part  thereof,  and upon
compliance with the terms of sale may hold,  retain,  possess and dispose of the
Collateral, in its absolute right without further accountability.  Secured Party
shall have the right to be  credited  on the  amount of its bid a  corresponding
amount of the Obligations as of the date of such sale.

                  (c) If, in the opinion of Secured Party, there is any question
that a public sale or  distribution  of any Collateral will violate any state or
federal  securities  law,  Secured  Party  (i) may  offer  and  sell  securities
privately to purchasers who will agree to take them for investment  purposes and
not with a view to distribution  and who will agree to imposition of restrictive
legends on the  certificates  representing  the security,  or (ii) may sell such
securities in an intrastate  offering  under Section  3(a)(11) of the Securities
Act of 1933,  and no sale so made in good faith by Secured Party shall be deemed
to be not "commercially reasonable" because so made.

                  (d)  Not  in  limitation  of  any  other   provision  of  this
Agreement,  Secured  Party shall have all rights and remedies of a secured party
under the Texas UCC.

         7.3  Application  of Proceeds.  Secured Party may apply the proceeds of
any foreclosure  sale hereunder or from any other  permitted  disposition of the
Collateral  or any part  thereof as  follows:  (a) first,  to the payment of all
reasonable  costs and expenses of any foreclosure  and collection  hereunder and
all proceedings in connection  therewith,  including reasonable attorneys' fees;
(b) then, to the  reimbursement of Secured Party for all  disbursements  made by
Secured Party for taxes,  assessments or liens superior to the Security Interest
and  which  Secured  Party  shall  deem  expedient  to  pay;  (c)  then,  to the
reimbursement of Secured Party of any other  disbursements made by Secured Party
in accordance with the terms hereof or under the Purchase Agreement or any Other
Agreement;  (d) then,  to or among the amounts of fees,  interest and  principal
then owing and unpaid in respect of the Obligations, in such priority as Secured
Party may determine in its  discretion;  and (e) the remainder of such proceeds,
if any,  shall be paid to Debtor.  If such  proceeds  shall be  insufficient  to
discharge the entire  Obligations,  Secured Party shall have any other available
legal recourse  against Debtor under,  or for the  performance  of, the Purchase
Agreement and any Other  Agreement  between  Debtor and Secured  Party,  for the
deficiency,  together with interest  thereon at the maximum rate permitted under
applicable law.

         7.4  Enforcement  of  Obligations.  Nothing in this Agreement or in any
other  document  or  agreement  shall  affect or impair  the  unconditional  and
absolute right of Secured Party to enforce the  Obligations as and when the same
shall become due in accordance with the terms of the Purchase Agreement or Other
Agreement.

                                  ARTICLE VIII

                             RIGHTS OF SECURED PARTY

         8.1  Subrogation.  Upon the occurrence of an Event of Default,  Secured
Party,  at its  election,  may  subrogate  to all of the  interest,  rights  and
remedies  of the  Debtor,  in respect  to any of the  Collateral  or  agreements
pertaining thereto.

         8.2 Secured Party  Appointed  Attorney-in-Fact.  Debtor hereby appoints
Secured Party as  attorney-in-fact  of Debtor,  with full authority in the place
and stead of Debtor and in the name of Debtor, Secured Party or otherwise,  from
time to time on Secured  Party's  discretion and upon the occurrence of an Event
of Default, to take any action and to execute any instrument which Secured Party
may deem  necessary or advisable to accomplish  the purposes of this  Agreement,
including without  limitation:  (a) to ask, demand,  collect,  sue for, recover,
compound, receive and give acquittance and receipts for moneys due and to become
due under or in respect of any of the Collateral;  (b) to receive,  endorse, and
collect  any  drafts or other  instruments,  documents  and  chattel  paper,  in
connection  with clause (a) of this  Section 8.2; (c) to file any claims or take
any action or institute any proceeding which Secured Party may deem necessary or
desirable for the  collection  of any of the  Collateral or otherwise to enforce
the rights of Secured Party against any of the Collateral; and (d) to assign and
transfer the  Collateral,  or any part  thereof,  absolutely  and to execute and
deliver  endorsements,   assignments,  conveyances,  bills  of  sale  and  other
instruments  with power to substitute  one or more persons or  corporation  with
like power.

         8.3  Performance  by Secured  Party.  If Debtor  fails to  perform  any
agreement  contained  herein,  Secured  Party may itself  perform,  or cause the
performance  of, such  agreement,  and the reasonable  expenses of Secured Party
incurred in connection  therewith  shall be payable by Debtor under Section 8.8.
In no  event,  however,  shall  Secured  Party  have any  obligation  or  duties
whatsoever to perform any covenant or agreement of Debtor contained herein,  and
any such performance by Secured Party shall be wholly discretionary with Secured
Party.

         8.4 Duties of Secured  Party.  The powers  conferred upon Secured Party
hereunder  are solely to protect its  interest in the  Collateral  and shall not
impose any duty upon it to exercise any such powers. Except for the safe custody
of any  Collateral  in its  possession  and the  accounting  for money  actually
received by it hereunder,  Secured Party shall have no duty as to any Collateral
or as to the taking of any  necessary  steps to preserve  rights  against  prior
parties or any other rights  pertaining to any Collateral.  Without limiting the
generality of the foregoing,  Secured Party shall not have any obligation,  duty
or  responsibility  to do any of the  following:  (a) ascertain any  maturities,
calls,  conversions,  exchanges,  offers, tenders or similar matters relating to
the  Collateral or informing  Debtor with respect to any such matters;  (b) fix,
preserve  or  exercise  any  right,  privilege  or option  (whether  conversion,
redemption or otherwise) with respect to the Collateral; (c) collect any amounts
payable  in  respect  of the  Collateral;  (d)  sell all or any  portion  of the
Collateral,  for any reason;  or (e) hold the Collateral for or on behalf of any
party other than Debtor.

         8.5 No  Liability  of Secured  Party.  Neither the  acceptance  of this
Agreement by Secured Party,  nor the exercise of any rights hereunder by Secured
Party,  shall be construed in any way as an  assumption  by Secured Party of any
obligations,  responsibilities,  or duties of Debtor arising in connection  with
the  Collateral  assigned  hereunder  or  otherwise  bind  Secured  Party to the
performance of any  obligations  respecting the  Collateral,  it being expressly
understood  that Secured  Party shall not be obligated to perform,  observe,  or
discharge  any  obligation,  responsibility,  duty,  or  liability  of Debtor in
respect of any of the Collateral,  including without limitation  appearing in or
defending any action, expending any money or incurring any expense in connection
therewith.  TO THE FULLEST EXTENT  PERMITTED BY APPLICABLE LAW, DEBTOR SHALL AND
DOES AGREE TO INDEMNIFY,  PROTECT,  DEFEND AND HOLD HARMLESS  SECURED PARTY, ITS
OFFICERS, DIRECTORS, REPRESENTATIVES, AGENTS, EMPLOYEES, LENDERS, SUCCESSORS AND
ASSIGNS,  FROM AND AGAINST ALL  LIABILITIES,  CLAIMS,  DAMAGES,  LOSSES,  FINES,
PENALTIES,  CAUSES OF ACTIONS,  SUITS,  JUDGMENTS AND EXPENSES  (INCLUDING COURT
COSTS,  ATTORNEY'S  FEES  AND  COST OF  INVESTIGATION)  OF ANY  NATURE,  KIND OR
DESCRIPTION  OF ANY PERSON OR ENTITY,  DIRECTLY OR  INDIRECTLY,  ARISING OUT OF,
CAUSED BY OR  RESULTING  FROM (IN WHOLE OR IN PART),  ANY  UNINTENTIONAL  ACT OR
OMISSION (OR  INTENTIONAL  ACT OR OMISSION SO LONG AS SUCH ACT OR OMISSION  DOES
NOT  CONSTITUTE  NEGLIGENCE)  OF SECURED  PARTY (OR  ANYONE  ACTING ON BEHALF OF
SECURED PARTY) IN CONNECTION WITH THE COLLATERAL.  THE FOREGOING INDEMNITY SHALL
SURVIVE THE EXPIRATION OR EARLIER TERMINATION OF THIS AGREEMENT.

         8.6 Right of Secured Party to Defend Action Affecting Security. Secured
Party  may,  at the  expense  of  Debtor,  appear in and  defend  any  action or
proceeding at law or in equity  purporting to affect  Secured  Party's  Security
Interest under this Agreement.

         8.7 Right of  Secured  Party to Prevent  or Remedy  Default.  If Debtor
shall fail to perform any of the covenants,  conditions and agreements  required
to be performed  and  observed by Debtor  under the Purchase  Agreement or Other
Agreement,  or in respect of the Collateral  (subject to any applicable  default
cure  period),  Secured  Party (a) may but shall  not be  obligated  to take any
action Secured Party deems  necessary or desirable to prevent or remedy any such
default by Debtor or otherwise to protect the Security  Interest,  and (b) shall
have the absolute and immediate  right to take  possession of the  Collateral or
any  part  thereof  (to  the  extent  Secured  Party  has not  previously  taken
possession)  to such  extent  and as often  as the  Secured  Party,  in its sole
discretion, deems necessary or desirable in order to prevent or to cure any such
default by Debtor,  or  otherwise  to protect the  security  of this  Agreement.
Secured Party may advance or expend such sums of money for the account of Debtor
as Secured Party in its sole discretion deems necessary for any such purpose.

         8.8 Secured Party's Expenses. All reasonable advances, costs, expenses,
charges and attorneys' fees which Secured Party may make, pay or incur under any
provision  of this  Agreement  for the  protection  of its  security  or for the
enforcement of any of its rights hereunder,  including,  without limitation,  in
foreclosure  proceedings commenced and subsequently  abandoned or in any dispute
or litigation in which Secured Party or the holder of any of the Obligations may
become involved by reason of or arising out of the Purchase Agreement, any Other
Agreement or the  Collateral,  shall be a part of the  Obligations  and shall be
paid by Debtor to Secured Party, upon demand, and shall bear interest until paid
at the maximum  rate of interest  permitted  by  applicable  law,  from the date
incurred by Secured Party until repaid by Debtor.

         8.9. Convertible  Collateral.  Secured Party may present for conversion
any  Collateral  which is  convertible  into any other  instrument or investment
security or a  combination  thereof with cash,  but Secured Party shall not have
any duty to present for conversion any Collateral  unless it shall have received
from Debtor  detailed  written  instructions to that effect at a time reasonably
far in advance of the final conversion date to make such conversion possible.

         8.10 Secured Party's Right of Set-Off.  Upon the happening of any event
entitling  Secured  Party to pursue any remedy  provided  herein,  or if Secured
Party shall be served with garnishment process in which Debtor shall be named as
defendant,  whether or not Debtor  shall be in  default  hereunder  at the time,
Secured Party may, but shall not be required to, set-off any indebtedness  owing
by  Secured  Party  to  Debtor  against  any of the  Obligations  without  first
resorting to the security hereunder and without prejudice to any other rights or
remedies of Secured Party or its Security Interest.

         8.11 Remedies.  No right or remedy herein  reserved to Secured Party is
intended to be exclusive  of any other right or remedy,  but each and every such
remedy shall be cumulative,  not in lieu of, but in addition to any other rights
or remedies given under this Agreement and all other security documents. Any and
all of Secured  Party's  rights and remedies may be exercised  from time to time
and as often as such exercise as deemed necessary or desirable by Secured Party.

         8.12 Debtor's Waivers.  Debtor waives notice of the creation,  advance,
increase,  existence,  extension,  or  renewal  of, and of any  indulgence  with
respect to, the  Obligations;  waives notice of intent to accelerate,  notice of
acceleration,  notice  of  intent  to  demand,  presentment,  demand,  notice of
dishonor,  and  protest;   waives  notice  of  the  amount  of  the  Obligations
outstanding  at any time,  notice of any change in  financial  condition  of any
person liable for the  Obligations  or any part thereof,  notice of any Event of
Default,  and all other  notices  respecting  the  Obligations;  and agrees that
maturity of the Obligations  and any part thereof may be accelerated,  extended,
or renewed one or more times by Secured Party in its discretion,  without notice
to Debtor.

         8.13 Other Parties and Other Collateral.  No renewal or extension of or
any other  indulgence  with respect to the  Obligations or any part thereof,  no
release  of any  security,  no  release  of any  person  (including  any  maker,
endorser,  guarantor,  or  surety)  liable  on  the  Obligations,  no  delay  in
enforcement  of payment,  and no delay or admission or lack of diligence or care
in exercising any right or power with respect to the Obligations or any security
therefor  or  guaranty  thereof or under this  Agreement  shall in other  manner
impair  or affect  the  rights  of  Secured  Party  under  the law,  under  this
Agreement,  or under any other  document or  agreement  pertaining  to the other
security for the  Obligations,  before  foreclosing  upon the Collateral for the
purpose of paying the Obligations.  Debtor waives any right to the benefit of or
to require or control application of any other security or proceeds thereof, and
Debtor  agrees that Secured  Party shall have no duty or obligation to Debtor to
apply to the Obligations any such other security or proceeds thereof.

         8.14 Relationship Among Secured Party.  Either or both of Secured Party
are  entitled  to enforce  any and all rights  granted to Secured  Party in this
Agreement,  and to take any other  action  allowed to be taken by Secured  Party
under this  Agreement.  Neither  Secured  Party is under any  obligation  to act
jointly or in concert with the other Secured Party  pursuant to this  Agreement.
Any action  required  to be taken by Debtor,  or notice  required to be given by
Debtor, shall be taken or given with respect to both of Secured Party.

                                   ARTICLE IX

                                  MISCELLANEOUS

     9.1 Terms  Commercially  Reasonable.  The terms of this Agreement  shall be
deemed commercially reasonable within the meaning of the Texas UCC.

         9.2 Notices.  Any notices or demands  required or permitted to be given
hereunder  shall be  deemed  sufficiently  given if in  writing  and  personally
delivered or mailed (with all postage and charges prepaid), addressed to Secured
Party or to Debtor their respective  addresses set forth below, or at such other
address as the above parties may from time to time  designate by written  notice
to the other given in  accordance  with this Section  9.2.  Any such notice,  if
personally  delivered or  transmitted  by telex or telegram,  shall be deemed to
have been given on the date so delivered or transmitted or, if mailed, be deemed
to have been given on the day after such  notice is placed in the United  States
mail in accordance with this Section 9.2.

                  Secured Party:  1301 Capital of Texas Hwy., Suite C-300
                                  Austin, Travis County, Texas 78746
                                  Attn: President

                  with copy to:   Timothy L. LaFrey, Esq.
                                  Akin, Gump, Strauss, Hauer & Feld, L.L.P.
                                  1900 Frost Bank Plaza
                                  816 Congress Avenue
                                  Austin, Texas 78701

                  Debtor:         David P. Bates III and Jane A. Bates
                                  1320 Canyon Side Avenue
                                  San Ramon, California   94583

         9.3 Parties Bound.  Secured Party's rights under this Agreement and the
Security Interest shall inure to the benefits of its successors and assigns, and
in the event of any  assignment  or  transfer of any of the  Obligations  or the
Collateral,  Secured  Party  thereafter  shall  be  fully  discharged  from  any
responsibility  with respect to the Collateral so assigned or  transferred,  but
Secured  Party shall  retain all rights and powers  hereby given with respect to
any of the  Obligations  or  Collateral  not so  assigned  or  transferred.  All
representations, warranties, and agreements of Debtor if more than one are joint
and several, and all shall be binding upon the personal representatives,  heirs,
successors, and assigns of Debtor.

         9.4 Waiver.  No delay of Secured Party in exercising any power or right
shall operate as a waiver thereof;  nor shall any single or partial  exercise of
any power or right preclude other or further exercise thereof or the exercise of
any other power or right.  No waiver by Secured Party of any right  hereunder of
any default by Debtor shall be binding upon Secured Party unless in writing, and
no failure by Secured  Party to exercise any power or right  hereunder or waiver
of any  default  by Debtor  shall  operate  as a waiver of any other or  further
exercise of such right or power of any further default.

         9.5 Agreement Continuing.  This Agreement shall constitute a continuing
agreement,  applying to all future as well as existing transactions,  whether or
not of the  character  contemplated  at the date of this  Agreement,  and if all
transactions between Secured Party and Debtor shall be closed at any time, shall
be equally  applicable to any new  transactions  thereafter.  Provisions of this
Agreement,  unless  by their  terms  exclusive,  shall be in  addition  to those
contained in any Other Agreement.

         9.6 Definitions. Unless the context indicated otherwise, definitions in
the Texas Business and Commerce Code ("Texas UCC") apply to words and phrases in
this Agreement; if Texas UCC definitions conflict, Chapter 9 definitions apply.

         9.7 Miscellaneous. In this Agreement, whenever the context so requires,
the neuter gender  includes the masculine and feminine,  and the singular number
includes  the plural and vice  versa.  The  headings  of  paragraphs  herein are
inserted only for convenience and shall in no way define,  describe or limit the
scope of intent of any  provisions  of this  Agreement.  No  change,  amendment,
modification,  cancellation,  or  discharge of any  provision of this  Agreement
shall be valid unless consented to in writing by Secured Party.

         9.8 Assignment of Secured  Party's  Interest.  Secured Party shall have
the right to assign all or any portion of its rights in this  Agreement  without
approval or consent.  Debtor may not assign this  Agreement or any of its rights
or obligations  hereunder  without the express prior written  consent of Secured
Party in each instance.

     9.9 Applicable  Laws.  THIS AGREEMENT SHALL BE GOVERNED BY AND CONSTRUED IN
ACCORDANCE  WITH THE LAWS OF THE STATE OF TEXAS AND THE  APPLICABLE  LAWS OF THE
UNITED STATES OF AMERICA.

     9.10 ENTIRE AGREEMENT.  THIS AGREEMENT AND THE PURCHASE AGREEMENT REPRESENT
THE FINAL AGREEMENT  BETWEEN THE PARTIES AND MAY NOT BE CONTRADICTED BY EVIDENCE
OF PRIOR,  CONTEMPORANEOUS,  OR SUBSEQUENT ORAL AGREEMENTS OF THE PARTIES. THERE
ARE NO UNWRITTEN ORAL AGREEMENTS BETWEEN THE PARTIES.

                            [Signature page follows]


<PAGE>


S-1



                                SIGNATURE PAGE TO

                                 ASSIGNMENT AND

                               SECURITY AGREEMENT

         EXECUTED this 1st day of September, 1999.

DEBTOR:

                                          /s/ David Bates

                                          Printed Name:  David Bates

                                          /s/ Jane A. Bates

                                          Printed Name:  Jane A. Bates


SECURED PARTY:                            Prime Medical Operating, Inc.

                                          By: /s/ Cheryl Williams

                                          Name: Cheryl Williams

                                          Title: Vice President


                                          Prime/BDR Acquisition, L.L.C.

                                          By: /s/ Cheryl Williams

                                          Name: Cheryl Williams

                                          Title: Vice President

<PAGE>



                        ASSIGNMENT AND SECURITY AGREEMENT

     THIS  ASSIGNMENT  AND SECURITY  AGREEMENT  (this  "Agreement")  is made and
entered into as of the 1st day of September,  1999, by and between Prime Medical
Operating, Inc., a Delaware corporation ("PMOI"), Prime/BDR Acquisition, L.L.C.,
a Delaware limited  liability  company ("Prime") (PMOI and Prime are referred to
herein,  jointly and  severally,  as "Secured  Party") and John Robert  Griffin,
M.D., Family Revocable Trust dated February 8, 1991 (the "Debtor").

                                    RECITALS:

         A. Debtor and Secured Party have  executed and  delivered  that certain
Stock  Purchase   Agreement  dated  as  of  September  1,  1999  (the  "Purchase
Agreement"),  pursuant to which  Secured  Party  purchased  from Debtor  certain
shares of the $0.01 par value  common  stock of Horizon  Vision  Center,  Inc, a
Nevada corporation ("Horizon").

         B. Secured Party has requested  that Debtor pledge the  Collateral  (as
defined below) to secure certain obligations and liabilities that Debtor may now
or hereafter have to Secured Party, including, without limitation, any indemnity
obligations arising under the Purchase Agreement.

     C. Debtor desires to enter into this Agreement as a material  inducement to
Secured  Party's  purchase  of  Debtor's  shares of Horizon  under the  Purchase
Agreement.

                                   AGREEMENT:

         NOW, THEREFORE, in consideration of the foregoing and the covenants and
agreements hereinafter set forth, and other good and valuable consideration, the
receipt and sufficiency of which Debtor  acknowledges,  Debtor and Secured Party
agree as follows:

                                    ARTICLE I

                       COLLATERAL AND SECURED OBLIGATIONS

         1.1 Grant of Security Interest.  Debtor hereby assigns,  transfers, and
pledges to Secured  Party,  and Debtor hereby grants to Secured Party a security
interest   in,   the   following   described   collateral   (collectively,   the
"Collateral"):

                  (a)  Shares  of  Horizon.  From  and  after  the  date of this
Agreement,  (i) all shares of the common  stock of Horizon  owned or acquired in
any manner by Debtor or Secured Party,  (collectively,  the "Shares"),  (ii) any
replacements,  substitutions,  or exchanges of the certificates representing the
Shares, and (iii) any and all options,  rescission rights,  registration rights,
conversion rights, subscription rights, contractual or quasi-contractual rights,
warrants,  redemption rights,  redemption proceeds, calls, preemptive rights and
all other rights and benefits pertaining to the Shares;

                  (b)  Accounts.  All  accounts  and  rights  now  or  hereafter
attributable to any of the Collateral  described in (a) above, and all rights of
Debtor now or hereafter arising under any agreement pertaining to the Collateral
described  in  (a)  above,   including  without  limitation  all  distributions,
proceeds, fees, dividends,  preferences,  payments or other benefits of whatever
nature  which  Debtor is now or may  hereafter  become  entitled to receive with
respect to any Collateral described in (a) above;

                  (c) Additional  Property.  "Collateral" shall also include the
following  property  (collectively,  the  "Additional  Property")  which  Debtor
becomes  entitled  to receive or shall  receive  in  connection  with any of the
Collateral  described in this Section 1.1: (i) any stock certificate,  including
without  limitation,  any  certificate  representing  a  stock  dividend  or any
certificate in connection with any recapitalization,  reclassification,  merger,
consolidation,  conversion,  sale of assets, combination of shares, stock split,
reverse  stock split or  spin-off;  (ii) any option,  warrant,  subscription  or
right,  whether as an addition to or in  substitution  of any of the  Collateral
described in this Section 1.1; (iii) any dividends or  distributions of any kind
whatsoever,  whether  distributable in cash,  stock or other property;  (iv) any
interest,  premium or principal  payments;  and (v) any conversion or redemption
proceeds; and

                  (d) Proceeds.  All proceeds (cash and non-cash) arising out of
the sale, exchange, collection or other disposition of all or any portion of the
Collateral  described in (a), (b), or (c) above,  including  without  limitation
proceeds in the form of stock, accounts, chattel paper, instruments,  documents,
goods, inventory and equipment.

The  security  interest in the  Collateral  hereby  granted by Debtor to Secured
Party may sometimes be referred to in this Agreement as the "Security Interest".

         1.2 Obligations.  This Agreement and the Security Interest shall secure
full and punctual payment and performance of the following indebtedness,  duties
and obligations (collectively, the "Obligations"):

                  (a) All liabilities and obligations of Debtor to Secured Party
(including, without limitation, any principal, interest, fees and other amounts,
and any  indemnity  obligations)  under and pursuant to the Purchase  Agreement,
this Agreement and/or any other contract or agreement  between Secured Party and
Debtor or any affiliate of Debtor (collectively, "Other Agreements"; and

                  (b) (i)  all  indebtedness,  obligations  and  liabilities  of
Debtor and/or any affiliate of Debtor to Secured Party of any kind or character,
now existing or hereafter arising, whether direct, indirect, related, unrelated,
fixed,  contingent,  liquidated,  unliquidated,  joint,  several  or  joint  and
several,  arising from,  connected with, or related to the Purchase Agreement or
any Other Agreement, or any other document, agreement, or instrument executed in
connection  either of them,  (ii) all accrued but unpaid  interest on any of the
indebtedness  described in (i) above, (iii) all obligations of Debtor and/or any
affiliate  of  Debtor  to  Secured  Party  under  any  documents  or  agreements
evidencing,  securing,  governing  and/or  pertaining  to all or any part of the
indebtedness  described  in (i) and (ii)  above,  (iv) all  costs  and  expenses
incurred by Secured Party in connection  with the collection and  administration
of all or any part of the  indebtedness  and obligations  described in (i), (ii)
and (iii) above or the protection or preservation  of, or realization  upon, the
collateral  securing  all or any  part of  such  indebtedness  and  obligations,
including  without  limitation  all  attorneys'  fees,  and  (v)  all  renewals,
extensions, modifications and rearrangements of the indebtedness and obligations
described in (i), (ii), (iii) and (iv) above.

                  (c) All sums now or  hereafter  loaned or  advanced by Secured
Party to  Debtor,  or  expended  by Secured  Party for the  account of Debtor or
otherwise owing by Debtor to Secured Party, in respect of the  Obligations,  and
all other sums  expended or advanced  by Secured  Party  pursuant to any term or
provision of this Agreement or any Other Agreement (i) to collect and/or enforce
the Obligations or (ii) to maintain, protect and preserve the Collateral.

                                   ARTICLE II

       DEBTOR'S REPRESENTATIONS AND WARRANTIES WITH RESPECT TO COLLATERAL

       Debtor hereby represents and warrants to Secured Party as follows:

         2.1 Ownership of Collateral.  Debtor has good and  marketable  title to
the Collateral  free and clear of any liens,  security  interests,  shareholders
agreement, calls, charge, or encumbrance,  except for this Security Interest. No
financing  statement or other  instrument  similar in effect covering all or any
part of the  Collateral is on file in any recording  office,  except as may have
been filed in favor of Secured Party relating to this Agreement.

         2.2  Power  &  Authority.  Debtor  has the  lawful  right,  power,  and
authority  to grant the Security  Interest in the  Collateral.  This  Agreement,
together  with all filings and other  actions  necessary or desirable to perfect
and protect such security interest,  which have been duly taken,  create a valid
and perfected first priority  security  interest in the Collateral  securing the
payment and performance of the Obligations.

         2.3  No  Agreements.  The  Shares  are  not  subject  to any  right  of
redemption,  or any  call or put  options,  voting  trust,  proxy,  shareholders
agreement,  right of first  refusal,  or any other  document or agreement  which
would in any way impair or adversely affect this Security Interest or the rights
of Secured Party under this Agreement.

         2.4  Securities.  Any  certificates  evidencing  securities  pledged as
Collateral  are valid and  genuine  and have not been  altered.  All  securities
pledged as Collateral have been duly  authorized and validly  issued,  are fully
paid and  non-assessable,  and were not issued in  violation  of the  preemptive
rights of any party or of any agreement by which Debtor or the issuer thereof is
bound.  No  restrictions  or  conditions  exist with  respect to the transfer or
voting of any securities pledged as Collateral.

                                   ARTICLE III

                  DEBTOR'S OTHER REPRESENTATIONS AND WARRANTIES

         3.1 Solvency of Debtor.  As of the date hereof,  (i) Debtor is solvent;
(ii) the fair saleable  value of Debtor's  assets exceeds  Debtor's  liabilities
(both fixed and contingent);  (iii) Debtor has sufficient capital to satisfy all
of Debtor's  obligations  as they  become due;  (iv) no  receiver,  trustee,  or
custodian has been appointed for, or taken  possession of, all or  substantially
all of the assets of Debtor,  either in a  proceeding  brought by Debtor or in a
proceeding  brought against Debtor;  (v) Debtor is not the subject of a petition
for relief under the United  States  Bankruptcy  Code or any similar  federal or
state insolvency law, including without limitation a petition filed by Debtor or
a petition filed by a third party seeking relief against Debtor; and (vi) Debtor
has no  intention  of filing a  petition  for  relief  under the  United  States
Bankruptcy  Code or any similar  federal or state  insolvency law, or of seeking
any other form of  creditor  relief,  within  the  two-year  period  immediately
following the date of this Agreement.

         3.2  Authority and  Compliance.  Debtor has full power and authority to
enter into this Agreement. Debtor has full power and authority to enter into and
perform its obligations  under the Purchase  Agreement and each Other Agreement.
No further  consent or approval is  required as a condition  to the  validity of
this  Agreement,  the Purchase  Agreement or any Other  Agreement.  Debtor is in
compliance with all applicable laws, ordinances,  statutes, orders, regulations,
judgments, writs, or decrees of any governmental entity to which it is subject.

         3.3 Binding Agreement.  This Agreement, the Purchase Agreement and each
Other Agreement  constitute valid and legally binding  obligations of Debtor, in
accordance  with  their  terms,  subject  to  (a)  the  applicable   bankruptcy,
insolvency,  reorganization,  moratorium,  and similar laws affecting creditors'
rights  generally  and  (b)  restrictions  imposed  by any  court  of  competent
jurisdiction on the enforcement of non-competition and exclusive use restrictive
covenants imposed under the Purchase Agreement or any Other Agreement.

         3.4 Litigation.  There are no proceedings  pending or, to the knowledge
of Debtor,  threatened before any court or  administrative  agency which will or
may have a material adverse effect on the financial  condition of Debtor or upon
Debtor's ability to perform its obligations  under this Agreement,  the Purchase
Agreement or any Other Agreement.

         3.5 No Conflicting Agreements.  There are no provisions of any existing
agreement,  mortgage,  indenture or contract  binding on Debtor or affecting its
property,  which  would  conflict  with or in any  way  prevent  the  execution,
delivery, or carrying out of the terms of this Agreement, the Purchase Agreement
or any Other Agreement.

         3.6  Ownership  of  Assets.  Debtor  has  good  and  full  title to the
Collateral,  and the  Collateral  is owned  free and  clear of  liens,  charges,
claims, security interests, and other encumbrances.

     3.7 Taxes. Debtor has filed all tax returns required to be filed by Debtor.

                                   ARTICLE IV

                  DEBTOR'S COVENANTS WITH RESPECT TO COLLATERAL

         Debtor  covenants  and agrees  that from the date  hereof and until the
payment  and  performance  in  full  of the  Obligations  unless  Secured  Party
otherwise consents in writing:

         4.1  Delivery of  Instruments  and/or  Certificates.  Contemporaneously
herewith,   Debtor  covenants  and  agrees  to  deliver  to  Secured  Party  any
certificates,   documents,   or  instruments   representing  or  evidencing  the
Collateral,  with Debtor's  endorsement  thereon and/or  accompanied by property
instruments of transfer and assignment duly executed in blank with, if requested
by Secured Party,  signatures  guaranteed by a member or member  organization in
good standing of an authorized Securities Transfer Agents Medallion Program, all
in form and substance satisfactory to Secured Party.

         4.2  Further  Assurances.   Debtor  will   contemporaneously  with  the
execution  hereof  and from  time to time  thereafter  at its  expense  promptly
execute and deliver all further  instruments  and documents and take all further
action  necessary or  appropriate or that Secured Party may request in order (i)
to perfect and protect the security  interest created or purported to be created
hereby and the first priority of such security interest,  (ii) to enable Secured
Party to exercise  and enforce its rights and  remedies  hereunder in respect of
the  Collateral,  and (iii) to otherwise  effect the purposes of this Agreement,
including  without  limitation:  (A)  executing  and  filing  any  financing  or
continuation  statements,  or any  amendments  thereto;  (B)  obtaining  written
confirmation  from the issuer of any  securities  pledged as  Collateral  of the
pledge of such securities,  in form and substance satisfactory to Secured Party;
(C)  cooperating  with Secured Party in registering the pledge of any securities
pledged as Collateral with the issuer of such securities;  (D) delivering notice
of Secured Party's security interest in any securities  pledged as Collateral to
any securities or financial  intermediary,  clearing  corporation or other party
required by Secured Party, in form and substance  satisfactory to Secured Party;
and  (E)  obtaining  written  confirmation  of  the  pledge  of  any  securities
constituting Collateral from any securities or financial intermediary,  clearing
corporation  or other party  required by Secured  Party,  in form and  substance
satisfactory to Secured Party.

         4.3 Additional Property. All Additional Property, as defined in Section
1.1(c)  above,  received by Debtor shall be received in trust for the benefit of
Secured Party.  All Additional  Property and all  certificates  or other written
instruments or documents  evidencing and/or representing the Additional Property
that is  received  by Debtor,  together  with such  instruments  of  transfer as
Secured Party may request,  shall  immediately be delivered to or deposited with
Secured Party and held by Secured  Party as  Collateral  under the terms of this
Agreement.  If the  Additional  Property  received  by Debtor and  delivered  to
Secured  Party  pursuant  to this  Section  shall  be  shares  of stock or other
securities,  such shares of stock or other  securities shall be duly endorsed in
blank or  accompanied  by proper  instruments  of transfer and  assignment  duly
executed in blank with, if requested by Secured Party,  signatures guaranteed by
a member or member  organization  in good standing of an  authorized  Securities
Transfer Agents  Medallion  Program,  all in form and substance  satisfactory to
Secured  Party.  Secured  Party  shall  be  deemed  to  have  possession  of any
Collateral in transit to Secured Party or its agent.

         4.4  Sale,  Transfer,  Encumbrance.  Debtor  will not  sell,  transfer,
mortgage,  or otherwise  encumber any  Collateral or impair the value thereof in
any manner without  Secured  Party's prior written  consent,  including  without
limitation by purchase, lease, barter, trade, payment deferral, or the creation,
assumption  or guarantee of  indebtedness  or other  lending of credit.  Secured
Party's written consent to any sale,  mortgage,  transfer,  or encumbrance shall
not be construed to be a waiver of this  provision in respect to any  subsequent
proposed sale, mortgage, transfer, or encumbrance.

         4.5 Liens. Neither Debtor nor any person acting on Debtor's behalf has,
or shall have any right, power, or authority to and shall not create,  incur, or
permit to be placed or  imposed,  upon the  Collateral,  any lien of any type or
nature whatsoever, other than the liens in favor of Secured Party.

         4.6 Matters or  Occurrences  Affecting  Collateral  or this  Agreement.
Debtor will promptly  notify Secured Party of any and all matters or occurrences
that may have a material adverse effect on the status or value of the Collateral
or this Agreement,  including  without  limitation the occurrence of an Event of
Default,  or an event  which,  with giving of notice or lapse of time,  or both,
would constitute an Event of Default.

     4.7  Agreements  Pertaining to  Collateral.  Debtor will not enter into any
type of contract or agreement  pertaining to any of the Collateral or in any way
transfer any voting rights pertaining to the Collateral to any person or entity.

         4.8 Dilution of Ownership.  As to any securities pledged as Collateral,
Debtor  will not  consent to or approve of the  issuance  of (i) any  additional
shares  of  any  class  of  securities  of  such  issuer,  (ii)  any  instrument
convertible  voluntarily  by  the  holder  thereof  or  automatically  upon  the
occurrence or  non-occurrence  of any event or condition  into, or  exchangeable
for, any such  securities,  or (iii) any warrants,  options,  contracts or other
commitments  entitling any third party to purchase or otherwise acquire any such
securities.  Notwithstanding  the  foregoing  or any  other  provision  of  this
Agreement to the contrary,  Debtor (i) shall comply with his  obligations  under
the  Purchase  Agreement  and each Other  Document,  and (ii) may consent to any
issuance of shares of Horizon if such  issuance has been  approved by a majority
of the Board of Directors of Horizon.

         4.9  Restrictions  on  Securities.  Debtor  will  not  enter  into  any
agreement  creating,  or otherwise permit to exist, any restriction or condition
upon the transfer,  voting or control of any  securities  pledged as Collateral,
except as consented to in writing by Secured Party. As to any securities pledged
as collateral, Debtor will not consent to or approve of any stock split, reverse
stock  split,  stock  dividend,  reclassification,   or  other  similar  act  or
transaction  regarding  the  Shares  unless  consented  to in writing by Secured
Party.

                                    ARTICLE V

                         DEBTOR'S AFFIRMATIVE COVENANTS

         Until payment and performance of all Obligations,  Debtor covenants and
agrees that it shall promptly  advise Secured Party in writing of any litigation
filed  against  Debtor  and of any  condition,  event or act which  comes to its
attention  that  would or might  have a  material  adverse  effect  on  Debtor's
financial condition or on Debtor's ability to perform the Obligations.

                                   ARTICLE VI

                               NEGATIVE COVENANTS

         Until payment and performance of all Obligations,  Debtor covenants and
agrees that Debtor will not, without the prior written consent of Secured Party:

     6.1 Liens. Grant, suffer, or permit liens on, or security interests in, the
Collateral.

         6.2  Violate  Other  Covenants.  Violate  or fail to  comply  with  any
covenants  or  agreements  regarding  other  debt  which  will or would with the
passage  of time or upon  demand  cause the  maturity  of any  other  debt to be
accelerated.

                                   ARTICLE VII

                              DEFAULT AND REMEDIES

     7.1 Events of Default.  An Event of Default  (herein so called) shall exist
if any one or more of the following events shall occur:

                  (a) The  failure  of Debtor to pay any amount  required  to be
paid under the Purchase Agreement (including,  without limitation, the indemnity
provisions  contained  therein),  or any other  amount  which  Debtor may now or
hereafter  owe to Secured Party under any Other  Agreement or otherwise,  within
(15) calendar days after such amount is due;

     (b) The failure of Debtor to pay any  Obligation  within (15) calendar days
after such amount is due; and

                  (c)  Debtor's  breach of a covenant in this  Agreement  or any
other  failure to perform  its  obligations  under this  Agreement  or any Other
Agreement.

     7.2 Secured Party's Remedies. Upon the occurrence of an Event of Default:

                  (a) Secured Party may declare the Obligations in whole or part
immediately due and may enforce payment and performance of the same and exercise
any rights under the Texas UCC,  rights and remedies of Secured Party under this
Agreement, or otherwise.

                  (b) Secured  Party may, at Secured  Party's  option and at the
expense of Debtor,  either in Secured Party's own right or in the name of Debtor
and in the same manner and to the same extent that Debtor  might  reasonably  so
act  if  this  Agreement  had  not  been  made:  (i) do  all  things  requisite,
convenient,  or  necessary  to enforce the  performance  and  observance  of all
rights,  remedies and privileges of Debtor arising from the  Collateral,  or any
part thereof,  including without limitation compromising,  waiving, excusing, or
in any manner releasing or discharging any obligation of any party to or arising
from the Collateral;  (ii) take possession of the books, papers,  chattel paper,
documents of title, and accounts of Debtor,  wherever  located,  relating to the
Collateral; (iii) sue or otherwise collect and receive money attributable to the
Collateral;  and (iv) exercise any other lawfully  available powers or remedies,
and do all other things  which  Secured  Party deems  requisite,  convenient  or
necessary  or which the  Secured  Party  deems  proper to protect  the  Security
Interest.

                  (b) Secured Party may foreclose  this  Agreement in the manner
now or  hereafter  provided  or  permitted  by law and may upon such  reasonable
notification  prior thereto as may be required by applicable  law (Debtor hereby
agreeing  that ten  days'  notice is  commercially  reasonable),  sell,  assign,
transfer,  or otherwise  dispose of the Collateral at public or private sale, in
whole  or in  part,  and  Secured  Party  may,  in its own  name or as  Debtor's
attorney-in-fact  effectively  assign and transfer the  Collateral,  or any part
thereof,   absolutely,  and  execute  and  deliver  all  necessary  assignments,
conveyances,  bills of sale, and other  instruments with power to substitute one
or more persons or  corporations  with like power.  Any such  foreclosure  sale,
assignment,  transfer,  or other  disposition  shall, to the extent permitted by
law,  be a  perpetual  bar,  both at law and in equity,  against  Debtor and all
persons and corporations  lawfully  claiming by or through or under Debtor.  Any
such foreclosure sale may be adjourned from time to time. Upon any sale, Secured
Party may bid for and purchase the  Collateral,  or any part  thereof,  and upon
compliance with the terms of sale may hold,  retain,  possess and dispose of the
Collateral, in its absolute right without further accountability.  Secured Party
shall have the right to be  credited  on the  amount of its bid a  corresponding
amount of the Obligations as of the date of such sale.

                  (c) If, in the opinion of Secured Party, there is any question
that a public sale or  distribution  of any Collateral will violate any state or
federal  securities  law,  Secured  Party  (i) may  offer  and  sell  securities
privately to purchasers who will agree to take them for investment  purposes and
not with a view to distribution  and who will agree to imposition of restrictive
legends on the  certificates  representing  the security,  or (ii) may sell such
securities in an intrastate  offering  under Section  3(a)(11) of the Securities
Act of 1933,  and no sale so made in good faith by Secured Party shall be deemed
to be not "commercially reasonable" because so made.

                  (d)  Not  in  limitation  of  any  other   provision  of  this
Agreement,  Secured  Party shall have all rights and remedies of a secured party
under the Texas UCC.

         7.3  Application  of Proceeds.  Secured Party may apply the proceeds of
any foreclosure  sale hereunder or from any other  permitted  disposition of the
Collateral  or any part  thereof as  follows:  (a) first,  to the payment of all
reasonable  costs and expenses of any foreclosure  and collection  hereunder and
all proceedings in connection  therewith,  including reasonable attorneys' fees;
(b) then, to the  reimbursement of Secured Party for all  disbursements  made by
Secured Party for taxes,  assessments or liens superior to the Security Interest
and  which  Secured  Party  shall  deem  expedient  to  pay;  (c)  then,  to the
reimbursement of Secured Party of any other  disbursements made by Secured Party
in accordance with the terms hereof or under the Purchase Agreement or any Other
Agreement;  (d) then,  to or among the amounts of fees,  interest and  principal
then owing and unpaid in respect of the Obligations, in such priority as Secured
Party may determine in its  discretion;  and (e) the remainder of such proceeds,
if any,  shall be paid to Debtor.  If such  proceeds  shall be  insufficient  to
discharge the entire  Obligations,  Secured Party shall have any other available
legal recourse  against Debtor under,  or for the  performance  of, the Purchase
Agreement and any Other  Agreement  between  Debtor and Secured  Party,  for the
deficiency,  together with interest  thereon at the maximum rate permitted under
applicable law.

         7.4  Enforcement  of  Obligations.  Nothing in this Agreement or in any
other  document  or  agreement  shall  affect or impair  the  unconditional  and
absolute right of Secured Party to enforce the  Obligations as and when the same
shall become due in accordance with the terms of the Purchase Agreement or Other
Agreement.

                                  ARTICLE VIII

                             RIGHTS OF SECURED PARTY

         8.1  Subrogation.  Upon the occurrence of an Event of Default,  Secured
Party,  at its  election,  may  subrogate  to all of the  interest,  rights  and
remedies  of the  Debtor,  in respect  to any of the  Collateral  or  agreements
pertaining thereto.

         8.2 Secured Party  Appointed  Attorney-in-Fact.  Debtor hereby appoints
Secured Party as  attorney-in-fact  of Debtor,  with full authority in the place
and stead of Debtor and in the name of Debtor, Secured Party or otherwise,  from
time to time on Secured  Party's  discretion and upon the occurrence of an Event
of Default, to take any action and to execute any instrument which Secured Party
may deem  necessary or advisable to accomplish  the purposes of this  Agreement,
including without  limitation:  (a) to ask, demand,  collect,  sue for, recover,
compound, receive and give acquittance and receipts for moneys due and to become
due under or in respect of any of the Collateral;  (b) to receive,  endorse, and
collect  any  drafts or other  instruments,  documents  and  chattel  paper,  in
connection  with clause (a) of this  Section 8.2; (c) to file any claims or take
any action or institute any proceeding which Secured Party may deem necessary or
desirable for the  collection  of any of the  Collateral or otherwise to enforce
the rights of Secured Party against any of the Collateral; and (d) to assign and
transfer the  Collateral,  or any part  thereof,  absolutely  and to execute and
deliver  endorsements,   assignments,  conveyances,  bills  of  sale  and  other
instruments  with power to substitute  one or more persons or  corporation  with
like power.

         8.3  Performance  by Secured  Party.  If Debtor  fails to  perform  any
agreement  contained  herein,  Secured  Party may itself  perform,  or cause the
performance  of, such  agreement,  and the reasonable  expenses of Secured Party
incurred in connection  therewith  shall be payable by Debtor under Section 8.8.
In no  event,  however,  shall  Secured  Party  have any  obligation  or  duties
whatsoever to perform any covenant or agreement of Debtor contained herein,  and
any such performance by Secured Party shall be wholly discretionary with Secured
Party.

         8.4 Duties of Secured  Party.  The powers  conferred upon Secured Party
hereunder  are solely to protect its  interest in the  Collateral  and shall not
impose any duty upon it to exercise any such powers. Except for the safe custody
of any  Collateral  in its  possession  and the  accounting  for money  actually
received by it hereunder,  Secured Party shall have no duty as to any Collateral
or as to the taking of any  necessary  steps to preserve  rights  against  prior
parties or any other rights  pertaining to any Collateral.  Without limiting the
generality of the foregoing,  Secured Party shall not have any obligation,  duty
or  responsibility  to do any of the  following:  (a) ascertain any  maturities,
calls,  conversions,  exchanges,  offers, tenders or similar matters relating to
the  Collateral or informing  Debtor with respect to any such matters;  (b) fix,
preserve  or  exercise  any  right,  privilege  or option  (whether  conversion,
redemption or otherwise) with respect to the Collateral; (c) collect any amounts
payable  in  respect  of the  Collateral;  (d)  sell all or any  portion  of the
Collateral,  for any reason;  or (e) hold the Collateral for or on behalf of any
party other than Debtor.

         8.5 No  Liability  of Secured  Party.  Neither the  acceptance  of this
Agreement by Secured Party,  nor the exercise of any rights hereunder by Secured
Party,  shall be construed in any way as an  assumption  by Secured Party of any
obligations,  responsibilities,  or duties of Debtor arising in connection  with
the  Collateral  assigned  hereunder  or  otherwise  bind  Secured  Party to the
performance of any  obligations  respecting the  Collateral,  it being expressly
understood  that Secured  Party shall not be obligated to perform,  observe,  or
discharge  any  obligation,  responsibility,  duty,  or  liability  of Debtor in
respect of any of the Collateral,  including without limitation  appearing in or
defending any action, expending any money or incurring any expense in connection
therewith.  TO THE FULLEST EXTENT  PERMITTED BY APPLICABLE LAW, DEBTOR SHALL AND
DOES AGREE TO INDEMNIFY,  PROTECT,  DEFEND AND HOLD HARMLESS  SECURED PARTY, ITS
OFFICERS, DIRECTORS, REPRESENTATIVES, AGENTS, EMPLOYEES, LENDERS, SUCCESSORS AND
ASSIGNS,  FROM AND AGAINST ALL  LIABILITIES,  CLAIMS,  DAMAGES,  LOSSES,  FINES,
PENALTIES,  CAUSES OF ACTIONS,  SUITS,  JUDGMENTS AND EXPENSES  (INCLUDING COURT
COSTS,  ATTORNEY'S  FEES  AND  COST OF  INVESTIGATION)  OF ANY  NATURE,  KIND OR
DESCRIPTION  OF ANY PERSON OR ENTITY,  DIRECTLY OR  INDIRECTLY,  ARISING OUT OF,
CAUSED BY OR  RESULTING  FROM (IN WHOLE OR IN PART),  ANY  UNINTENTIONAL  ACT OR
OMISSION (OR  INTENTIONAL  ACT OR OMISSION SO LONG AS SUCH ACT OR OMISSION  DOES
NOT  CONSTITUTE  NEGLIGENCE)  OF SECURED  PARTY (OR  ANYONE  ACTING ON BEHALF OF
SECURED PARTY) IN CONNECTION WITH THE COLLATERAL.  THE FOREGOING INDEMNITY SHALL
SURVIVE THE EXPIRATION OR EARLIER TERMINATION OF THIS AGREEMENT.

         8.6 Right of Secured Party to Defend Action Affecting Security. Secured
Party  may,  at the  expense  of  Debtor,  appear in and  defend  any  action or
proceeding at law or in equity  purporting to affect  Secured  Party's  Security
Interest under this Agreement.

         8.7 Right of  Secured  Party to Prevent  or Remedy  Default.  If Debtor
shall fail to perform any of the covenants,  conditions and agreements  required
to be performed  and  observed by Debtor  under the Purchase  Agreement or Other
Agreement,  or in respect of the Collateral  (subject to any applicable  default
cure  period),  Secured  Party (a) may but shall  not be  obligated  to take any
action Secured Party deems  necessary or desirable to prevent or remedy any such
default by Debtor or otherwise to protect the Security  Interest,  and (b) shall
have the absolute and immediate  right to take  possession of the  Collateral or
any  part  thereof  (to  the  extent  Secured  Party  has not  previously  taken
possession)  to such  extent  and as often  as the  Secured  Party,  in its sole
discretion, deems necessary or desirable in order to prevent or to cure any such
default by Debtor,  or  otherwise  to protect the  security  of this  Agreement.
Secured Party may advance or expend such sums of money for the account of Debtor
as Secured Party in its sole discretion deems necessary for any such purpose.

         8.8 Secured Party's Expenses. All reasonable advances, costs, expenses,
charges and attorneys' fees which Secured Party may make, pay or incur under any
provision  of this  Agreement  for the  protection  of its  security  or for the
enforcement of any of its rights hereunder,  including,  without limitation,  in
foreclosure  proceedings commenced and subsequently  abandoned or in any dispute
or litigation in which Secured Party or the holder of any of the Obligations may
become involved by reason of or arising out of the Purchase Agreement, any Other
Agreement or the  Collateral,  shall be a part of the  Obligations  and shall be
paid by Debtor to Secured Party, upon demand, and shall bear interest until paid
at the maximum  rate of interest  permitted  by  applicable  law,  from the date
incurred by Secured Party until repaid by Debtor.

         8.9. Convertible  Collateral.  Secured Party may present for conversion
any  Collateral  which is  convertible  into any other  instrument or investment
security or a  combination  thereof with cash,  but Secured Party shall not have
any duty to present for conversion any Collateral  unless it shall have received
from Debtor  detailed  written  instructions to that effect at a time reasonably
far in advance of the final conversion date to make such conversion possible.

         8.10 Secured Party's Right of Set-Off.  Upon the happening of any event
entitling  Secured  Party to pursue any remedy  provided  herein,  or if Secured
Party shall be served with garnishment process in which Debtor shall be named as
defendant,  whether or not Debtor  shall be in  default  hereunder  at the time,
Secured Party may, but shall not be required to, set-off any indebtedness  owing
by  Secured  Party  to  Debtor  against  any of the  Obligations  without  first
resorting to the security hereunder and without prejudice to any other rights or
remedies of Secured Party or its Security Interest.

         8.11 Remedies.  No right or remedy herein  reserved to Secured Party is
intended to be exclusive  of any other right or remedy,  but each and every such
remedy shall be cumulative,  not in lieu of, but in addition to any other rights
or remedies given under this Agreement and all other security documents. Any and
all of Secured  Party's  rights and remedies may be exercised  from time to time
and as often as such exercise as deemed necessary or desirable by Secured Party.

         8.12 Debtor's Waivers.  Debtor waives notice of the creation,  advance,
increase,  existence,  extension,  or  renewal  of, and of any  indulgence  with
respect to, the  Obligations;  waives notice of intent to accelerate,  notice of
acceleration,  notice  of  intent  to  demand,  presentment,  demand,  notice of
dishonor,  and  protest;   waives  notice  of  the  amount  of  the  Obligations
outstanding  at any time,  notice of any change in  financial  condition  of any
person liable for the  Obligations  or any part thereof,  notice of any Event of
Default,  and all other  notices  respecting  the  Obligations;  and agrees that
maturity of the Obligations  and any part thereof may be accelerated,  extended,
or renewed one or more times by Secured Party in its discretion,  without notice
to Debtor.

         8.13 Other Parties and Other Collateral.  No renewal or extension of or
any other  indulgence  with respect to the  Obligations or any part thereof,  no
release  of any  security,  no  release  of any  person  (including  any  maker,
endorser,  guarantor,  or  surety)  liable  on  the  Obligations,  no  delay  in
enforcement  of payment,  and no delay or admission or lack of diligence or care
in exercising any right or power with respect to the Obligations or any security
therefor  or  guaranty  thereof or under this  Agreement  shall in other  manner
impair  or affect  the  rights  of  Secured  Party  under  the law,  under  this
Agreement,  or under any other  document or  agreement  pertaining  to the other
security for the  Obligations,  before  foreclosing  upon the Collateral for the
purpose of paying the Obligations.  Debtor waives any right to the benefit of or
to require or control application of any other security or proceeds thereof, and
Debtor  agrees that Secured  Party shall have no duty or obligation to Debtor to
apply to the Obligations any such other security or proceeds thereof.

         8.14 Relationship Among Secured Party.  Either or both of Secured Party
are  entitled  to enforce  any and all rights  granted to Secured  Party in this
Agreement,  and to take any other  action  allowed to be taken by Secured  Party
under this  Agreement.  Neither  Secured  Party is under any  obligation  to act
jointly or in concert with the other Secured Party  pursuant to this  Agreement.
Any action  required  to be taken by Debtor,  or notice  required to be given by
Debtor, shall be taken or given with respect to both of Secured Party.

                                   ARTICLE IX

                                  MISCELLANEOUS

     9.1 Terms  Commercially  Reasonable.  The terms of this Agreement  shall be
deemed commercially reasonable within the meaning of the Texas UCC.

         9.2 Notices.  Any notices or demands  required or permitted to be given
hereunder  shall be  deemed  sufficiently  given if in  writing  and  personally
delivered or mailed (with all postage and charges prepaid), addressed to Secured
Party or to Debtor their respective  addresses set forth below, or at such other
address as the above parties may from time to time  designate by written  notice
to the other given in  accordance  with this Section  9.2.  Any such notice,  if
personally  delivered or  transmitted  by telex or telegram,  shall be deemed to
have been given on the date so delivered or transmitted or, if mailed, be deemed
to have been given on the day after such  notice is placed in the United  States
mail in accordance with this Section 9.2.

                  Secured Party:  1301 Capital of Texas Hwy., Suite C-300
                                  Austin, Travis County, Texas 78746
                                  Attn: President

                  with copy to:   Timothy L. LaFrey, Esq.
                                  Akin, Gump, Strauss, Hauer & Feld, L.L.P.
                                  1900 Frost Bank Plaza
                                  816 Congress Avenue
                                  Austin, Texas 78701

                  Debtor:         John Robert Griffin, M.D., Trustee under the
                                  Family Revocable Trust dated February 8, 1991
                                  4913 Puma Way
                                  Carmichael, California   95608

         9.3 Parties Bound.  Secured Party's rights under this Agreement and the
Security Interest shall inure to the benefits of its successors and assigns, and
in the event of any  assignment  or  transfer of any of the  Obligations  or the
Collateral,  Secured  Party  thereafter  shall  be  fully  discharged  from  any
responsibility  with respect to the Collateral so assigned or  transferred,  but
Secured  Party shall  retain all rights and powers  hereby given with respect to
any of the  Obligations  or  Collateral  not so  assigned  or  transferred.  All
representations, warranties, and agreements of Debtor if more than one are joint
and several, and all shall be binding upon the personal representatives,  heirs,
successors, and assigns of Debtor.

         9.4 Waiver.  No delay of Secured Party in exercising any power or right
shall operate as a waiver thereof;  nor shall any single or partial  exercise of
any power or right preclude other or further exercise thereof or the exercise of
any other power or right.  No waiver by Secured Party of any right  hereunder of
any default by Debtor shall be binding upon Secured Party unless in writing, and
no failure by Secured  Party to exercise any power or right  hereunder or waiver
of any  default  by Debtor  shall  operate  as a waiver of any other or  further
exercise of such right or power of any further default.

         9.5 Agreement Continuing.  This Agreement shall constitute a continuing
agreement,  applying to all future as well as existing transactions,  whether or
not of the  character  contemplated  at the date of this  Agreement,  and if all
transactions between Secured Party and Debtor shall be closed at any time, shall
be equally  applicable to any new  transactions  thereafter.  Provisions of this
Agreement,  unless  by their  terms  exclusive,  shall be in  addition  to those
contained in any Other Agreement.

         9.6 Definitions. Unless the context indicated otherwise, definitions in
the Texas Business and Commerce Code ("Texas UCC") apply to words and phrases in
this Agreement; if Texas UCC definitions conflict, Chapter 9 definitions apply.

         9.7 Miscellaneous. In this Agreement, whenever the context so requires,
the neuter gender  includes the masculine and feminine,  and the singular number
includes  the plural and vice  versa.  The  headings  of  paragraphs  herein are
inserted only for convenience and shall in no way define,  describe or limit the
scope of intent of any  provisions  of this  Agreement.  No  change,  amendment,
modification,  cancellation,  or  discharge of any  provision of this  Agreement
shall be valid unless consented to in writing by Secured Party.

         9.8 Assignment of Secured  Party's  Interest.  Secured Party shall have
the right to assign all or any portion of its rights in this  Agreement  without
approval or consent.  Debtor may not assign this  Agreement or any of its rights
or obligations  hereunder  without the express prior written  consent of Secured
Party in each instance.

     9.9 Applicable  Laws.  THIS AGREEMENT SHALL BE GOVERNED BY AND CONSTRUED IN
ACCORDANCE  WITH THE LAWS OF THE STATE OF TEXAS AND THE  APPLICABLE  LAWS OF THE
UNITED STATES OF AMERICA.

     9.10 ENTIRE AGREEMENT.  THIS AGREEMENT AND THE PURCHASE AGREEMENT REPRESENT
THE FINAL AGREEMENT  BETWEEN THE PARTIES AND MAY NOT BE CONTRADICTED BY EVIDENCE
OF PRIOR,  CONTEMPORANEOUS,  OR SUBSEQUENT ORAL AGREEMENTS OF THE PARTIES. THERE
ARE NO UNWRITTEN ORAL AGREEMENTS BETWEEN THE PARTIES.

                            [Signature page follows]


<PAGE>


S-1



                                SIGNATURE PAGE TO

                                 ASSIGNMENT AND

                               SECURITY AGREEMENT

         EXECUTED this 1st day of September, 1999.


DEBTOR:                            /s/ John Robert Griffin, M.D., Trustee

                                   Printed Name: John Robert Griffin, M.D.,
                                   Trustee under the John Robert Griffin, M.D.
                                   Family Revocable Trust dated February 8, 1991


SECURED PARTY:                     Prime Medical Operating, Inc.

                                   By: /s/ Cheryl Williams

                                   Name: Cheryl Williams

                                   Title: Vice President


                                   Prime/BDR Acquisition, L.L.C.

                                   By: /s/ Cheryl Williams

                                   Name: Cheryl Williams

                                   Title: Vice President

<PAGE>



                        ASSIGNMENT AND SECURITY AGREEMENT

     THIS  ASSIGNMENT  AND SECURITY  AGREEMENT  (this  "Agreement")  is made and
entered into as of the 1st day of September,  1999, by and between Prime Medical
Operating, Inc., a Delaware corporation ("PMOI"), Prime/BDR Acquisition, L.L.C.,
a Delaware limited  liability  company ("Prime") (PMOI and Prime are referred to
herein,  jointly and severally,  as "Secured  Party") and Christian K. Kim, M.D.
(the "Debtor").
                                    RECITALS:

         A. Debtor and Secured Party have  executed and  delivered  that certain
Stock  Purchase   Agreement  dated  as  of  September  1,  1999  (the  "Purchase
Agreement"),  pursuant to which  Secured  Party  purchased  from Debtor  certain
shares of the $0.01 par value  common  stock of Horizon  Vision  Center,  Inc, a
Nevada corporation ("Horizon").

         B. Secured Party has requested  that Debtor pledge the  Collateral  (as
defined below) to secure certain obligations and liabilities that Debtor may now
or hereafter have to Secured Party, including, without limitation, any indemnity
obligations arising under the Purchase Agreement.

     C. Debtor desires to enter into this Agreement as a material  inducement to
Secured  Party's  purchase  of  Debtor's  shares of Horizon  under the  Purchase
Agreement.

                                   AGREEMENT:

         NOW, THEREFORE, in consideration of the foregoing and the covenants and
agreements hereinafter set forth, and other good and valuable consideration, the
receipt and sufficiency of which Debtor  acknowledges,  Debtor and Secured Party
agree as follows:

                                    ARTICLE I

                       COLLATERAL AND SECURED OBLIGATIONS

         1.1 Grant of Security Interest.  Debtor hereby assigns,  transfers, and
pledges to Secured  Party,  and Debtor hereby grants to Secured Party a security
interest   in,   the   following   described   collateral   (collectively,   the
"Collateral"):

                  (a)  Shares  of  Horizon.  From  and  after  the  date of this
Agreement,  (i) all shares of the common  stock of Horizon  owned or acquired in
any manner by Debtor or Secured Party,  (collectively,  the "Shares"),  (ii) any
replacements,  substitutions,  or exchanges of the certificates representing the
Shares, and (iii) any and all options,  rescission rights,  registration rights,
conversion rights, subscription rights, contractual or quasi-contractual rights,
warrants,  redemption rights,  redemption proceeds, calls, preemptive rights and
all other rights and benefits pertaining to the Shares;

                  (b)  Accounts.  All  accounts  and  rights  now  or  hereafter
attributable to any of the Collateral  described in (a) above, and all rights of
Debtor now or hereafter arising under any agreement pertaining to the Collateral
described  in  (a)  above,   including  without  limitation  all  distributions,
proceeds, fees, dividends,  preferences,  payments or other benefits of whatever
nature  which  Debtor is now or may  hereafter  become  entitled to receive with
respect to any Collateral described in (a) above;

                  (c) Additional  Property.  "Collateral" shall also include the
following  property  (collectively,  the  "Additional  Property")  which  Debtor
becomes  entitled  to receive or shall  receive  in  connection  with any of the
Collateral  described in this Section 1.1: (i) any stock certificate,  including
without  limitation,  any  certificate  representing  a  stock  dividend  or any
certificate in connection with any recapitalization,  reclassification,  merger,
consolidation,  conversion,  sale of assets, combination of shares, stock split,
reverse  stock split or  spin-off;  (ii) any option,  warrant,  subscription  or
right,  whether as an addition to or in  substitution  of any of the  Collateral
described in this Section 1.1; (iii) any dividends or  distributions of any kind
whatsoever,  whether  distributable in cash,  stock or other property;  (iv) any
interest,  premium or principal  payments;  and (v) any conversion or redemption
proceeds; and

                  (d) Proceeds.  All proceeds (cash and non-cash) arising out of
the sale, exchange, collection or other disposition of all or any portion of the
Collateral  described in (a), (b), or (c) above,  including  without  limitation
proceeds in the form of stock, accounts, chattel paper, instruments,  documents,
goods, inventory and equipment.

The  security  interest in the  Collateral  hereby  granted by Debtor to Secured
Party may sometimes be referred to in this Agreement as the "Security Interest".

         1.2 Obligations.  This Agreement and the Security Interest shall secure
full and punctual payment and performance of the following indebtedness,  duties
and obligations (collectively, the "Obligations"):

                  (a) All liabilities and obligations of Debtor to Secured Party
(including, without limitation, any principal, interest, fees and other amounts,
and any  indemnity  obligations)  under and pursuant to the Purchase  Agreement,
this Agreement and/or any other contract or agreement  between Secured Party and
Debtor or any affiliate of Debtor (collectively, "Other Agreements"; and

                  (b) (i)  all  indebtedness,  obligations  and  liabilities  of
Debtor and/or any affiliate of Debtor to Secured Party of any kind or character,
now existing or hereafter arising, whether direct, indirect, related, unrelated,
fixed,  contingent,  liquidated,  unliquidated,  joint,  several  or  joint  and
several,  arising from,  connected with, or related to the Purchase Agreement or
any Other Agreement, or any other document, agreement, or instrument executed in
connection  either of them,  (ii) all accrued but unpaid  interest on any of the
indebtedness  described in (i) above, (iii) all obligations of Debtor and/or any
affiliate  of  Debtor  to  Secured  Party  under  any  documents  or  agreements
evidencing,  securing,  governing  and/or  pertaining  to all or any part of the
indebtedness  described  in (i) and (ii)  above,  (iv) all  costs  and  expenses
incurred by Secured Party in connection  with the collection and  administration
of all or any part of the  indebtedness  and obligations  described in (i), (ii)
and (iii) above or the protection or preservation  of, or realization  upon, the
collateral  securing  all or any  part of  such  indebtedness  and  obligations,
including  without  limitation  all  attorneys'  fees,  and  (v)  all  renewals,
extensions, modifications and rearrangements of the indebtedness and obligations
described in (i), (ii), (iii) and (iv) above.

                  (c) All sums now or  hereafter  loaned or  advanced by Secured
Party to  Debtor,  or  expended  by Secured  Party for the  account of Debtor or
otherwise owing by Debtor to Secured Party, in respect of the  Obligations,  and
all other sums  expended or advanced  by Secured  Party  pursuant to any term or
provision of this Agreement or any Other Agreement (i) to collect and/or enforce
the Obligations or (ii) to maintain, protect and preserve the Collateral.

                                   ARTICLE II

       DEBTOR'S REPRESENTATIONS AND WARRANTIES WITH RESPECT TO COLLATERAL

       Debtor hereby represents and warrants to Secured Party as follows:

         2.1 Ownership of Collateral.  Debtor has good and  marketable  title to
the Collateral  free and clear of any liens,  security  interests,  shareholders
agreement, calls, charge, or encumbrance,  except for this Security Interest. No
financing  statement or other  instrument  similar in effect covering all or any
part of the  Collateral is on file in any recording  office,  except as may have
been filed in favor of Secured Party relating to this Agreement.

         2.2  Power  &  Authority.  Debtor  has the  lawful  right,  power,  and
authority  to grant the Security  Interest in the  Collateral.  This  Agreement,
together  with all filings and other  actions  necessary or desirable to perfect
and protect such security interest,  which have been duly taken,  create a valid
and perfected first priority  security  interest in the Collateral  securing the
payment and performance of the Obligations.

         2.3  No  Agreements.  The  Shares  are  not  subject  to any  right  of
redemption,  or any  call or put  options,  voting  trust,  proxy,  shareholders
agreement,  right of first  refusal,  or any other  document or agreement  which
would in any way impair or adversely affect this Security Interest or the rights
of Secured Party under this Agreement.

         2.4  Securities.  Any  certificates  evidencing  securities  pledged as
Collateral  are valid and  genuine  and have not been  altered.  All  securities
pledged as Collateral have been duly  authorized and validly  issued,  are fully
paid and  non-assessable,  and were not issued in  violation  of the  preemptive
rights of any party or of any agreement by which Debtor or the issuer thereof is
bound.  No  restrictions  or  conditions  exist with  respect to the transfer or
voting of any securities pledged as Collateral.

                                   ARTICLE III

                  DEBTOR'S OTHER REPRESENTATIONS AND WARRANTIES

         3.1 Solvency of Debtor.  As of the date hereof,  (i) Debtor is solvent;
(ii) the fair saleable  value of Debtor's  assets exceeds  Debtor's  liabilities
(both fixed and contingent);  (iii) Debtor has sufficient capital to satisfy all
of Debtor's  obligations  as they  become due;  (iv) no  receiver,  trustee,  or
custodian has been appointed for, or taken  possession of, all or  substantially
all of the assets of Debtor,  either in a  proceeding  brought by Debtor or in a
proceeding  brought against Debtor;  (v) Debtor is not the subject of a petition
for relief under the United  States  Bankruptcy  Code or any similar  federal or
state insolvency law, including without limitation a petition filed by Debtor or
a petition filed by a third party seeking relief against Debtor; and (vi) Debtor
has no  intention  of filing a  petition  for  relief  under the  United  States
Bankruptcy  Code or any similar  federal or state  insolvency law, or of seeking
any other form of  creditor  relief,  within  the  two-year  period  immediately
following the date of this Agreement.

         3.2  Authority and  Compliance.  Debtor has full power and authority to
enter into this Agreement. Debtor has full power and authority to enter into and
perform its obligations  under the Purchase  Agreement and each Other Agreement.
No further  consent or approval is  required as a condition  to the  validity of
this  Agreement,  the Purchase  Agreement or any Other  Agreement.  Debtor is in
compliance with all applicable laws, ordinances,  statutes, orders, regulations,
judgments, writs, or decrees of any governmental entity to which it is subject.

         3.3 Binding Agreement.  This Agreement, the Purchase Agreement and each
Other Agreement  constitute valid and legally binding  obligations of Debtor, in
accordance  with  their  terms,  subject  to  (a)  the  applicable   bankruptcy,
insolvency,  reorganization,  moratorium,  and similar laws affecting creditors'
rights  generally  and  (b)  restrictions  imposed  by any  court  of  competent
jurisdiction on the enforcement of non-competition and exclusive use restrictive
covenants imposed under the Purchase Agreement or any Other Agreement.

         3.4 Litigation.  There are no proceedings  pending or, to the knowledge
of Debtor,  threatened before any court or  administrative  agency which will or
may have a material adverse effect on the financial  condition of Debtor or upon
Debtor's ability to perform its obligations  under this Agreement,  the Purchase
Agreement or any Other Agreement.

         3.5 No Conflicting Agreements.  There are no provisions of any existing
agreement,  mortgage,  indenture or contract  binding on Debtor or affecting its
property,  which  would  conflict  with or in any  way  prevent  the  execution,
delivery, or carrying out of the terms of this Agreement, the Purchase Agreement
or any Other Agreement.

         3.6  Ownership  of  Assets.  Debtor  has  good  and  full  title to the
Collateral,  and the  Collateral  is owned  free and  clear of  liens,  charges,
claims, security interests, and other encumbrances.

     3.7 Taxes. Debtor has filed all tax returns required to be filed by Debtor.

                                   ARTICLE IV

                  DEBTOR'S COVENANTS WITH RESPECT TO COLLATERAL

         Debtor  covenants  and agrees  that from the date  hereof and until the
payment  and  performance  in  full  of the  Obligations  unless  Secured  Party
otherwise consents in writing:

         4.1  Delivery of  Instruments  and/or  Certificates.  Contemporaneously
herewith,   Debtor  covenants  and  agrees  to  deliver  to  Secured  Party  any
certificates,   documents,   or  instruments   representing  or  evidencing  the
Collateral,  with Debtor's  endorsement  thereon and/or  accompanied by property
instruments of transfer and assignment duly executed in blank with, if requested
by Secured Party,  signatures  guaranteed by a member or member  organization in
good standing of an authorized Securities Transfer Agents Medallion Program, all
in form and substance satisfactory to Secured Party.

         4.2  Further  Assurances.   Debtor  will   contemporaneously  with  the
execution  hereof  and from  time to time  thereafter  at its  expense  promptly
execute and deliver all further  instruments  and documents and take all further
action  necessary or  appropriate or that Secured Party may request in order (i)
to perfect and protect the security  interest created or purported to be created
hereby and the first priority of such security interest,  (ii) to enable Secured
Party to exercise  and enforce its rights and  remedies  hereunder in respect of
the  Collateral,  and (iii) to otherwise  effect the purposes of this Agreement,
including  without  limitation:  (A)  executing  and  filing  any  financing  or
continuation  statements,  or any  amendments  thereto;  (B)  obtaining  written
confirmation  from the issuer of any  securities  pledged as  Collateral  of the
pledge of such securities,  in form and substance satisfactory to Secured Party;
(C)  cooperating  with Secured Party in registering the pledge of any securities
pledged as Collateral with the issuer of such securities;  (D) delivering notice
of Secured Party's security interest in any securities  pledged as Collateral to
any securities or financial  intermediary,  clearing  corporation or other party
required by Secured Party, in form and substance  satisfactory to Secured Party;
and  (E)  obtaining  written  confirmation  of  the  pledge  of  any  securities
constituting Collateral from any securities or financial intermediary,  clearing
corporation  or other party  required by Secured  Party,  in form and  substance
satisfactory to Secured Party.

         4.3 Additional Property. All Additional Property, as defined in Section
1.1(c)  above,  received by Debtor shall be received in trust for the benefit of
Secured Party.  All Additional  Property and all  certificates  or other written
instruments or documents  evidencing and/or representing the Additional Property
that is  received  by Debtor,  together  with such  instruments  of  transfer as
Secured Party may request,  shall  immediately be delivered to or deposited with
Secured Party and held by Secured  Party as  Collateral  under the terms of this
Agreement.  If the  Additional  Property  received  by Debtor and  delivered  to
Secured  Party  pursuant  to this  Section  shall  be  shares  of stock or other
securities,  such shares of stock or other  securities shall be duly endorsed in
blank or  accompanied  by proper  instruments  of transfer and  assignment  duly
executed in blank with, if requested by Secured Party,  signatures guaranteed by
a member or member  organization  in good standing of an  authorized  Securities
Transfer Agents  Medallion  Program,  all in form and substance  satisfactory to
Secured  Party.  Secured  Party  shall  be  deemed  to  have  possession  of any
Collateral in transit to Secured Party or its agent.

         4.4  Sale,  Transfer,  Encumbrance.  Debtor  will not  sell,  transfer,
mortgage,  or otherwise  encumber any  Collateral or impair the value thereof in
any manner without  Secured  Party's prior written  consent,  including  without
limitation by purchase, lease, barter, trade, payment deferral, or the creation,
assumption  or guarantee of  indebtedness  or other  lending of credit.  Secured
Party's written consent to any sale,  mortgage,  transfer,  or encumbrance shall
not be construed to be a waiver of this  provision in respect to any  subsequent
proposed sale, mortgage, transfer, or encumbrance.

         4.5 Liens. Neither Debtor nor any person acting on Debtor's behalf has,
or shall have any right, power, or authority to and shall not create,  incur, or
permit to be placed or  imposed,  upon the  Collateral,  any lien of any type or
nature whatsoever, other than the liens in favor of Secured Party.

         4.6 Matters or  Occurrences  Affecting  Collateral  or this  Agreement.
Debtor will promptly  notify Secured Party of any and all matters or occurrences
that may have a material adverse effect on the status or value of the Collateral
or this Agreement,  including  without  limitation the occurrence of an Event of
Default,  or an event  which,  with giving of notice or lapse of time,  or both,
would constitute an Event of Default.

     4.7  Agreements  Pertaining to  Collateral.  Debtor will not enter into any
type of contract or agreement  pertaining to any of the Collateral or in any way
transfer any voting rights pertaining to the Collateral to any person or entity.

         4.8 Dilution of Ownership.  As to any securities pledged as Collateral,
Debtor  will not  consent to or approve of the  issuance  of (i) any  additional
shares  of  any  class  of  securities  of  such  issuer,  (ii)  any  instrument
convertible  voluntarily  by  the  holder  thereof  or  automatically  upon  the
occurrence or  non-occurrence  of any event or condition  into, or  exchangeable
for, any such  securities,  or (iii) any warrants,  options,  contracts or other
commitments  entitling any third party to purchase or otherwise acquire any such
securities.  Notwithstanding  the  foregoing  or any  other  provision  of  this
Agreement to the contrary,  Debtor (i) shall comply with his  obligations  under
the  Purchase  Agreement  and each Other  Document,  and (ii) may consent to any
issuance of shares of Horizon if such  issuance has been  approved by a majority
of the Board of Directors of Horizon.

         4.9  Restrictions  on  Securities.  Debtor  will  not  enter  into  any
agreement  creating,  or otherwise permit to exist, any restriction or condition
upon the transfer,  voting or control of any  securities  pledged as Collateral,
except as consented to in writing by Secured Party. As to any securities pledged
as collateral, Debtor will not consent to or approve of any stock split, reverse
stock  split,  stock  dividend,  reclassification,   or  other  similar  act  or
transaction  regarding  the  Shares  unless  consented  to in writing by Secured
Party.

                                    ARTICLE V

                         DEBTOR'S AFFIRMATIVE COVENANTS

         Until payment and performance of all Obligations,  Debtor covenants and
agrees that it shall promptly  advise Secured Party in writing of any litigation
filed  against  Debtor  and of any  condition,  event or act which  comes to its
attention  that  would or might  have a  material  adverse  effect  on  Debtor's
financial condition or on Debtor's ability to perform the Obligations.

                                   ARTICLE VI

                               NEGATIVE COVENANTS

         Until payment and performance of all Obligations,  Debtor covenants and
agrees that Debtor will not, without the prior written consent of Secured Party:

     6.1 Liens. Grant, suffer, or permit liens on, or security interests in, the
Collateral.

         6.2  Violate  Other  Covenants.  Violate  or fail to  comply  with  any
covenants  or  agreements  regarding  other  debt  which  will or would with the
passage  of time or upon  demand  cause the  maturity  of any  other  debt to be
accelerated.

                                   ARTICLE VII

                              DEFAULT AND REMEDIES

     7.1 Events of Default.  An Event of Default  (herein so called) shall exist
if any one or more of the following events shall occur:

                  (a) The  failure  of Debtor to pay any amount  required  to be
paid under the Purchase Agreement (including,  without limitation, the indemnity
provisions  contained  therein),  or any other  amount  which  Debtor may now or
hereafter  owe to Secured Party under any Other  Agreement or otherwise,  within
(15) calendar days after such amount is due;

     (b) The failure of Debtor to pay any  Obligation  within (15) calendar days
after such amount is due; and

                  (c)  Debtor's  breach of a covenant in this  Agreement  or any
other  failure to perform  its  obligations  under this  Agreement  or any Other
Agreement.

     7.2 Secured Party's Remedies. Upon the occurrence of an Event of Default:

                  (a) Secured Party may declare the Obligations in whole or part
immediately due and may enforce payment and performance of the same and exercise
any rights under the Texas UCC,  rights and remedies of Secured Party under this
Agreement, or otherwise.

                  (b) Secured  Party may, at Secured  Party's  option and at the
expense of Debtor,  either in Secured Party's own right or in the name of Debtor
and in the same manner and to the same extent that Debtor  might  reasonably  so
act  if  this  Agreement  had  not  been  made:  (i) do  all  things  requisite,
convenient,  or  necessary  to enforce the  performance  and  observance  of all
rights,  remedies and privileges of Debtor arising from the  Collateral,  or any
part thereof,  including without limitation compromising,  waiving, excusing, or
in any manner releasing or discharging any obligation of any party to or arising
from the Collateral;  (ii) take possession of the books, papers,  chattel paper,
documents of title, and accounts of Debtor,  wherever  located,  relating to the
Collateral; (iii) sue or otherwise collect and receive money attributable to the
Collateral;  and (iv) exercise any other lawfully  available powers or remedies,
and do all other things  which  Secured  Party deems  requisite,  convenient  or
necessary  or which the  Secured  Party  deems  proper to protect  the  Security
Interest.

                  (b) Secured Party may foreclose  this  Agreement in the manner
now or  hereafter  provided  or  permitted  by law and may upon such  reasonable
notification  prior thereto as may be required by applicable  law (Debtor hereby
agreeing  that ten  days'  notice is  commercially  reasonable),  sell,  assign,
transfer,  or otherwise  dispose of the Collateral at public or private sale, in
whole  or in  part,  and  Secured  Party  may,  in its own  name or as  Debtor's
attorney-in-fact  effectively  assign and transfer the  Collateral,  or any part
thereof,   absolutely,  and  execute  and  deliver  all  necessary  assignments,
conveyances,  bills of sale, and other  instruments with power to substitute one
or more persons or  corporations  with like power.  Any such  foreclosure  sale,
assignment,  transfer,  or other  disposition  shall, to the extent permitted by
law,  be a  perpetual  bar,  both at law and in equity,  against  Debtor and all
persons and corporations  lawfully  claiming by or through or under Debtor.  Any
such foreclosure sale may be adjourned from time to time. Upon any sale, Secured
Party may bid for and purchase the  Collateral,  or any part  thereof,  and upon
compliance with the terms of sale may hold,  retain,  possess and dispose of the
Collateral, in its absolute right without further accountability.  Secured Party
shall have the right to be  credited  on the  amount of its bid a  corresponding
amount of the Obligations as of the date of such sale.

                  (c) If, in the opinion of Secured Party, there is any question
that a public sale or  distribution  of any Collateral will violate any state or
federal  securities  law,  Secured  Party  (i) may  offer  and  sell  securities
privately to purchasers who will agree to take them for investment  purposes and
not with a view to distribution  and who will agree to imposition of restrictive
legends on the  certificates  representing  the security,  or (ii) may sell such
securities in an intrastate  offering  under Section  3(a)(11) of the Securities
Act of 1933,  and no sale so made in good faith by Secured Party shall be deemed
to be not "commercially reasonable" because so made.

                  (d)  Not  in  limitation  of  any  other   provision  of  this
Agreement,  Secured  Party shall have all rights and remedies of a secured party
under the Texas UCC.

         7.3  Application  of Proceeds.  Secured Party may apply the proceeds of
any foreclosure  sale hereunder or from any other  permitted  disposition of the
Collateral  or any part  thereof as  follows:  (a) first,  to the payment of all
reasonable  costs and expenses of any foreclosure  and collection  hereunder and
all proceedings in connection  therewith,  including reasonable attorneys' fees;
(b) then, to the  reimbursement of Secured Party for all  disbursements  made by
Secured Party for taxes,  assessments or liens superior to the Security Interest
and  which  Secured  Party  shall  deem  expedient  to  pay;  (c)  then,  to the
reimbursement of Secured Party of any other  disbursements made by Secured Party
in accordance with the terms hereof or under the Purchase Agreement or any Other
Agreement;  (d) then,  to or among the amounts of fees,  interest and  principal
then owing and unpaid in respect of the Obligations, in such priority as Secured
Party may determine in its  discretion;  and (e) the remainder of such proceeds,
if any,  shall be paid to Debtor.  If such  proceeds  shall be  insufficient  to
discharge the entire  Obligations,  Secured Party shall have any other available
legal recourse  against Debtor under,  or for the  performance  of, the Purchase
Agreement and any Other  Agreement  between  Debtor and Secured  Party,  for the
deficiency,  together with interest  thereon at the maximum rate permitted under
applicable law.

         7.4  Enforcement  of  Obligations.  Nothing in this Agreement or in any
other  document  or  agreement  shall  affect or impair  the  unconditional  and
absolute right of Secured Party to enforce the  Obligations as and when the same
shall become due in accordance with the terms of the Purchase Agreement or Other
Agreement.

                                  ARTICLE VIII

                             RIGHTS OF SECURED PARTY

         8.1  Subrogation.  Upon the occurrence of an Event of Default,  Secured
Party,  at its  election,  may  subrogate  to all of the  interest,  rights  and
remedies  of the  Debtor,  in respect  to any of the  Collateral  or  agreements
pertaining thereto.

         8.2 Secured Party  Appointed  Attorney-in-Fact.  Debtor hereby appoints
Secured Party as  attorney-in-fact  of Debtor,  with full authority in the place
and stead of Debtor and in the name of Debtor, Secured Party or otherwise,  from
time to time on Secured  Party's  discretion and upon the occurrence of an Event
of Default, to take any action and to execute any instrument which Secured Party
may deem  necessary or advisable to accomplish  the purposes of this  Agreement,
including without  limitation:  (a) to ask, demand,  collect,  sue for, recover,
compound, receive and give acquittance and receipts for moneys due and to become
due under or in respect of any of the Collateral;  (b) to receive,  endorse, and
collect  any  drafts or other  instruments,  documents  and  chattel  paper,  in
connection  with clause (a) of this  Section 8.2; (c) to file any claims or take
any action or institute any proceeding which Secured Party may deem necessary or
desirable for the  collection  of any of the  Collateral or otherwise to enforce
the rights of Secured Party against any of the Collateral; and (d) to assign and
transfer the  Collateral,  or any part  thereof,  absolutely  and to execute and
deliver  endorsements,   assignments,  conveyances,  bills  of  sale  and  other
instruments  with power to substitute  one or more persons or  corporation  with
like power.

         8.3  Performance  by Secured  Party.  If Debtor  fails to  perform  any
agreement  contained  herein,  Secured  Party may itself  perform,  or cause the
performance  of, such  agreement,  and the reasonable  expenses of Secured Party
incurred in connection  therewith  shall be payable by Debtor under Section 8.8.
In no  event,  however,  shall  Secured  Party  have any  obligation  or  duties
whatsoever to perform any covenant or agreement of Debtor contained herein,  and
any such performance by Secured Party shall be wholly discretionary with Secured
Party.

         8.4 Duties of Secured  Party.  The powers  conferred upon Secured Party
hereunder  are solely to protect its  interest in the  Collateral  and shall not
impose any duty upon it to exercise any such powers. Except for the safe custody
of any  Collateral  in its  possession  and the  accounting  for money  actually
received by it hereunder,  Secured Party shall have no duty as to any Collateral
or as to the taking of any  necessary  steps to preserve  rights  against  prior
parties or any other rights  pertaining to any Collateral.  Without limiting the
generality of the foregoing,  Secured Party shall not have any obligation,  duty
or  responsibility  to do any of the  following:  (a) ascertain any  maturities,
calls,  conversions,  exchanges,  offers, tenders or similar matters relating to
the  Collateral or informing  Debtor with respect to any such matters;  (b) fix,
preserve  or  exercise  any  right,  privilege  or option  (whether  conversion,
redemption or otherwise) with respect to the Collateral; (c) collect any amounts
payable  in  respect  of the  Collateral;  (d)  sell all or any  portion  of the
Collateral,  for any reason;  or (e) hold the Collateral for or on behalf of any
party other than Debtor.

         8.5 No  Liability  of Secured  Party.  Neither the  acceptance  of this
Agreement by Secured Party,  nor the exercise of any rights hereunder by Secured
Party,  shall be construed in any way as an  assumption  by Secured Party of any
obligations,  responsibilities,  or duties of Debtor arising in connection  with
the  Collateral  assigned  hereunder  or  otherwise  bind  Secured  Party to the
performance of any  obligations  respecting the  Collateral,  it being expressly
understood  that Secured  Party shall not be obligated to perform,  observe,  or
discharge  any  obligation,  responsibility,  duty,  or  liability  of Debtor in
respect of any of the Collateral,  including without limitation  appearing in or
defending any action, expending any money or incurring any expense in connection
therewith.  TO THE FULLEST EXTENT  PERMITTED BY APPLICABLE LAW, DEBTOR SHALL AND
DOES AGREE TO INDEMNIFY,  PROTECT,  DEFEND AND HOLD HARMLESS  SECURED PARTY, ITS
OFFICERS, DIRECTORS, REPRESENTATIVES, AGENTS, EMPLOYEES, LENDERS, SUCCESSORS AND
ASSIGNS,  FROM AND AGAINST ALL  LIABILITIES,  CLAIMS,  DAMAGES,  LOSSES,  FINES,
PENALTIES,  CAUSES OF ACTIONS,  SUITS,  JUDGMENTS AND EXPENSES  (INCLUDING COURT
COSTS,  ATTORNEY'S  FEES  AND  COST OF  INVESTIGATION)  OF ANY  NATURE,  KIND OR
DESCRIPTION  OF ANY PERSON OR ENTITY,  DIRECTLY OR  INDIRECTLY,  ARISING OUT OF,
CAUSED BY OR  RESULTING  FROM (IN WHOLE OR IN PART),  ANY  UNINTENTIONAL  ACT OR
OMISSION (OR  INTENTIONAL  ACT OR OMISSION SO LONG AS SUCH ACT OR OMISSION  DOES
NOT  CONSTITUTE  NEGLIGENCE)  OF SECURED  PARTY (OR  ANYONE  ACTING ON BEHALF OF
SECURED PARTY) IN CONNECTION WITH THE COLLATERAL.  THE FOREGOING INDEMNITY SHALL
SURVIVE THE EXPIRATION OR EARLIER TERMINATION OF THIS AGREEMENT.

         8.6 Right of Secured Party to Defend Action Affecting Security. Secured
Party  may,  at the  expense  of  Debtor,  appear in and  defend  any  action or
proceeding at law or in equity  purporting to affect  Secured  Party's  Security
Interest under this Agreement.

         8.7 Right of  Secured  Party to Prevent  or Remedy  Default.  If Debtor
shall fail to perform any of the covenants,  conditions and agreements  required
to be performed  and  observed by Debtor  under the Purchase  Agreement or Other
Agreement,  or in respect of the Collateral  (subject to any applicable  default
cure  period),  Secured  Party (a) may but shall  not be  obligated  to take any
action Secured Party deems  necessary or desirable to prevent or remedy any such
default by Debtor or otherwise to protect the Security  Interest,  and (b) shall
have the absolute and immediate  right to take  possession of the  Collateral or
any  part  thereof  (to  the  extent  Secured  Party  has not  previously  taken
possession)  to such  extent  and as often  as the  Secured  Party,  in its sole
discretion, deems necessary or desirable in order to prevent or to cure any such
default by Debtor,  or  otherwise  to protect the  security  of this  Agreement.
Secured Party may advance or expend such sums of money for the account of Debtor
as Secured Party in its sole discretion deems necessary for any such purpose.

         8.8 Secured Party's Expenses. All reasonable advances, costs, expenses,
charges and attorneys' fees which Secured Party may make, pay or incur under any
provision  of this  Agreement  for the  protection  of its  security  or for the
enforcement of any of its rights hereunder,  including,  without limitation,  in
foreclosure  proceedings commenced and subsequently  abandoned or in any dispute
or litigation in which Secured Party or the holder of any of the Obligations may
become involved by reason of or arising out of the Purchase Agreement, any Other
Agreement or the  Collateral,  shall be a part of the  Obligations  and shall be
paid by Debtor to Secured Party, upon demand, and shall bear interest until paid
at the maximum  rate of interest  permitted  by  applicable  law,  from the date
incurred by Secured Party until repaid by Debtor.

         8.9. Convertible  Collateral.  Secured Party may present for conversion
any  Collateral  which is  convertible  into any other  instrument or investment
security or a  combination  thereof with cash,  but Secured Party shall not have
any duty to present for conversion any Collateral  unless it shall have received
from Debtor  detailed  written  instructions to that effect at a time reasonably
far in advance of the final conversion date to make such conversion possible.

         8.10 Secured Party's Right of Set-Off.  Upon the happening of any event
entitling  Secured  Party to pursue any remedy  provided  herein,  or if Secured
Party shall be served with garnishment process in which Debtor shall be named as
defendant,  whether or not Debtor  shall be in  default  hereunder  at the time,
Secured Party may, but shall not be required to, set-off any indebtedness  owing
by  Secured  Party  to  Debtor  against  any of the  Obligations  without  first
resorting to the security hereunder and without prejudice to any other rights or
remedies of Secured Party or its Security Interest.

         8.11 Remedies.  No right or remedy herein  reserved to Secured Party is
intended to be exclusive  of any other right or remedy,  but each and every such
remedy shall be cumulative,  not in lieu of, but in addition to any other rights
or remedies given under this Agreement and all other security documents. Any and
all of Secured  Party's  rights and remedies may be exercised  from time to time
and as often as such exercise as deemed necessary or desirable by Secured Party.

         8.12 Debtor's Waivers.  Debtor waives notice of the creation,  advance,
increase,  existence,  extension,  or  renewal  of, and of any  indulgence  with
respect to, the  Obligations;  waives notice of intent to accelerate,  notice of
acceleration,  notice  of  intent  to  demand,  presentment,  demand,  notice of
dishonor,  and  protest;   waives  notice  of  the  amount  of  the  Obligations
outstanding  at any time,  notice of any change in  financial  condition  of any
person liable for the  Obligations  or any part thereof,  notice of any Event of
Default,  and all other  notices  respecting  the  Obligations;  and agrees that
maturity of the Obligations  and any part thereof may be accelerated,  extended,
or renewed one or more times by Secured Party in its discretion,  without notice
to Debtor.

         8.13 Other Parties and Other Collateral.  No renewal or extension of or
any other  indulgence  with respect to the  Obligations or any part thereof,  no
release  of any  security,  no  release  of any  person  (including  any  maker,
endorser,  guarantor,  or  surety)  liable  on  the  Obligations,  no  delay  in
enforcement  of payment,  and no delay or admission or lack of diligence or care
in exercising any right or power with respect to the Obligations or any security
therefor  or  guaranty  thereof or under this  Agreement  shall in other  manner
impair  or affect  the  rights  of  Secured  Party  under  the law,  under  this
Agreement,  or under any other  document or  agreement  pertaining  to the other
security for the  Obligations,  before  foreclosing  upon the Collateral for the
purpose of paying the Obligations.  Debtor waives any right to the benefit of or
to require or control application of any other security or proceeds thereof, and
Debtor  agrees that Secured  Party shall have no duty or obligation to Debtor to
apply to the Obligations any such other security or proceeds thereof.

         8.14 Relationship Among Secured Party.  Either or both of Secured Party
are  entitled  to enforce  any and all rights  granted to Secured  Party in this
Agreement,  and to take any other  action  allowed to be taken by Secured  Party
under this  Agreement.  Neither  Secured  Party is under any  obligation  to act
jointly or in concert with the other Secured Party  pursuant to this  Agreement.
Any action  required  to be taken by Debtor,  or notice  required to be given by
Debtor, shall be taken or given with respect to both of Secured Party.

                                   ARTICLE IX

                                  MISCELLANEOUS

     9.1 Terms  Commercially  Reasonable.  The terms of this Agreement  shall be
deemed commercially reasonable within the meaning of the Texas UCC.

         9.2 Notices.  Any notices or demands  required or permitted to be given
hereunder  shall be  deemed  sufficiently  given if in  writing  and  personally
delivered or mailed (with all postage and charges prepaid), addressed to Secured
Party or to Debtor their respective  addresses set forth below, or at such other
address as the above parties may from time to time  designate by written  notice
to the other given in  accordance  with this Section  9.2.  Any such notice,  if
personally  delivered or  transmitted  by telex or telegram,  shall be deemed to
have been given on the date so delivered or transmitted or, if mailed, be deemed
to have been given on the day after such  notice is placed in the United  States
mail in accordance with this Section 9.2.

                  Secured Party:  1301 Capital of Texas Hwy., Suite C-300
                                  Austin, Travis County, Texas 78746
                                  Attn: President

                  with copy to:   Timothy L. LaFrey, Esq.
                                  Akin, Gump, Strauss, Hauer & Feld, L.L.P.
                                  1900 Frost Bank Plaza
                                  816 Congress Avenue
                                  Austin, Texas 78701

                  Debtor:         Christian K. Kim, M.D.
                                  40 Presidio Drive
                                  Novato, California   94949

         9.3 Parties Bound.  Secured Party's rights under this Agreement and the
Security Interest shall inure to the benefits of its successors and assigns, and
in the event of any  assignment  or  transfer of any of the  Obligations  or the
Collateral,  Secured  Party  thereafter  shall  be  fully  discharged  from  any
responsibility  with respect to the Collateral so assigned or  transferred,  but
Secured  Party shall  retain all rights and powers  hereby given with respect to
any of the  Obligations  or  Collateral  not so  assigned  or  transferred.  All
representations, warranties, and agreements of Debtor if more than one are joint
and several, and all shall be binding upon the personal representatives,  heirs,
successors, and assigns of Debtor.

         9.4 Waiver.  No delay of Secured Party in exercising any power or right
shall operate as a waiver thereof;  nor shall any single or partial  exercise of
any power or right preclude other or further exercise thereof or the exercise of
any other power or right.  No waiver by Secured Party of any right  hereunder of
any default by Debtor shall be binding upon Secured Party unless in writing, and
no failure by Secured  Party to exercise any power or right  hereunder or waiver
of any  default  by Debtor  shall  operate  as a waiver of any other or  further
exercise of such right or power of any further default.

         9.5 Agreement Continuing.  This Agreement shall constitute a continuing
agreement,  applying to all future as well as existing transactions,  whether or
not of the  character  contemplated  at the date of this  Agreement,  and if all
transactions between Secured Party and Debtor shall be closed at any time, shall
be equally  applicable to any new  transactions  thereafter.  Provisions of this
Agreement,  unless  by their  terms  exclusive,  shall be in  addition  to those
contained in any Other Agreement.

         9.6 Definitions. Unless the context indicated otherwise, definitions in
the Texas Business and Commerce Code ("Texas UCC") apply to words and phrases in
this Agreement; if Texas UCC definitions conflict, Chapter 9 definitions apply.

         9.7 Miscellaneous. In this Agreement, whenever the context so requires,
the neuter gender  includes the masculine and feminine,  and the singular number
includes  the plural and vice  versa.  The  headings  of  paragraphs  herein are
inserted only for convenience and shall in no way define,  describe or limit the
scope of intent of any  provisions  of this  Agreement.  No  change,  amendment,
modification,  cancellation,  or  discharge of any  provision of this  Agreement
shall be valid unless consented to in writing by Secured Party.

         9.8 Assignment of Secured  Party's  Interest.  Secured Party shall have
the right to assign all or any portion of its rights in this  Agreement  without
approval or consent.  Debtor may not assign this  Agreement or any of its rights
or obligations  hereunder  without the express prior written  consent of Secured
Party in each instance.

     9.9 Applicable  Laws.  THIS AGREEMENT SHALL BE GOVERNED BY AND CONSTRUED IN
ACCORDANCE  WITH THE LAWS OF THE STATE OF TEXAS AND THE  APPLICABLE  LAWS OF THE
UNITED STATES OF AMERICA.

     9.10 ENTIRE AGREEMENT.  THIS AGREEMENT AND THE PURCHASE AGREEMENT REPRESENT
THE FINAL AGREEMENT  BETWEEN THE PARTIES AND MAY NOT BE CONTRADICTED BY EVIDENCE
OF PRIOR,  CONTEMPORANEOUS,  OR SUBSEQUENT ORAL AGREEMENTS OF THE PARTIES. THERE
ARE NO UNWRITTEN ORAL AGREEMENTS BETWEEN THE PARTIES.

                            [Signature page follows]


<PAGE>


S-1



                                SIGNATURE PAGE TO

                                 ASSIGNMENT AND

                               SECURITY AGREEMENT

         EXECUTED this 1st day of September, 1999.



DEBTOR:                                   /s/ Christian K. Kim, M.D.

                                          Printed Name: Christian K. Kim, M.D.


SECURED PARTY:                            Prime Medical Operating, Inc.

                                          By: /s/ Cheryl Williams

                                          Name: Cheryl Williams

                                          Title: Vice President


                                          Prime/BDR Acquisition, L.L.C.

                                          By: /s/ Cheryl Williams

                                          Name: Cheryl Williams

                                          Title: Secretary and Manager

<PAGE>



                        ASSIGNMENT AND SECURITY AGREEMENT

     THIS  ASSIGNMENT  AND SECURITY  AGREEMENT  (this  "Agreement")  is made and
entered into as of the 1st day of September,  1999, by and between Prime Medical
Operating, Inc., a Delaware corporation ("PMOI"), Prime/BDR Acquisition, L.L.C.,
a Delaware limited  liability  company ("Prime") (PMOI and Prime are referred to
herein,  jointly and severally,  as "Secured  Party") and Mark R. Mandel,  M.D.,
Trustee under the Trust Agreement dated April 12, 1989 (the "Debtor").

                                    RECITALS:

         A. Debtor and Secured Party have  executed and  delivered  that certain
Stock  Purchase   Agreement  dated  as  of  September  1,  1999  (the  "Purchase
Agreement"),  pursuant to which  Secured  Party  purchased  from Debtor  certain
shares of the $0.01 par value  common  stock of Horizon  Vision  Center,  Inc, a
Nevada corporation ("Horizon").

         B. Secured Party has requested  that Debtor pledge the  Collateral  (as
defined below) to secure certain obligations and liabilities that Debtor may now
or hereafter have to Secured Party, including, without limitation, any indemnity
obligations arising under the Purchase Agreement.

     C. Debtor desires to enter into this Agreement as a material  inducement to
Secured  Party's  purchase  of  Debtor's  shares of Horizon  under the  Purchase
Agreement.

                                   AGREEMENT:

         NOW, THEREFORE, in consideration of the foregoing and the covenants and
agreements hereinafter set forth, and other good and valuable consideration, the
receipt and sufficiency of which Debtor  acknowledges,  Debtor and Secured Party
agree as follows:

                                    ARTICLE I

                       COLLATERAL AND SECURED OBLIGATIONS

         1.1 Grant of Security Interest.  Debtor hereby assigns,  transfers, and
pledges to Secured  Party,  and Debtor hereby grants to Secured Party a security
interest   in,   the   following   described   collateral   (collectively,   the
"Collateral"):

                  (a)  Shares  of  Horizon.  From  and  after  the  date of this
Agreement,  (i) all shares of the common  stock of Horizon  owned or acquired in
any manner by Debtor or Secured Party,  (collectively,  the "Shares"),  (ii) any
replacements,  substitutions,  or exchanges of the certificates representing the
Shares, and (iii) any and all options,  rescission rights,  registration rights,
conversion rights, subscription rights, contractual or quasi-contractual rights,
warrants,  redemption rights,  redemption proceeds, calls, preemptive rights and
all other rights and benefits pertaining to the Shares;

                  (b)  Accounts.  All  accounts  and  rights  now  or  hereafter
attributable to any of the Collateral  described in (a) above, and all rights of
Debtor now or hereafter arising under any agreement pertaining to the Collateral
described  in  (a)  above,   including  without  limitation  all  distributions,
proceeds, fees, dividends,  preferences,  payments or other benefits of whatever
nature  which  Debtor is now or may  hereafter  become  entitled to receive with
respect to any Collateral described in (a) above;

                  (c) Additional  Property.  "Collateral" shall also include the
following  property  (collectively,  the  "Additional  Property")  which  Debtor
becomes  entitled  to receive or shall  receive  in  connection  with any of the
Collateral  described in this Section 1.1: (i) any stock certificate,  including
without  limitation,  any  certificate  representing  a  stock  dividend  or any
certificate in connection with any recapitalization,  reclassification,  merger,
consolidation,  conversion,  sale of assets, combination of shares, stock split,
reverse  stock split or  spin-off;  (ii) any option,  warrant,  subscription  or
right,  whether as an addition to or in  substitution  of any of the  Collateral
described in this Section 1.1; (iii) any dividends or  distributions of any kind
whatsoever,  whether  distributable in cash,  stock or other property;  (iv) any
interest,  premium or principal  payments;  and (v) any conversion or redemption
proceeds; and

                  (d) Proceeds.  All proceeds (cash and non-cash) arising out of
the sale, exchange, collection or other disposition of all or any portion of the
Collateral  described in (a), (b), or (c) above,  including  without  limitation
proceeds in the form of stock, accounts, chattel paper, instruments,  documents,
goods, inventory and equipment.

The  security  interest in the  Collateral  hereby  granted by Debtor to Secured
Party may sometimes be referred to in this Agreement as the "Security Interest".

         1.2 Obligations.  This Agreement and the Security Interest shall secure
full and punctual payment and performance of the following indebtedness,  duties
and obligations (collectively, the "Obligations"):

                  (a) All liabilities and obligations of Debtor to Secured Party
(including, without limitation, any principal, interest, fees and other amounts,
and any  indemnity  obligations)  under and pursuant to the Purchase  Agreement,
this Agreement and/or any other contract or agreement  between Secured Party and
Debtor or any affiliate of Debtor (collectively, "Other Agreements"; and

                  (b) (i)  all  indebtedness,  obligations  and  liabilities  of
Debtor and/or any affiliate of Debtor to Secured Party of any kind or character,
now existing or hereafter arising, whether direct, indirect, related, unrelated,
fixed,  contingent,  liquidated,  unliquidated,  joint,  several  or  joint  and
several,  arising from,  connected with, or related to the Purchase Agreement or
any Other Agreement, or any other document, agreement, or instrument executed in
connection  either of them,  (ii) all accrued but unpaid  interest on any of the
indebtedness  described in (i) above, (iii) all obligations of Debtor and/or any
affiliate  of  Debtor  to  Secured  Party  under  any  documents  or  agreements
evidencing,  securing,  governing  and/or  pertaining  to all or any part of the
indebtedness  described  in (i) and (ii)  above,  (iv) all  costs  and  expenses
incurred by Secured Party in connection  with the collection and  administration
of all or any part of the  indebtedness  and obligations  described in (i), (ii)
and (iii) above or the protection or preservation  of, or realization  upon, the
collateral  securing  all or any  part of  such  indebtedness  and  obligations,
including  without  limitation  all  attorneys'  fees,  and  (v)  all  renewals,
extensions, modifications and rearrangements of the indebtedness and obligations
described in (i), (ii), (iii) and (iv) above.

                  (c) All sums now or  hereafter  loaned or  advanced by Secured
Party to  Debtor,  or  expended  by Secured  Party for the  account of Debtor or
otherwise owing by Debtor to Secured Party, in respect of the  Obligations,  and
all other sums  expended or advanced  by Secured  Party  pursuant to any term or
provision of this Agreement or any Other Agreement (i) to collect and/or enforce
the Obligations or (ii) to maintain, protect and preserve the Collateral.

                                   ARTICLE II

       DEBTOR'S REPRESENTATIONS AND WARRANTIES WITH RESPECT TO COLLATERAL

       Debtor hereby represents and warrants to Secured Party as follows:

         2.1 Ownership of Collateral.  Debtor has good and  marketable  title to
the Collateral  free and clear of any liens,  security  interests,  shareholders
agreement, calls, charge, or encumbrance,  except for this Security Interest. No
financing  statement or other  instrument  similar in effect covering all or any
part of the  Collateral is on file in any recording  office,  except as may have
been filed in favor of Secured Party relating to this Agreement.

         2.2  Power  &  Authority.  Debtor  has the  lawful  right,  power,  and
authority  to grant the Security  Interest in the  Collateral.  This  Agreement,
together  with all filings and other  actions  necessary or desirable to perfect
and protect such security interest,  which have been duly taken,  create a valid
and perfected first priority  security  interest in the Collateral  securing the
payment and performance of the Obligations.

         2.3  No  Agreements.  The  Shares  are  not  subject  to any  right  of
redemption,  or any  call or put  options,  voting  trust,  proxy,  shareholders
agreement,  right of first  refusal,  or any other  document or agreement  which
would in any way impair or adversely affect this Security Interest or the rights
of Secured Party under this Agreement.

         2.4  Securities.  Any  certificates  evidencing  securities  pledged as
Collateral  are valid and  genuine  and have not been  altered.  All  securities
pledged as Collateral have been duly  authorized and validly  issued,  are fully
paid and  non-assessable,  and were not issued in  violation  of the  preemptive
rights of any party or of any agreement by which Debtor or the issuer thereof is
bound.  No  restrictions  or  conditions  exist with  respect to the transfer or
voting of any securities pledged as Collateral.

                                   ARTICLE III

                  DEBTOR'S OTHER REPRESENTATIONS AND WARRANTIES

         3.1 Solvency of Debtor.  As of the date hereof,  (i) Debtor is solvent;
(ii) the fair saleable  value of Debtor's  assets exceeds  Debtor's  liabilities
(both fixed and contingent);  (iii) Debtor has sufficient capital to satisfy all
of Debtor's  obligations  as they  become due;  (iv) no  receiver,  trustee,  or
custodian has been appointed for, or taken  possession of, all or  substantially
all of the assets of Debtor,  either in a  proceeding  brought by Debtor or in a
proceeding  brought against Debtor;  (v) Debtor is not the subject of a petition
for relief under the United  States  Bankruptcy  Code or any similar  federal or
state insolvency law, including without limitation a petition filed by Debtor or
a petition filed by a third party seeking relief against Debtor; and (vi) Debtor
has no  intention  of filing a  petition  for  relief  under the  United  States
Bankruptcy  Code or any similar  federal or state  insolvency law, or of seeking
any other form of  creditor  relief,  within  the  two-year  period  immediately
following the date of this Agreement.

         3.2  Authority and  Compliance.  Debtor has full power and authority to
enter into this Agreement. Debtor has full power and authority to enter into and
perform its obligations  under the Purchase  Agreement and each Other Agreement.
No further  consent or approval is  required as a condition  to the  validity of
this  Agreement,  the Purchase  Agreement or any Other  Agreement.  Debtor is in
compliance with all applicable laws, ordinances,  statutes, orders, regulations,
judgments, writs, or decrees of any governmental entity to which it is subject.

         3.3 Binding Agreement.  This Agreement, the Purchase Agreement and each
Other Agreement  constitute valid and legally binding  obligations of Debtor, in
accordance  with  their  terms,  subject  to  (a)  the  applicable   bankruptcy,
insolvency,  reorganization,  moratorium,  and similar laws affecting creditors'
rights  generally  and  (b)  restrictions  imposed  by any  court  of  competent
jurisdiction on the enforcement of non-competition and exclusive use restrictive
covenants imposed under the Purchase Agreement or any Other Agreement.

         3.4 Litigation.  There are no proceedings  pending or, to the knowledge
of Debtor,  threatened before any court or  administrative  agency which will or
may have a material adverse effect on the financial  condition of Debtor or upon
Debtor's ability to perform its obligations  under this Agreement,  the Purchase
Agreement or any Other Agreement.

         3.5 No Conflicting Agreements.  There are no provisions of any existing
agreement,  mortgage,  indenture or contract  binding on Debtor or affecting its
property,  which  would  conflict  with or in any  way  prevent  the  execution,
delivery, or carrying out of the terms of this Agreement, the Purchase Agreement
or any Other Agreement.

         3.6  Ownership  of  Assets.  Debtor  has  good  and  full  title to the
Collateral,  and the  Collateral  is owned  free and  clear of  liens,  charges,
claims, security interests, and other encumbrances.

     3.7 Taxes. Debtor has filed all tax returns required to be filed by Debtor.

                                   ARTICLE IV

                  DEBTOR'S COVENANTS WITH RESPECT TO COLLATERAL

         Debtor  covenants  and agrees  that from the date  hereof and until the
payment  and  performance  in  full  of the  Obligations  unless  Secured  Party
otherwise consents in writing:

         4.1  Delivery of  Instruments  and/or  Certificates.  Contemporaneously
herewith,   Debtor  covenants  and  agrees  to  deliver  to  Secured  Party  any
certificates,   documents,   or  instruments   representing  or  evidencing  the
Collateral,  with Debtor's  endorsement  thereon and/or  accompanied by property
instruments of transfer and assignment duly executed in blank with, if requested
by Secured Party,  signatures  guaranteed by a member or member  organization in
good standing of an authorized Securities Transfer Agents Medallion Program, all
in form and substance satisfactory to Secured Party.

         4.2  Further  Assurances.   Debtor  will   contemporaneously  with  the
execution  hereof  and from  time to time  thereafter  at its  expense  promptly
execute and deliver all further  instruments  and documents and take all further
action  necessary or  appropriate or that Secured Party may request in order (i)
to perfect and protect the security  interest created or purported to be created
hereby and the first priority of such security interest,  (ii) to enable Secured
Party to exercise  and enforce its rights and  remedies  hereunder in respect of
the  Collateral,  and (iii) to otherwise  effect the purposes of this Agreement,
including  without  limitation:  (A)  executing  and  filing  any  financing  or
continuation  statements,  or any  amendments  thereto;  (B)  obtaining  written
confirmation  from the issuer of any  securities  pledged as  Collateral  of the
pledge of such securities,  in form and substance satisfactory to Secured Party;
(C)  cooperating  with Secured Party in registering the pledge of any securities
pledged as Collateral with the issuer of such securities;  (D) delivering notice
of Secured Party's security interest in any securities  pledged as Collateral to
any securities or financial  intermediary,  clearing  corporation or other party
required by Secured Party, in form and substance  satisfactory to Secured Party;
and  (E)  obtaining  written  confirmation  of  the  pledge  of  any  securities
constituting Collateral from any securities or financial intermediary,  clearing
corporation  or other party  required by Secured  Party,  in form and  substance
satisfactory to Secured Party.

         4.3 Additional Property. All Additional Property, as defined in Section
1.1(c)  above,  received by Debtor shall be received in trust for the benefit of
Secured Party.  All Additional  Property and all  certificates  or other written
instruments or documents  evidencing and/or representing the Additional Property
that is  received  by Debtor,  together  with such  instruments  of  transfer as
Secured Party may request,  shall  immediately be delivered to or deposited with
Secured Party and held by Secured  Party as  Collateral  under the terms of this
Agreement.  If the  Additional  Property  received  by Debtor and  delivered  to
Secured  Party  pursuant  to this  Section  shall  be  shares  of stock or other
securities,  such shares of stock or other  securities shall be duly endorsed in
blank or  accompanied  by proper  instruments  of transfer and  assignment  duly
executed in blank with, if requested by Secured Party,  signatures guaranteed by
a member or member  organization  in good standing of an  authorized  Securities
Transfer Agents  Medallion  Program,  all in form and substance  satisfactory to
Secured  Party.  Secured  Party  shall  be  deemed  to  have  possession  of any
Collateral in transit to Secured Party or its agent.

         4.4  Sale,  Transfer,  Encumbrance.  Debtor  will not  sell,  transfer,
mortgage,  or otherwise  encumber any  Collateral or impair the value thereof in
any manner without  Secured  Party's prior written  consent,  including  without
limitation by purchase, lease, barter, trade, payment deferral, or the creation,
assumption  or guarantee of  indebtedness  or other  lending of credit.  Secured
Party's written consent to any sale,  mortgage,  transfer,  or encumbrance shall
not be construed to be a waiver of this  provision in respect to any  subsequent
proposed sale, mortgage, transfer, or encumbrance.

         4.5 Liens. Neither Debtor nor any person acting on Debtor's behalf has,
or shall have any right, power, or authority to and shall not create,  incur, or
permit to be placed or  imposed,  upon the  Collateral,  any lien of any type or
nature whatsoever, other than the liens in favor of Secured Party.

         4.6 Matters or  Occurrences  Affecting  Collateral  or this  Agreement.
Debtor will promptly  notify Secured Party of any and all matters or occurrences
that may have a material adverse effect on the status or value of the Collateral
or this Agreement,  including  without  limitation the occurrence of an Event of
Default,  or an event  which,  with giving of notice or lapse of time,  or both,
would constitute an Event of Default.

     4.7  Agreements  Pertaining to  Collateral.  Debtor will not enter into any
type of contract or agreement  pertaining to any of the Collateral or in any way
transfer any voting rights pertaining to the Collateral to any person or entity.

         4.8 Dilution of Ownership.  As to any securities pledged as Collateral,
Debtor  will not  consent to or approve of the  issuance  of (i) any  additional
shares  of  any  class  of  securities  of  such  issuer,  (ii)  any  instrument
convertible  voluntarily  by  the  holder  thereof  or  automatically  upon  the
occurrence or  non-occurrence  of any event or condition  into, or  exchangeable
for, any such  securities,  or (iii) any warrants,  options,  contracts or other
commitments  entitling any third party to purchase or otherwise acquire any such
securities.  Notwithstanding  the  foregoing  or any  other  provision  of  this
Agreement to the contrary,  Debtor (i) shall comply with his  obligations  under
the  Purchase  Agreement  and each Other  Document,  and (ii) may consent to any
issuance of shares of Horizon if such  issuance has been  approved by a majority
of the Board of Directors of Horizon.

         4.9  Restrictions  on  Securities.  Debtor  will  not  enter  into  any
agreement  creating,  or otherwise permit to exist, any restriction or condition
upon the transfer,  voting or control of any  securities  pledged as Collateral,
except as consented to in writing by Secured Party. As to any securities pledged
as collateral, Debtor will not consent to or approve of any stock split, reverse
stock  split,  stock  dividend,  reclassification,   or  other  similar  act  or
transaction  regarding  the  Shares  unless  consented  to in writing by Secured
Party.

                                    ARTICLE V

                         DEBTOR'S AFFIRMATIVE COVENANTS

         Until payment and performance of all Obligations,  Debtor covenants and
agrees that it shall promptly  advise Secured Party in writing of any litigation
filed  against  Debtor  and of any  condition,  event or act which  comes to its
attention  that  would or might  have a  material  adverse  effect  on  Debtor's
financial condition or on Debtor's ability to perform the Obligations.

                                   ARTICLE VI

                               NEGATIVE COVENANTS

         Until payment and performance of all Obligations,  Debtor covenants and
agrees that Debtor will not, without the prior written consent of Secured Party:

     6.1 Liens. Grant, suffer, or permit liens on, or security interests in, the
Collateral.

         6.2  Violate  Other  Covenants.  Violate  or fail to  comply  with  any
covenants  or  agreements  regarding  other  debt  which  will or would with the
passage  of time or upon  demand  cause the  maturity  of any  other  debt to be
accelerated.

                                   ARTICLE VII

                              DEFAULT AND REMEDIES

     7.1 Events of Default.  An Event of Default  (herein so called) shall exist
if any one or more of the following events shall occur:

                  (a) The  failure  of Debtor to pay any amount  required  to be
paid under the Purchase Agreement (including,  without limitation, the indemnity
provisions  contained  therein),  or any other  amount  which  Debtor may now or
hereafter  owe to Secured Party under any Other  Agreement or otherwise,  within
(15) calendar days after such amount is due;

     (b) The failure of Debtor to pay any  Obligation  within (15) calendar days
after such amount is due; and

                  (c)  Debtor's  breach of a covenant in this  Agreement  or any
other  failure to perform  its  obligations  under this  Agreement  or any Other
Agreement.

     7.2 Secured Party's Remedies. Upon the occurrence of an Event of Default:

                  (a) Secured Party may declare the Obligations in whole or part
immediately due and may enforce payment and performance of the same and exercise
any rights under the Texas UCC,  rights and remedies of Secured Party under this
Agreement, or otherwise.

                  (b) Secured  Party may, at Secured  Party's  option and at the
expense of Debtor,  either in Secured Party's own right or in the name of Debtor
and in the same manner and to the same extent that Debtor  might  reasonably  so
act  if  this  Agreement  had  not  been  made:  (i) do  all  things  requisite,
convenient,  or  necessary  to enforce the  performance  and  observance  of all
rights,  remedies and privileges of Debtor arising from the  Collateral,  or any
part thereof,  including without limitation compromising,  waiving, excusing, or
in any manner releasing or discharging any obligation of any party to or arising
from the Collateral;  (ii) take possession of the books, papers,  chattel paper,
documents of title, and accounts of Debtor,  wherever  located,  relating to the
Collateral; (iii) sue or otherwise collect and receive money attributable to the
Collateral;  and (iv) exercise any other lawfully  available powers or remedies,
and do all other things  which  Secured  Party deems  requisite,  convenient  or
necessary  or which the  Secured  Party  deems  proper to protect  the  Security
Interest.

                  (b) Secured Party may foreclose  this  Agreement in the manner
now or  hereafter  provided  or  permitted  by law and may upon such  reasonable
notification  prior thereto as may be required by applicable  law (Debtor hereby
agreeing  that ten  days'  notice is  commercially  reasonable),  sell,  assign,
transfer,  or otherwise  dispose of the Collateral at public or private sale, in
whole  or in  part,  and  Secured  Party  may,  in its own  name or as  Debtor's
attorney-in-fact  effectively  assign and transfer the  Collateral,  or any part
thereof,   absolutely,  and  execute  and  deliver  all  necessary  assignments,
conveyances,  bills of sale, and other  instruments with power to substitute one
or more persons or  corporations  with like power.  Any such  foreclosure  sale,
assignment,  transfer,  or other  disposition  shall, to the extent permitted by
law,  be a  perpetual  bar,  both at law and in equity,  against  Debtor and all
persons and corporations  lawfully  claiming by or through or under Debtor.  Any
such foreclosure sale may be adjourned from time to time. Upon any sale, Secured
Party may bid for and purchase the  Collateral,  or any part  thereof,  and upon
compliance with the terms of sale may hold,  retain,  possess and dispose of the
Collateral, in its absolute right without further accountability.  Secured Party
shall have the right to be  credited  on the  amount of its bid a  corresponding
amount of the Obligations as of the date of such sale.

                  (c) If, in the opinion of Secured Party, there is any question
that a public sale or  distribution  of any Collateral will violate any state or
federal  securities  law,  Secured  Party  (i) may  offer  and  sell  securities
privately to purchasers who will agree to take them for investment  purposes and
not with a view to distribution  and who will agree to imposition of restrictive
legends on the  certificates  representing  the security,  or (ii) may sell such
securities in an intrastate  offering  under Section  3(a)(11) of the Securities
Act of 1933,  and no sale so made in good faith by Secured Party shall be deemed
to be not "commercially reasonable" because so made.

                  (d)  Not  in  limitation  of  any  other   provision  of  this
Agreement,  Secured  Party shall have all rights and remedies of a secured party
under the Texas UCC.

         7.3  Application  of Proceeds.  Secured Party may apply the proceeds of
any foreclosure  sale hereunder or from any other  permitted  disposition of the
Collateral  or any part  thereof as  follows:  (a) first,  to the payment of all
reasonable  costs and expenses of any foreclosure  and collection  hereunder and
all proceedings in connection  therewith,  including reasonable attorneys' fees;
(b) then, to the  reimbursement of Secured Party for all  disbursements  made by
Secured Party for taxes,  assessments or liens superior to the Security Interest
and  which  Secured  Party  shall  deem  expedient  to  pay;  (c)  then,  to the
reimbursement of Secured Party of any other  disbursements made by Secured Party
in accordance with the terms hereof or under the Purchase Agreement or any Other
Agreement;  (d) then,  to or among the amounts of fees,  interest and  principal
then owing and unpaid in respect of the Obligations, in such priority as Secured
Party may determine in its  discretion;  and (e) the remainder of such proceeds,
if any,  shall be paid to Debtor.  If such  proceeds  shall be  insufficient  to
discharge the entire  Obligations,  Secured Party shall have any other available
legal recourse  against Debtor under,  or for the  performance  of, the Purchase
Agreement and any Other  Agreement  between  Debtor and Secured  Party,  for the
deficiency,  together with interest  thereon at the maximum rate permitted under
applicable law.

         7.4  Enforcement  of  Obligations.  Nothing in this Agreement or in any
other  document  or  agreement  shall  affect or impair  the  unconditional  and
absolute right of Secured Party to enforce the  Obligations as and when the same
shall become due in accordance with the terms of the Purchase Agreement or Other
Agreement.

                                  ARTICLE VIII

                             RIGHTS OF SECURED PARTY

         8.1  Subrogation.  Upon the occurrence of an Event of Default,  Secured
Party,  at its  election,  may  subrogate  to all of the  interest,  rights  and
remedies  of the  Debtor,  in respect  to any of the  Collateral  or  agreements
pertaining thereto.

         8.2 Secured Party  Appointed  Attorney-in-Fact.  Debtor hereby appoints
Secured Party as  attorney-in-fact  of Debtor,  with full authority in the place
and stead of Debtor and in the name of Debtor, Secured Party or otherwise,  from
time to time on Secured  Party's  discretion and upon the occurrence of an Event
of Default, to take any action and to execute any instrument which Secured Party
may deem  necessary or advisable to accomplish  the purposes of this  Agreement,
including without  limitation:  (a) to ask, demand,  collect,  sue for, recover,
compound, receive and give acquittance and receipts for moneys due and to become
due under or in respect of any of the Collateral;  (b) to receive,  endorse, and
collect  any  drafts or other  instruments,  documents  and  chattel  paper,  in
connection  with clause (a) of this  Section 8.2; (c) to file any claims or take
any action or institute any proceeding which Secured Party may deem necessary or
desirable for the  collection  of any of the  Collateral or otherwise to enforce
the rights of Secured Party against any of the Collateral; and (d) to assign and
transfer the  Collateral,  or any part  thereof,  absolutely  and to execute and
deliver  endorsements,   assignments,  conveyances,  bills  of  sale  and  other
instruments  with power to substitute  one or more persons or  corporation  with
like power.

         8.3  Performance  by Secured  Party.  If Debtor  fails to  perform  any
agreement  contained  herein,  Secured  Party may itself  perform,  or cause the
performance  of, such  agreement,  and the reasonable  expenses of Secured Party
incurred in connection  therewith  shall be payable by Debtor under Section 8.8.
In no  event,  however,  shall  Secured  Party  have any  obligation  or  duties
whatsoever to perform any covenant or agreement of Debtor contained herein,  and
any such performance by Secured Party shall be wholly discretionary with Secured
Party.

         8.4 Duties of Secured  Party.  The powers  conferred upon Secured Party
hereunder  are solely to protect its  interest in the  Collateral  and shall not
impose any duty upon it to exercise any such powers. Except for the safe custody
of any  Collateral  in its  possession  and the  accounting  for money  actually
received by it hereunder,  Secured Party shall have no duty as to any Collateral
or as to the taking of any  necessary  steps to preserve  rights  against  prior
parties or any other rights  pertaining to any Collateral.  Without limiting the
generality of the foregoing,  Secured Party shall not have any obligation,  duty
or  responsibility  to do any of the  following:  (a) ascertain any  maturities,
calls,  conversions,  exchanges,  offers, tenders or similar matters relating to
the  Collateral or informing  Debtor with respect to any such matters;  (b) fix,
preserve  or  exercise  any  right,  privilege  or option  (whether  conversion,
redemption or otherwise) with respect to the Collateral; (c) collect any amounts
payable  in  respect  of the  Collateral;  (d)  sell all or any  portion  of the
Collateral,  for any reason;  or (e) hold the Collateral for or on behalf of any
party other than Debtor.

         8.5 No  Liability  of Secured  Party.  Neither the  acceptance  of this
Agreement by Secured Party,  nor the exercise of any rights hereunder by Secured
Party,  shall be construed in any way as an  assumption  by Secured Party of any
obligations,  responsibilities,  or duties of Debtor arising in connection  with
the  Collateral  assigned  hereunder  or  otherwise  bind  Secured  Party to the
performance of any  obligations  respecting the  Collateral,  it being expressly
understood  that Secured  Party shall not be obligated to perform,  observe,  or
discharge  any  obligation,  responsibility,  duty,  or  liability  of Debtor in
respect of any of the Collateral,  including without limitation  appearing in or
defending any action, expending any money or incurring any expense in connection
therewith.  TO THE FULLEST EXTENT  PERMITTED BY APPLICABLE LAW, DEBTOR SHALL AND
DOES AGREE TO INDEMNIFY,  PROTECT,  DEFEND AND HOLD HARMLESS  SECURED PARTY, ITS
OFFICERS, DIRECTORS, REPRESENTATIVES, AGENTS, EMPLOYEES, LENDERS, SUCCESSORS AND
ASSIGNS,  FROM AND AGAINST ALL  LIABILITIES,  CLAIMS,  DAMAGES,  LOSSES,  FINES,
PENALTIES,  CAUSES OF ACTIONS,  SUITS,  JUDGMENTS AND EXPENSES  (INCLUDING COURT
COSTS,  ATTORNEY'S  FEES  AND  COST OF  INVESTIGATION)  OF ANY  NATURE,  KIND OR
DESCRIPTION  OF ANY PERSON OR ENTITY,  DIRECTLY OR  INDIRECTLY,  ARISING OUT OF,
CAUSED BY OR  RESULTING  FROM (IN WHOLE OR IN PART),  ANY  UNINTENTIONAL  ACT OR
OMISSION (OR  INTENTIONAL  ACT OR OMISSION SO LONG AS SUCH ACT OR OMISSION  DOES
NOT  CONSTITUTE  NEGLIGENCE)  OF SECURED  PARTY (OR  ANYONE  ACTING ON BEHALF OF
SECURED PARTY) IN CONNECTION WITH THE COLLATERAL.  THE FOREGOING INDEMNITY SHALL
SURVIVE THE EXPIRATION OR EARLIER TERMINATION OF THIS AGREEMENT.

         8.6 Right of Secured Party to Defend Action Affecting Security. Secured
Party  may,  at the  expense  of  Debtor,  appear in and  defend  any  action or
proceeding at law or in equity  purporting to affect  Secured  Party's  Security
Interest under this Agreement.

         8.7 Right of  Secured  Party to Prevent  or Remedy  Default.  If Debtor
shall fail to perform any of the covenants,  conditions and agreements  required
to be performed  and  observed by Debtor  under the Purchase  Agreement or Other
Agreement,  or in respect of the Collateral  (subject to any applicable  default
cure  period),  Secured  Party (a) may but shall  not be  obligated  to take any
action Secured Party deems  necessary or desirable to prevent or remedy any such
default by Debtor or otherwise to protect the Security  Interest,  and (b) shall
have the absolute and immediate  right to take  possession of the  Collateral or
any  part  thereof  (to  the  extent  Secured  Party  has not  previously  taken
possession)  to such  extent  and as often  as the  Secured  Party,  in its sole
discretion, deems necessary or desirable in order to prevent or to cure any such
default by Debtor,  or  otherwise  to protect the  security  of this  Agreement.
Secured Party may advance or expend such sums of money for the account of Debtor
as Secured Party in its sole discretion deems necessary for any such purpose.

         8.8 Secured Party's Expenses. All reasonable advances, costs, expenses,
charges and attorneys' fees which Secured Party may make, pay or incur under any
provision  of this  Agreement  for the  protection  of its  security  or for the
enforcement of any of its rights hereunder,  including,  without limitation,  in
foreclosure  proceedings commenced and subsequently  abandoned or in any dispute
or litigation in which Secured Party or the holder of any of the Obligations may
become involved by reason of or arising out of the Purchase Agreement, any Other
Agreement or the  Collateral,  shall be a part of the  Obligations  and shall be
paid by Debtor to Secured Party, upon demand, and shall bear interest until paid
at the maximum  rate of interest  permitted  by  applicable  law,  from the date
incurred by Secured Party until repaid by Debtor.

         8.9. Convertible  Collateral.  Secured Party may present for conversion
any  Collateral  which is  convertible  into any other  instrument or investment
security or a  combination  thereof with cash,  but Secured Party shall not have
any duty to present for conversion any Collateral  unless it shall have received
from Debtor  detailed  written  instructions to that effect at a time reasonably
far in advance of the final conversion date to make such conversion possible.

         8.10 Secured Party's Right of Set-Off.  Upon the happening of any event
entitling  Secured  Party to pursue any remedy  provided  herein,  or if Secured
Party shall be served with garnishment process in which Debtor shall be named as
defendant,  whether or not Debtor  shall be in  default  hereunder  at the time,
Secured Party may, but shall not be required to, set-off any indebtedness  owing
by  Secured  Party  to  Debtor  against  any of the  Obligations  without  first
resorting to the security hereunder and without prejudice to any other rights or
remedies of Secured Party or its Security Interest.

         8.11 Remedies.  No right or remedy herein  reserved to Secured Party is
intended to be exclusive  of any other right or remedy,  but each and every such
remedy shall be cumulative,  not in lieu of, but in addition to any other rights
or remedies given under this Agreement and all other security documents. Any and
all of Secured  Party's  rights and remedies may be exercised  from time to time
and as often as such exercise as deemed necessary or desirable by Secured Party.

         8.12 Debtor's Waivers.  Debtor waives notice of the creation,  advance,
increase,  existence,  extension,  or  renewal  of, and of any  indulgence  with
respect to, the  Obligations;  waives notice of intent to accelerate,  notice of
acceleration,  notice  of  intent  to  demand,  presentment,  demand,  notice of
dishonor,  and  protest;   waives  notice  of  the  amount  of  the  Obligations
outstanding  at any time,  notice of any change in  financial  condition  of any
person liable for the  Obligations  or any part thereof,  notice of any Event of
Default,  and all other  notices  respecting  the  Obligations;  and agrees that
maturity of the Obligations  and any part thereof may be accelerated,  extended,
or renewed one or more times by Secured Party in its discretion,  without notice
to Debtor.

         8.13 Other Parties and Other Collateral.  No renewal or extension of or
any other  indulgence  with respect to the  Obligations or any part thereof,  no
release  of any  security,  no  release  of any  person  (including  any  maker,
endorser,  guarantor,  or  surety)  liable  on  the  Obligations,  no  delay  in
enforcement  of payment,  and no delay or admission or lack of diligence or care
in exercising any right or power with respect to the Obligations or any security
therefor  or  guaranty  thereof or under this  Agreement  shall in other  manner
impair  or affect  the  rights  of  Secured  Party  under  the law,  under  this
Agreement,  or under any other  document or  agreement  pertaining  to the other
security for the  Obligations,  before  foreclosing  upon the Collateral for the
purpose of paying the Obligations.  Debtor waives any right to the benefit of or
to require or control application of any other security or proceeds thereof, and
Debtor  agrees that Secured  Party shall have no duty or obligation to Debtor to
apply to the Obligations any such other security or proceeds thereof.

         8.14 Relationship Among Secured Party.  Either or both of Secured Party
are  entitled  to enforce  any and all rights  granted to Secured  Party in this
Agreement,  and to take any other  action  allowed to be taken by Secured  Party
under this  Agreement.  Neither  Secured  Party is under any  obligation  to act
jointly or in concert with the other Secured Party  pursuant to this  Agreement.
Any action  required  to be taken by Debtor,  or notice  required to be given by
Debtor, shall be taken or given with respect to both of Secured Party.

                                   ARTICLE IX

                                  MISCELLANEOUS

     9.1 Terms  Commercially  Reasonable.  The terms of this Agreement  shall be
deemed commercially reasonable within the meaning of the Texas UCC.

         9.2 Notices.  Any notices or demands  required or permitted to be given
hereunder  shall be  deemed  sufficiently  given if in  writing  and  personally
delivered or mailed (with all postage and charges prepaid), addressed to Secured
Party or to Debtor their respective  addresses set forth below, or at such other
address as the above parties may from time to time  designate by written  notice
to the other given in  accordance  with this Section  9.2.  Any such notice,  if
personally  delivered or  transmitted  by telex or telegram,  shall be deemed to
have been given on the date so delivered or transmitted or, if mailed, be deemed
to have been given on the day after such  notice is placed in the United  States
mail in accordance with this Section 9.2.

                  Secured Party:  1301 Capital of Texas Hwy., Suite C-300
                                  Austin, Travis County, Texas 78746
                                  Attn: President

                  with copy to:   Timothy L. LaFrey, Esq.
                                  Akin, Gump, Strauss, Hauer & Feld, L.L.P.
                                  1900 Frost Bank Plaza
                                  816 Congress Avenue
                                  Austin, Texas 78701

                  Debtor:         Mark R. Mandel, M.D., Trustee under Trust
                                  Agreement dated April 12, 1989
                                  680 Brewer Road
                                  Hillsborough, California   94010

         9.3 Parties Bound.  Secured Party's rights under this Agreement and the
Security Interest shall inure to the benefits of its successors and assigns, and
in the event of any  assignment  or  transfer of any of the  Obligations  or the
Collateral,  Secured  Party  thereafter  shall  be  fully  discharged  from  any
responsibility  with respect to the Collateral so assigned or  transferred,  but
Secured  Party shall  retain all rights and powers  hereby given with respect to
any of the  Obligations  or  Collateral  not so  assigned  or  transferred.  All
representations, warranties, and agreements of Debtor if more than one are joint
and several, and all shall be binding upon the personal representatives,  heirs,
successors, and assigns of Debtor.

         9.4 Waiver.  No delay of Secured Party in exercising any power or right
shall operate as a waiver thereof;  nor shall any single or partial  exercise of
any power or right preclude other or further exercise thereof or the exercise of
any other power or right.  No waiver by Secured Party of any right  hereunder of
any default by Debtor shall be binding upon Secured Party unless in writing, and
no failure by Secured  Party to exercise any power or right  hereunder or waiver
of any  default  by Debtor  shall  operate  as a waiver of any other or  further
exercise of such right or power of any further default.

         9.5 Agreement Continuing.  This Agreement shall constitute a continuing
agreement,  applying to all future as well as existing transactions,  whether or
not of the  character  contemplated  at the date of this  Agreement,  and if all
transactions between Secured Party and Debtor shall be closed at any time, shall
be equally  applicable to any new  transactions  thereafter.  Provisions of this
Agreement,  unless  by their  terms  exclusive,  shall be in  addition  to those
contained in any Other Agreement.

         9.6 Definitions. Unless the context indicated otherwise, definitions in
the Texas Business and Commerce Code ("Texas UCC") apply to words and phrases in
this Agreement; if Texas UCC definitions conflict, Chapter 9 definitions apply.

         9.7 Miscellaneous. In this Agreement, whenever the context so requires,
the neuter gender  includes the masculine and feminine,  and the singular number
includes  the plural and vice  versa.  The  headings  of  paragraphs  herein are
inserted only for convenience and shall in no way define,  describe or limit the
scope of intent of any  provisions  of this  Agreement.  No  change,  amendment,
modification,  cancellation,  or  discharge of any  provision of this  Agreement
shall be valid unless consented to in writing by Secured Party.

         9.8 Assignment of Secured  Party's  Interest.  Secured Party shall have
the right to assign all or any portion of its rights in this  Agreement  without
approval or consent.  Debtor may not assign this  Agreement or any of its rights
or obligations  hereunder  without the express prior written  consent of Secured
Party in each instance.

     9.9 Applicable  Laws.  THIS AGREEMENT SHALL BE GOVERNED BY AND CONSTRUED IN
ACCORDANCE  WITH THE LAWS OF THE STATE OF TEXAS AND THE  APPLICABLE  LAWS OF THE
UNITED STATES OF AMERICA.

     9.10 ENTIRE AGREEMENT.  THIS AGREEMENT AND THE PURCHASE AGREEMENT REPRESENT
THE FINAL AGREEMENT  BETWEEN THE PARTIES AND MAY NOT BE CONTRADICTED BY EVIDENCE
OF PRIOR,  CONTEMPORANEOUS,  OR SUBSEQUENT ORAL AGREEMENTS OF THE PARTIES. THERE
ARE NO UNWRITTEN ORAL AGREEMENTS BETWEEN THE PARTIES.

                            [Signature page follows]


<PAGE>


S-1



                                SIGNATURE PAGE TO

                                 ASSIGNMENT AND

                               SECURITY AGREEMENT

         EXECUTED this 1st day of September, 1999.



DEBTOR:                                   /s/ Mark R. Mandel, M.D.

                                          Printed Name: Mark R. Mandel, M.D.,
                                          under Trustee Agreement dated
                                          April 12, 1989

SECURED PARTY:                            Prime Medical Operating, Inc.

                                          By: /s/ Cheryl Williams

                                          Name: Cheryl Williams

                                          Title: Vice President


                                          Prime/BDR Acquisition, L.L.C.

                                          By: /s/ Cheryl Williams

                                          Name: Cheryl Williams

                                          Title: Vice President

<PAGE>



                        ASSIGNMENT AND SECURITY AGREEMENT

     THIS  ASSIGNMENT  AND SECURITY  AGREEMENT  (this  "Agreement")  is made and
entered into as of the 1st day of September,  1999, by and between Prime Medical
Operating, Inc., a Delaware corporation ("PMOI"), Prime/BDR Acquisition, L.L.C.,
a Delaware limited  liability  company ("Prime") (PMOI and Prime are referred to
herein,  jointly  and  severally,  as  "Secured  Party")  and D.  Brent Reed and
Carellyn S. Reed (collectively and individually referred to as the "Debtor").

                                    RECITALS:

         A. Debtor and Secured Party have  executed and  delivered  that certain
Stock  Purchase   Agreement  dated  as  of  September  1,  1999  (the  "Purchase
Agreement"),  pursuant to which  Secured  Party  purchased  from Debtor  certain
shares of the $0.01 par value  common  stock of Horizon  Vision  Center,  Inc, a
Nevada corporation ("Horizon").

         B. Secured Party has requested  that Debtor pledge the  Collateral  (as
defined below) to secure certain obligations and liabilities that Debtor may now
or hereafter have to Secured Party, including, without limitation, any indemnity
obligations arising under the Purchase Agreement.

     C. Debtor desires to enter into this Agreement as a material  inducement to
Secured  Party's  purchase  of  Debtor's  shares of Horizon  under the  Purchase
Agreement.

                                   AGREEMENT:

         NOW, THEREFORE, in consideration of the foregoing and the covenants and
agreements hereinafter set forth, and other good and valuable consideration, the
receipt and sufficiency of which Debtor  acknowledges,  Debtor and Secured Party
agree as follows:

                                    ARTICLE I

                       COLLATERAL AND SECURED OBLIGATIONS

         1.1 Grant of Security Interest.  Debtor hereby assigns,  transfers, and
pledges to Secured  Party,  and Debtor hereby grants to Secured Party a security
interest   in,   the   following   described   collateral   (collectively,   the
"Collateral"):

                  (a)  Shares  of  Horizon.  From  and  after  the  date of this
Agreement,  (i) all shares of the common  stock of Horizon  owned or acquired in
any manner by Debtor or Secured Party,  (collectively,  the "Shares"),  (ii) any
replacements,  substitutions,  or exchanges of the certificates representing the
Shares, and (iii) any and all options,  rescission rights,  registration rights,
conversion rights, subscription rights, contractual or quasi-contractual rights,
warrants,  redemption rights,  redemption proceeds, calls, preemptive rights and
all other rights and benefits pertaining to the Shares;

                  (b)  Accounts.  All  accounts  and  rights  now  or  hereafter
attributable to any of the Collateral  described in (a) above, and all rights of
Debtor now or hereafter arising under any agreement pertaining to the Collateral
described  in  (a)  above,   including  without  limitation  all  distributions,
proceeds, fees, dividends,  preferences,  payments or other benefits of whatever
nature  which  Debtor is now or may  hereafter  become  entitled to receive with
respect to any Collateral described in (a) above;

                  (c) Additional  Property.  "Collateral" shall also include the
following  property  (collectively,  the  "Additional  Property")  which  Debtor
becomes  entitled  to receive or shall  receive  in  connection  with any of the
Collateral  described in this Section 1.1: (i) any stock certificate,  including
without  limitation,  any  certificate  representing  a  stock  dividend  or any
certificate in connection with any recapitalization,  reclassification,  merger,
consolidation,  conversion,  sale of assets, combination of shares, stock split,
reverse  stock split or  spin-off;  (ii) any option,  warrant,  subscription  or
right,  whether as an addition to or in  substitution  of any of the  Collateral
described in this Section 1.1; (iii) any dividends or  distributions of any kind
whatsoever,  whether  distributable in cash,  stock or other property;  (iv) any
interest,  premium or principal  payments;  and (v) any conversion or redemption
proceeds; and

                  (d) Proceeds.  All proceeds (cash and non-cash) arising out of
the sale, exchange, collection or other disposition of all or any portion of the
Collateral  described in (a), (b), or (c) above,  including  without  limitation
proceeds in the form of stock, accounts, chattel paper, instruments,  documents,
goods, inventory and equipment.

The  security  interest in the  Collateral  hereby  granted by Debtor to Secured
Party may sometimes be referred to in this Agreement as the "Security Interest".

         1.2 Obligations.  This Agreement and the Security Interest shall secure
full and punctual payment and performance of the following indebtedness,  duties
and obligations (collectively, the "Obligations"):

                  (a) All liabilities and obligations of Debtor to Secured Party
(including, without limitation, any principal, interest, fees and other amounts,
and any  indemnity  obligations)  under and pursuant to the Purchase  Agreement,
this Agreement and/or any other contract or agreement  between Secured Party and
Debtor or any affiliate of Debtor (collectively, "Other Agreements"; and

                  (b) (i)  all  indebtedness,  obligations  and  liabilities  of
Debtor and/or any affiliate of Debtor to Secured Party of any kind or character,
now existing or hereafter arising, whether direct, indirect, related, unrelated,
fixed,  contingent,  liquidated,  unliquidated,  joint,  several  or  joint  and
several,  arising from,  connected with, or related to the Purchase Agreement or
any Other Agreement, or any other document, agreement, or instrument executed in
connection  either of them,  (ii) all accrued but unpaid  interest on any of the
indebtedness  described in (i) above, (iii) all obligations of Debtor and/or any
affiliate  of  Debtor  to  Secured  Party  under  any  documents  or  agreements
evidencing,  securing,  governing  and/or  pertaining  to all or any part of the
indebtedness  described  in (i) and (ii)  above,  (iv) all  costs  and  expenses
incurred by Secured Party in connection  with the collection and  administration
of all or any part of the  indebtedness  and obligations  described in (i), (ii)
and (iii) above or the protection or preservation  of, or realization  upon, the
collateral  securing  all or any  part of  such  indebtedness  and  obligations,
including  without  limitation  all  attorneys'  fees,  and  (v)  all  renewals,
extensions, modifications and rearrangements of the indebtedness and obligations
described in (i), (ii), (iii) and (iv) above.

                  (c) All sums now or  hereafter  loaned or  advanced by Secured
Party to  Debtor,  or  expended  by Secured  Party for the  account of Debtor or
otherwise owing by Debtor to Secured Party, in respect of the  Obligations,  and
all other sums  expended or advanced  by Secured  Party  pursuant to any term or
provision of this Agreement or any Other Agreement (i) to collect and/or enforce
the Obligations or (ii) to maintain, protect and preserve the Collateral.

                                   ARTICLE II

       DEBTOR'S REPRESENTATIONS AND WARRANTIES WITH RESPECT TO COLLATERAL

       Debtor hereby represents and warrants to Secured Party as follows:

         2.1 Ownership of Collateral.  Debtor has good and  marketable  title to
the Collateral  free and clear of any liens,  security  interests,  shareholders
agreement, calls, charge, or encumbrance,  except for this Security Interest. No
financing  statement or other  instrument  similar in effect covering all or any
part of the  Collateral is on file in any recording  office,  except as may have
been filed in favor of Secured Party relating to this Agreement.

         2.2  Power  &  Authority.  Debtor  has the  lawful  right,  power,  and
authority  to grant the Security  Interest in the  Collateral.  This  Agreement,
together  with all filings and other  actions  necessary or desirable to perfect
and protect such security interest,  which have been duly taken,  create a valid
and perfected first priority  security  interest in the Collateral  securing the
payment and performance of the Obligations.

         2.3  No  Agreements.  The  Shares  are  not  subject  to any  right  of
redemption,  or any  call or put  options,  voting  trust,  proxy,  shareholders
agreement,  right of first  refusal,  or any other  document or agreement  which
would in any way impair or adversely affect this Security Interest or the rights
of Secured Party under this Agreement.

         2.4  Securities.  Any  certificates  evidencing  securities  pledged as
Collateral  are valid and  genuine  and have not been  altered.  All  securities
pledged as Collateral have been duly  authorized and validly  issued,  are fully
paid and  non-assessable,  and were not issued in  violation  of the  preemptive
rights of any party or of any agreement by which Debtor or the issuer thereof is
bound.  No  restrictions  or  conditions  exist with  respect to the transfer or
voting of any securities pledged as Collateral.

                                   ARTICLE III

                  DEBTOR'S OTHER REPRESENTATIONS AND WARRANTIES

         3.1 Solvency of Debtor.  As of the date hereof,  (i) Debtor is solvent;
(ii) the fair saleable  value of Debtor's  assets exceeds  Debtor's  liabilities
(both fixed and contingent);  (iii) Debtor has sufficient capital to satisfy all
of Debtor's  obligations  as they  become due;  (iv) no  receiver,  trustee,  or
custodian has been appointed for, or taken  possession of, all or  substantially
all of the assets of Debtor,  either in a  proceeding  brought by Debtor or in a
proceeding  brought against Debtor;  (v) Debtor is not the subject of a petition
for relief under the United  States  Bankruptcy  Code or any similar  federal or
state insolvency law, including without limitation a petition filed by Debtor or
a petition filed by a third party seeking relief against Debtor; and (vi) Debtor
has no  intention  of filing a  petition  for  relief  under the  United  States
Bankruptcy  Code or any similar  federal or state  insolvency law, or of seeking
any other form of  creditor  relief,  within  the  two-year  period  immediately
following the date of this Agreement.

         3.2  Authority and  Compliance.  Debtor has full power and authority to
enter into this Agreement. Debtor has full power and authority to enter into and
perform its obligations  under the Purchase  Agreement and each Other Agreement.
No further  consent or approval is  required as a condition  to the  validity of
this  Agreement,  the Purchase  Agreement or any Other  Agreement.  Debtor is in
compliance with all applicable laws, ordinances,  statutes, orders, regulations,
judgments, writs, or decrees of any governmental entity to which it is subject.

         3.3 Binding Agreement.  This Agreement, the Purchase Agreement and each
Other Agreement  constitute valid and legally binding  obligations of Debtor, in
accordance  with  their  terms,  subject  to  (a)  the  applicable   bankruptcy,
insolvency,  reorganization,  moratorium,  and similar laws affecting creditors'
rights  generally  and  (b)  restrictions  imposed  by any  court  of  competent
jurisdiction on the enforcement of non-competition and exclusive use restrictive
covenants imposed under the Purchase Agreement or any Other Agreement.

         3.4 Litigation.  There are no proceedings  pending or, to the knowledge
of Debtor,  threatened before any court or  administrative  agency which will or
may have a material adverse effect on the financial  condition of Debtor or upon
Debtor's ability to perform its obligations  under this Agreement,  the Purchase
Agreement or any Other Agreement.

         3.5 No Conflicting Agreements.  There are no provisions of any existing
agreement,  mortgage,  indenture or contract  binding on Debtor or affecting its
property,  which  would  conflict  with or in any  way  prevent  the  execution,
delivery, or carrying out of the terms of this Agreement, the Purchase Agreement
or any Other Agreement.

         3.6  Ownership  of  Assets.  Debtor  has  good  and  full  title to the
Collateral,  and the  Collateral  is owned  free and  clear of  liens,  charges,
claims, security interests, and other encumbrances.

     3.7 Taxes. Debtor has filed all tax returns required to be filed by Debtor.

                                   ARTICLE IV

                  DEBTOR'S COVENANTS WITH RESPECT TO COLLATERAL

         Debtor  covenants  and agrees  that from the date  hereof and until the
payment  and  performance  in  full  of the  Obligations  unless  Secured  Party
otherwise consents in writing:

         4.1  Delivery of  Instruments  and/or  Certificates.  Contemporaneously
herewith,   Debtor  covenants  and  agrees  to  deliver  to  Secured  Party  any
certificates,   documents,   or  instruments   representing  or  evidencing  the
Collateral,  with Debtor's  endorsement  thereon and/or  accompanied by property
instruments of transfer and assignment duly executed in blank with, if requested
by Secured Party,  signatures  guaranteed by a member or member  organization in
good standing of an authorized Securities Transfer Agents Medallion Program, all
in form and substance satisfactory to Secured Party.

         4.2  Further  Assurances.   Debtor  will   contemporaneously  with  the
execution  hereof  and from  time to time  thereafter  at its  expense  promptly
execute and deliver all further  instruments  and documents and take all further
action  necessary or  appropriate or that Secured Party may request in order (i)
to perfect and protect the security  interest created or purported to be created
hereby and the first priority of such security interest,  (ii) to enable Secured
Party to exercise  and enforce its rights and  remedies  hereunder in respect of
the  Collateral,  and (iii) to otherwise  effect the purposes of this Agreement,
including  without  limitation:  (A)  executing  and  filing  any  financing  or
continuation  statements,  or any  amendments  thereto;  (B)  obtaining  written
confirmation  from the issuer of any  securities  pledged as  Collateral  of the
pledge of such securities,  in form and substance satisfactory to Secured Party;
(C)  cooperating  with Secured Party in registering the pledge of any securities
pledged as Collateral with the issuer of such securities;  (D) delivering notice
of Secured Party's security interest in any securities  pledged as Collateral to
any securities or financial  intermediary,  clearing  corporation or other party
required by Secured Party, in form and substance  satisfactory to Secured Party;
and  (E)  obtaining  written  confirmation  of  the  pledge  of  any  securities
constituting Collateral from any securities or financial intermediary,  clearing
corporation  or other party  required by Secured  Party,  in form and  substance
satisfactory to Secured Party.

         4.3 Additional Property. All Additional Property, as defined in Section
1.1(c)  above,  received by Debtor shall be received in trust for the benefit of
Secured Party.  All Additional  Property and all  certificates  or other written
instruments or documents  evidencing and/or representing the Additional Property
that is  received  by Debtor,  together  with such  instruments  of  transfer as
Secured Party may request,  shall  immediately be delivered to or deposited with
Secured Party and held by Secured  Party as  Collateral  under the terms of this
Agreement.  If the  Additional  Property  received  by Debtor and  delivered  to
Secured  Party  pursuant  to this  Section  shall  be  shares  of stock or other
securities,  such shares of stock or other  securities shall be duly endorsed in
blank or  accompanied  by proper  instruments  of transfer and  assignment  duly
executed in blank with, if requested by Secured Party,  signatures guaranteed by
a member or member  organization  in good standing of an  authorized  Securities
Transfer Agents  Medallion  Program,  all in form and substance  satisfactory to
Secured  Party.  Secured  Party  shall  be  deemed  to  have  possession  of any
Collateral in transit to Secured Party or its agent.

         4.4  Sale,  Transfer,  Encumbrance.  Debtor  will not  sell,  transfer,
mortgage,  or otherwise  encumber any  Collateral or impair the value thereof in
any manner without  Secured  Party's prior written  consent,  including  without
limitation by purchase, lease, barter, trade, payment deferral, or the creation,
assumption  or guarantee of  indebtedness  or other  lending of credit.  Secured
Party's written consent to any sale,  mortgage,  transfer,  or encumbrance shall
not be construed to be a waiver of this  provision in respect to any  subsequent
proposed sale, mortgage, transfer, or encumbrance.

         4.5 Liens. Neither Debtor nor any person acting on Debtor's behalf has,
or shall have any right, power, or authority to and shall not create,  incur, or
permit to be placed or  imposed,  upon the  Collateral,  any lien of any type or
nature whatsoever, other than the liens in favor of Secured Party.

         4.6 Matters or  Occurrences  Affecting  Collateral  or this  Agreement.
Debtor will promptly  notify Secured Party of any and all matters or occurrences
that may have a material adverse effect on the status or value of the Collateral
or this Agreement,  including  without  limitation the occurrence of an Event of
Default,  or an event  which,  with giving of notice or lapse of time,  or both,
would constitute an Event of Default.

     4.7  Agreements  Pertaining to  Collateral.  Debtor will not enter into any
type of contract or agreement  pertaining to any of the Collateral or in any way
transfer any voting rights pertaining to the Collateral to any person or entity.

         4.8 Dilution of Ownership.  As to any securities pledged as Collateral,
Debtor  will not  consent to or approve of the  issuance  of (i) any  additional
shares  of  any  class  of  securities  of  such  issuer,  (ii)  any  instrument
convertible  voluntarily  by  the  holder  thereof  or  automatically  upon  the
occurrence or  non-occurrence  of any event or condition  into, or  exchangeable
for, any such  securities,  or (iii) any warrants,  options,  contracts or other
commitments  entitling any third party to purchase or otherwise acquire any such
securities.  Notwithstanding  the  foregoing  or any  other  provision  of  this
Agreement to the contrary,  Debtor (i) shall comply with his  obligations  under
the  Purchase  Agreement  and each Other  Document,  and (ii) may consent to any
issuance of shares of Horizon if such  issuance has been  approved by a majority
of the Board of Directors of Horizon.

         4.9  Restrictions  on  Securities.  Debtor  will  not  enter  into  any
agreement  creating,  or otherwise permit to exist, any restriction or condition
upon the transfer,  voting or control of any  securities  pledged as Collateral,
except as consented to in writing by Secured Party. As to any securities pledged
as collateral, Debtor will not consent to or approve of any stock split, reverse
stock  split,  stock  dividend,  reclassification,   or  other  similar  act  or
transaction  regarding  the  Shares  unless  consented  to in writing by Secured
Party.

                                    ARTICLE V

                         DEBTOR'S AFFIRMATIVE COVENANTS

         Until payment and performance of all Obligations,  Debtor covenants and
agrees that it shall promptly  advise Secured Party in writing of any litigation
filed  against  Debtor  and of any  condition,  event or act which  comes to its
attention  that  would or might  have a  material  adverse  effect  on  Debtor's
financial condition or on Debtor's ability to perform the Obligations.

                                   ARTICLE VI

                               NEGATIVE COVENANTS

         Until payment and performance of all Obligations,  Debtor covenants and
agrees that Debtor will not, without the prior written consent of Secured Party:

     6.1 Liens. Grant, suffer, or permit liens on, or security interests in, the
Collateral.

         6.2  Violate  Other  Covenants.  Violate  or fail to  comply  with  any
covenants  or  agreements  regarding  other  debt  which  will or would with the
passage  of time or upon  demand  cause the  maturity  of any  other  debt to be
accelerated.

                                   ARTICLE VII

                              DEFAULT AND REMEDIES

     7.1 Events of Default.  An Event of Default  (herein so called) shall exist
if any one or more of the following events shall occur:

                  (a) The  failure  of Debtor to pay any amount  required  to be
paid under the Purchase Agreement (including,  without limitation, the indemnity
provisions  contained  therein),  or any other  amount  which  Debtor may now or
hereafter  owe to Secured Party under any Other  Agreement or otherwise,  within
(15) calendar days after such amount is due;

     (b) The failure of Debtor to pay any  Obligation  within (15) calendar days
after such amount is due; and

                  (c)  Debtor's  breach of a covenant in this  Agreement  or any
other  failure to perform  its  obligations  under this  Agreement  or any Other
Agreement.

     7.2 Secured Party's Remedies. Upon the occurrence of an Event of Default:

                  (a) Secured Party may declare the Obligations in whole or part
immediately due and may enforce payment and performance of the same and exercise
any rights under the Texas UCC,  rights and remedies of Secured Party under this
Agreement, or otherwise.

                  (b) Secured  Party may, at Secured  Party's  option and at the
expense of Debtor,  either in Secured Party's own right or in the name of Debtor
and in the same manner and to the same extent that Debtor  might  reasonably  so
act  if  this  Agreement  had  not  been  made:  (i) do  all  things  requisite,
convenient,  or  necessary  to enforce the  performance  and  observance  of all
rights,  remedies and privileges of Debtor arising from the  Collateral,  or any
part thereof,  including without limitation compromising,  waiving, excusing, or
in any manner releasing or discharging any obligation of any party to or arising
from the Collateral;  (ii) take possession of the books, papers,  chattel paper,
documents of title, and accounts of Debtor,  wherever  located,  relating to the
Collateral; (iii) sue or otherwise collect and receive money attributable to the
Collateral;  and (iv) exercise any other lawfully  available powers or remedies,
and do all other things  which  Secured  Party deems  requisite,  convenient  or
necessary  or which the  Secured  Party  deems  proper to protect  the  Security
Interest.

                  (b) Secured Party may foreclose  this  Agreement in the manner
now or  hereafter  provided  or  permitted  by law and may upon such  reasonable
notification  prior thereto as may be required by applicable  law (Debtor hereby
agreeing  that ten  days'  notice is  commercially  reasonable),  sell,  assign,
transfer,  or otherwise  dispose of the Collateral at public or private sale, in
whole  or in  part,  and  Secured  Party  may,  in its own  name or as  Debtor's
attorney-in-fact  effectively  assign and transfer the  Collateral,  or any part
thereof,   absolutely,  and  execute  and  deliver  all  necessary  assignments,
conveyances,  bills of sale, and other  instruments with power to substitute one
or more persons or  corporations  with like power.  Any such  foreclosure  sale,
assignment,  transfer,  or other  disposition  shall, to the extent permitted by
law,  be a  perpetual  bar,  both at law and in equity,  against  Debtor and all
persons and corporations  lawfully  claiming by or through or under Debtor.  Any
such foreclosure sale may be adjourned from time to time. Upon any sale, Secured
Party may bid for and purchase the  Collateral,  or any part  thereof,  and upon
compliance with the terms of sale may hold,  retain,  possess and dispose of the
Collateral, in its absolute right without further accountability.  Secured Party
shall have the right to be  credited  on the  amount of its bid a  corresponding
amount of the Obligations as of the date of such sale.

                  (c) If, in the opinion of Secured Party, there is any question
that a public sale or  distribution  of any Collateral will violate any state or
federal  securities  law,  Secured  Party  (i) may  offer  and  sell  securities
privately to purchasers who will agree to take them for investment  purposes and
not with a view to distribution  and who will agree to imposition of restrictive
legends on the  certificates  representing  the security,  or (ii) may sell such
securities in an intrastate  offering  under Section  3(a)(11) of the Securities
Act of 1933,  and no sale so made in good faith by Secured Party shall be deemed
to be not "commercially reasonable" because so made.

                  (d)  Not  in  limitation  of  any  other   provision  of  this
Agreement,  Secured  Party shall have all rights and remedies of a secured party
under the Texas UCC.

         7.3  Application  of Proceeds.  Secured Party may apply the proceeds of
any foreclosure  sale hereunder or from any other  permitted  disposition of the
Collateral  or any part  thereof as  follows:  (a) first,  to the payment of all
reasonable  costs and expenses of any foreclosure  and collection  hereunder and
all proceedings in connection  therewith,  including reasonable attorneys' fees;
(b) then, to the  reimbursement of Secured Party for all  disbursements  made by
Secured Party for taxes,  assessments or liens superior to the Security Interest
and  which  Secured  Party  shall  deem  expedient  to  pay;  (c)  then,  to the
reimbursement of Secured Party of any other  disbursements made by Secured Party
in accordance with the terms hereof or under the Purchase Agreement or any Other
Agreement;  (d) then,  to or among the amounts of fees,  interest and  principal
then owing and unpaid in respect of the Obligations, in such priority as Secured
Party may determine in its  discretion;  and (e) the remainder of such proceeds,
if any,  shall be paid to Debtor.  If such  proceeds  shall be  insufficient  to
discharge the entire  Obligations,  Secured Party shall have any other available
legal recourse  against Debtor under,  or for the  performance  of, the Purchase
Agreement and any Other  Agreement  between  Debtor and Secured  Party,  for the
deficiency,  together with interest  thereon at the maximum rate permitted under
applicable law.

         7.4  Enforcement  of  Obligations.  Nothing in this Agreement or in any
other  document  or  agreement  shall  affect or impair  the  unconditional  and
absolute right of Secured Party to enforce the  Obligations as and when the same
shall become due in accordance with the terms of the Purchase Agreement or Other
Agreement.

                                  ARTICLE VIII

                             RIGHTS OF SECURED PARTY

         8.1  Subrogation.  Upon the occurrence of an Event of Default,  Secured
Party,  at its  election,  may  subrogate  to all of the  interest,  rights  and
remedies  of the  Debtor,  in respect  to any of the  Collateral  or  agreements
pertaining thereto.

         8.2 Secured Party  Appointed  Attorney-in-Fact.  Debtor hereby appoints
Secured Party as  attorney-in-fact  of Debtor,  with full authority in the place
and stead of Debtor and in the name of Debtor, Secured Party or otherwise,  from
time to time on Secured  Party's  discretion and upon the occurrence of an Event
of Default, to take any action and to execute any instrument which Secured Party
may deem  necessary or advisable to accomplish  the purposes of this  Agreement,
including without  limitation:  (a) to ask, demand,  collect,  sue for, recover,
compound, receive and give acquittance and receipts for moneys due and to become
due under or in respect of any of the Collateral;  (b) to receive,  endorse, and
collect  any  drafts or other  instruments,  documents  and  chattel  paper,  in
connection  with clause (a) of this  Section 8.2; (c) to file any claims or take
any action or institute any proceeding which Secured Party may deem necessary or
desirable for the  collection  of any of the  Collateral or otherwise to enforce
the rights of Secured Party against any of the Collateral; and (d) to assign and
transfer the  Collateral,  or any part  thereof,  absolutely  and to execute and
deliver  endorsements,   assignments,  conveyances,  bills  of  sale  and  other
instruments  with power to substitute  one or more persons or  corporation  with
like power.

         8.3  Performance  by Secured  Party.  If Debtor  fails to  perform  any
agreement  contained  herein,  Secured  Party may itself  perform,  or cause the
performance  of, such  agreement,  and the reasonable  expenses of Secured Party
incurred in connection  therewith  shall be payable by Debtor under Section 8.8.
In no  event,  however,  shall  Secured  Party  have any  obligation  or  duties
whatsoever to perform any covenant or agreement of Debtor contained herein,  and
any such performance by Secured Party shall be wholly discretionary with Secured
Party.

         8.4 Duties of Secured  Party.  The powers  conferred upon Secured Party
hereunder  are solely to protect its  interest in the  Collateral  and shall not
impose any duty upon it to exercise any such powers. Except for the safe custody
of any  Collateral  in its  possession  and the  accounting  for money  actually
received by it hereunder,  Secured Party shall have no duty as to any Collateral
or as to the taking of any  necessary  steps to preserve  rights  against  prior
parties or any other rights  pertaining to any Collateral.  Without limiting the
generality of the foregoing,  Secured Party shall not have any obligation,  duty
or  responsibility  to do any of the  following:  (a) ascertain any  maturities,
calls,  conversions,  exchanges,  offers, tenders or similar matters relating to
the  Collateral or informing  Debtor with respect to any such matters;  (b) fix,
preserve  or  exercise  any  right,  privilege  or option  (whether  conversion,
redemption or otherwise) with respect to the Collateral; (c) collect any amounts
payable  in  respect  of the  Collateral;  (d)  sell all or any  portion  of the
Collateral,  for any reason;  or (e) hold the Collateral for or on behalf of any
party other than Debtor.

         8.5 No  Liability  of Secured  Party.  Neither the  acceptance  of this
Agreement by Secured Party,  nor the exercise of any rights hereunder by Secured
Party,  shall be construed in any way as an  assumption  by Secured Party of any
obligations,  responsibilities,  or duties of Debtor arising in connection  with
the  Collateral  assigned  hereunder  or  otherwise  bind  Secured  Party to the
performance of any  obligations  respecting the  Collateral,  it being expressly
understood  that Secured  Party shall not be obligated to perform,  observe,  or
discharge  any  obligation,  responsibility,  duty,  or  liability  of Debtor in
respect of any of the Collateral,  including without limitation  appearing in or
defending any action, expending any money or incurring any expense in connection
therewith.  TO THE FULLEST EXTENT  PERMITTED BY APPLICABLE LAW, DEBTOR SHALL AND
DOES AGREE TO INDEMNIFY,  PROTECT,  DEFEND AND HOLD HARMLESS  SECURED PARTY, ITS
OFFICERS, DIRECTORS, REPRESENTATIVES, AGENTS, EMPLOYEES, LENDERS, SUCCESSORS AND
ASSIGNS,  FROM AND AGAINST ALL  LIABILITIES,  CLAIMS,  DAMAGES,  LOSSES,  FINES,
PENALTIES,  CAUSES OF ACTIONS,  SUITS,  JUDGMENTS AND EXPENSES  (INCLUDING COURT
COSTS,  ATTORNEY'S  FEES  AND  COST OF  INVESTIGATION)  OF ANY  NATURE,  KIND OR
DESCRIPTION  OF ANY PERSON OR ENTITY,  DIRECTLY OR  INDIRECTLY,  ARISING OUT OF,
CAUSED BY OR  RESULTING  FROM (IN WHOLE OR IN PART),  ANY  UNINTENTIONAL  ACT OR
OMISSION (OR  INTENTIONAL  ACT OR OMISSION SO LONG AS SUCH ACT OR OMISSION  DOES
NOT  CONSTITUTE  NEGLIGENCE)  OF SECURED  PARTY (OR  ANYONE  ACTING ON BEHALF OF
SECURED PARTY) IN CONNECTION WITH THE COLLATERAL.  THE FOREGOING INDEMNITY SHALL
SURVIVE THE EXPIRATION OR EARLIER TERMINATION OF THIS AGREEMENT.

         8.6 Right of Secured Party to Defend Action Affecting Security. Secured
Party  may,  at the  expense  of  Debtor,  appear in and  defend  any  action or
proceeding at law or in equity  purporting to affect  Secured  Party's  Security
Interest under this Agreement.

         8.7 Right of  Secured  Party to Prevent  or Remedy  Default.  If Debtor
shall fail to perform any of the covenants,  conditions and agreements  required
to be performed  and  observed by Debtor  under the Purchase  Agreement or Other
Agreement,  or in respect of the Collateral  (subject to any applicable  default
cure  period),  Secured  Party (a) may but shall  not be  obligated  to take any
action Secured Party deems  necessary or desirable to prevent or remedy any such
default by Debtor or otherwise to protect the Security  Interest,  and (b) shall
have the absolute and immediate  right to take  possession of the  Collateral or
any  part  thereof  (to  the  extent  Secured  Party  has not  previously  taken
possession)  to such  extent  and as often  as the  Secured  Party,  in its sole
discretion, deems necessary or desirable in order to prevent or to cure any such
default by Debtor,  or  otherwise  to protect the  security  of this  Agreement.
Secured Party may advance or expend such sums of money for the account of Debtor
as Secured Party in its sole discretion deems necessary for any such purpose.

         8.8 Secured Party's Expenses. All reasonable advances, costs, expenses,
charges and attorneys' fees which Secured Party may make, pay or incur under any
provision  of this  Agreement  for the  protection  of its  security  or for the
enforcement of any of its rights hereunder,  including,  without limitation,  in
foreclosure  proceedings commenced and subsequently  abandoned or in any dispute
or litigation in which Secured Party or the holder of any of the Obligations may
become involved by reason of or arising out of the Purchase Agreement, any Other
Agreement or the  Collateral,  shall be a part of the  Obligations  and shall be
paid by Debtor to Secured Party, upon demand, and shall bear interest until paid
at the maximum  rate of interest  permitted  by  applicable  law,  from the date
incurred by Secured Party until repaid by Debtor.

         8.9. Convertible  Collateral.  Secured Party may present for conversion
any  Collateral  which is  convertible  into any other  instrument or investment
security or a  combination  thereof with cash,  but Secured Party shall not have
any duty to present for conversion any Collateral  unless it shall have received
from Debtor  detailed  written  instructions to that effect at a time reasonably
far in advance of the final conversion date to make such conversion possible.

         8.10 Secured Party's Right of Set-Off.  Upon the happening of any event
entitling  Secured  Party to pursue any remedy  provided  herein,  or if Secured
Party shall be served with garnishment process in which Debtor shall be named as
defendant,  whether or not Debtor  shall be in  default  hereunder  at the time,
Secured Party may, but shall not be required to, set-off any indebtedness  owing
by  Secured  Party  to  Debtor  against  any of the  Obligations  without  first
resorting to the security hereunder and without prejudice to any other rights or
remedies of Secured Party or its Security Interest.

         8.11 Remedies.  No right or remedy herein  reserved to Secured Party is
intended to be exclusive  of any other right or remedy,  but each and every such
remedy shall be cumulative,  not in lieu of, but in addition to any other rights
or remedies given under this Agreement and all other security documents. Any and
all of Secured  Party's  rights and remedies may be exercised  from time to time
and as often as such exercise as deemed necessary or desirable by Secured Party.

         8.12 Debtor's Waivers.  Debtor waives notice of the creation,  advance,
increase,  existence,  extension,  or  renewal  of, and of any  indulgence  with
respect to, the  Obligations;  waives notice of intent to accelerate,  notice of
acceleration,  notice  of  intent  to  demand,  presentment,  demand,  notice of
dishonor,  and  protest;   waives  notice  of  the  amount  of  the  Obligations
outstanding  at any time,  notice of any change in  financial  condition  of any
person liable for the  Obligations  or any part thereof,  notice of any Event of
Default,  and all other  notices  respecting  the  Obligations;  and agrees that
maturity of the Obligations  and any part thereof may be accelerated,  extended,
or renewed one or more times by Secured Party in its discretion,  without notice
to Debtor.

         8.13 Other Parties and Other Collateral.  No renewal or extension of or
any other  indulgence  with respect to the  Obligations or any part thereof,  no
release  of any  security,  no  release  of any  person  (including  any  maker,
endorser,  guarantor,  or  surety)  liable  on  the  Obligations,  no  delay  in
enforcement  of payment,  and no delay or admission or lack of diligence or care
in exercising any right or power with respect to the Obligations or any security
therefor  or  guaranty  thereof or under this  Agreement  shall in other  manner
impair  or affect  the  rights  of  Secured  Party  under  the law,  under  this
Agreement,  or under any other  document or  agreement  pertaining  to the other
security for the  Obligations,  before  foreclosing  upon the Collateral for the
purpose of paying the Obligations.  Debtor waives any right to the benefit of or
to require or control application of any other security or proceeds thereof, and
Debtor  agrees that Secured  Party shall have no duty or obligation to Debtor to
apply to the Obligations any such other security or proceeds thereof.

         8.14 Relationship Among Secured Party.  Either or both of Secured Party
are  entitled  to enforce  any and all rights  granted to Secured  Party in this
Agreement,  and to take any other  action  allowed to be taken by Secured  Party
under this  Agreement.  Neither  Secured  Party is under any  obligation  to act
jointly or in concert with the other Secured Party  pursuant to this  Agreement.
Any action  required  to be taken by Debtor,  or notice  required to be given by
Debtor, shall be taken or given with respect to both of Secured Party.

                                   ARTICLE IX

                                  MISCELLANEOUS

     9.1 Terms  Commercially  Reasonable.  The terms of this Agreement  shall be
deemed commercially reasonable within the meaning of the Texas UCC.

         9.2 Notices.  Any notices or demands  required or permitted to be given
hereunder  shall be  deemed  sufficiently  given if in  writing  and  personally
delivered or mailed (with all postage and charges prepaid), addressed to Secured
Party or to Debtor their respective  addresses set forth below, or at such other
address as the above parties may from time to time  designate by written  notice
to the other given in  accordance  with this Section  9.2.  Any such notice,  if
personally  delivered or  transmitted  by telex or telegram,  shall be deemed to
have been given on the date so delivered or transmitted or, if mailed, be deemed
to have been given on the day after such  notice is placed in the United  States
mail in accordance with this Section 9.2.

                  Secured Party:  1301 Capital of Texas Hwy., Suite C-300
                                  Austin, Travis County, Texas 78746
                                  Attn: President

                  with copy to:   Timothy L. LaFrey, Esq.
                                  Akin, Gump, Strauss, Hauer & Feld, L.L.P.
                                  1900 Frost Bank Plaza
                                  816 Congress Avenue
                                  Austin, Texas 78701

                  Debtor:         D. Brent Reed and Carellyn S. Reed
                                  157 Cascade Falls Drive
                                  Folsom, California   95630

         9.3 Parties Bound.  Secured Party's rights under this Agreement and the
Security Interest shall inure to the benefits of its successors and assigns, and
in the event of any  assignment  or  transfer of any of the  Obligations  or the
Collateral,  Secured  Party  thereafter  shall  be  fully  discharged  from  any
responsibility  with respect to the Collateral so assigned or  transferred,  but
Secured  Party shall  retain all rights and powers  hereby given with respect to
any of the  Obligations  or  Collateral  not so  assigned  or  transferred.  All
representations, warranties, and agreements of Debtor if more than one are joint
and several, and all shall be binding upon the personal representatives,  heirs,
successors, and assigns of Debtor.

         9.4 Waiver.  No delay of Secured Party in exercising any power or right
shall operate as a waiver thereof;  nor shall any single or partial  exercise of
any power or right preclude other or further exercise thereof or the exercise of
any other power or right.  No waiver by Secured Party of any right  hereunder of
any default by Debtor shall be binding upon Secured Party unless in writing, and
no failure by Secured  Party to exercise any power or right  hereunder or waiver
of any  default  by Debtor  shall  operate  as a waiver of any other or  further
exercise of such right or power of any further default.

         9.5 Agreement Continuing.  This Agreement shall constitute a continuing
agreement,  applying to all future as well as existing transactions,  whether or
not of the  character  contemplated  at the date of this  Agreement,  and if all
transactions between Secured Party and Debtor shall be closed at any time, shall
be equally  applicable to any new  transactions  thereafter.  Provisions of this
Agreement,  unless  by their  terms  exclusive,  shall be in  addition  to those
contained in any Other Agreement.

         9.6 Definitions. Unless the context indicated otherwise, definitions in
the Texas Business and Commerce Code ("Texas UCC") apply to words and phrases in
this Agreement; if Texas UCC definitions conflict, Chapter 9 definitions apply.

         9.7 Miscellaneous. In this Agreement, whenever the context so requires,
the neuter gender  includes the masculine and feminine,  and the singular number
includes  the plural and vice  versa.  The  headings  of  paragraphs  herein are
inserted only for convenience and shall in no way define,  describe or limit the
scope of intent of any  provisions  of this  Agreement.  No  change,  amendment,
modification,  cancellation,  or  discharge of any  provision of this  Agreement
shall be valid unless consented to in writing by Secured Party.

         9.8 Assignment of Secured  Party's  Interest.  Secured Party shall have
the right to assign all or any portion of its rights in this  Agreement  without
approval or consent.  Debtor may not assign this  Agreement or any of its rights
or obligations  hereunder  without the express prior written  consent of Secured
Party in each instance.

     9.9 Applicable  Laws.  THIS AGREEMENT SHALL BE GOVERNED BY AND CONSTRUED IN
ACCORDANCE  WITH THE LAWS OF THE STATE OF TEXAS AND THE  APPLICABLE  LAWS OF THE
UNITED STATES OF AMERICA.

     9.10 ENTIRE AGREEMENT.  THIS AGREEMENT AND THE PURCHASE AGREEMENT REPRESENT
THE FINAL AGREEMENT  BETWEEN THE PARTIES AND MAY NOT BE CONTRADICTED BY EVIDENCE
OF PRIOR,  CONTEMPORANEOUS,  OR SUBSEQUENT ORAL AGREEMENTS OF THE PARTIES. THERE
ARE NO UNWRITTEN ORAL AGREEMENTS BETWEEN THE PARTIES.

                            [Signature page follows]


<PAGE>


S-1



                                SIGNATURE PAGE TO

                                 ASSIGNMENT AND

                               SECURITY AGREEMENT

         EXECUTED this 1st day of September, 1999.



DEBTOR:                                   /s/ D. Brent Reed

                                          Printed Name: D. Brent Reed

                                          /s/ Carellyn S. Reed

                                          Printed Name: Carellyn S. Reed

SECURED PARTY:                            Prime Medical Operating, Inc.

                                          By: /s/ Cheryl Williams

                                          Name: Cheryl Williams

                                          Title: Vice President


                                          Prime/BDR Acquisition, L.L.C.

                                          By: /s/ Cheryl Williams

                                          Name: Cheryl Williams

                                          Title: Vice President

<PAGE>



                        ASSIGNMENT AND SECURITY AGREEMENT

     THIS  ASSIGNMENT  AND SECURITY  AGREEMENT  (this  "Agreement")  is made and
entered into as of the 1st day of September,  1999, by and between Prime Medical
Operating, Inc., a Delaware corporation ("PMOI"), Prime/BDR Acquisition, L.L.C.,
a Delaware limited  liability  company ("Prime") (PMOI and Prime are referred to
herein, jointly and severally,  as "Secured Party") and Bradley J. Sandler, M.D.
(the "Debtor").

                                    RECITALS:

         A. Debtor and Secured Party have  executed and  delivered  that certain
Stock  Purchase   Agreement  dated  as  of  September  1,  1999  (the  "Purchase
Agreement"),  pursuant to which  Secured  Party  purchased  from Debtor  certain
shares of the $0.01 par value  common  stock of Horizon  Vision  Center,  Inc, a
Nevada corporation ("Horizon").

         B. Secured Party has requested  that Debtor pledge the  Collateral  (as
defined below) to secure certain obligations and liabilities that Debtor may now
or hereafter have to Secured Party, including, without limitation, any indemnity
obligations arising under the Purchase Agreement.

     C. Debtor desires to enter into this Agreement as a material  inducement to
Secured  Party's  purchase  of  Debtor's  shares of Horizon  under the  Purchase
Agreement.

                                   AGREEMENT:

         NOW, THEREFORE, in consideration of the foregoing and the covenants and
agreements hereinafter set forth, and other good and valuable consideration, the
receipt and sufficiency of which Debtor  acknowledges,  Debtor and Secured Party
agree as follows:

                                    ARTICLE I

                       COLLATERAL AND SECURED OBLIGATIONS

         1.1 Grant of Security Interest.  Debtor hereby assigns,  transfers, and
pledges to Secured  Party,  and Debtor hereby grants to Secured Party a security
interest   in,   the   following   described   collateral   (collectively,   the
"Collateral"):

                  (a)  Shares  of  Horizon.  From  and  after  the  date of this
Agreement,  (i) all shares of the common  stock of Horizon  owned or acquired in
any manner by Debtor or Secured Party,  (collectively,  the "Shares"),  (ii) any
replacements,  substitutions,  or exchanges of the certificates representing the
Shares, and (iii) any and all options,  rescission rights,  registration rights,
conversion rights, subscription rights, contractual or quasi-contractual rights,
warrants,  redemption rights,  redemption proceeds, calls, preemptive rights and
all other rights and benefits pertaining to the Shares;

                  (b)  Accounts.  All  accounts  and  rights  now  or  hereafter
attributable to any of the Collateral  described in (a) above, and all rights of
Debtor now or hereafter arising under any agreement pertaining to the Collateral
described  in  (a)  above,   including  without  limitation  all  distributions,
proceeds, fees, dividends,  preferences,  payments or other benefits of whatever
nature  which  Debtor is now or may  hereafter  become  entitled to receive with
respect to any Collateral described in (a) above;

                  (c) Additional  Property.  "Collateral" shall also include the
following  property  (collectively,  the  "Additional  Property")  which  Debtor
becomes  entitled  to receive or shall  receive  in  connection  with any of the
Collateral  described in this Section 1.1: (i) any stock certificate,  including
without  limitation,  any  certificate  representing  a  stock  dividend  or any
certificate in connection with any recapitalization,  reclassification,  merger,
consolidation,  conversion,  sale of assets, combination of shares, stock split,
reverse  stock split or  spin-off;  (ii) any option,  warrant,  subscription  or
right,  whether as an addition to or in  substitution  of any of the  Collateral
described in this Section 1.1; (iii) any dividends or  distributions of any kind
whatsoever,  whether  distributable in cash,  stock or other property;  (iv) any
interest,  premium or principal  payments;  and (v) any conversion or redemption
proceeds; and

                  (d) Proceeds.  All proceeds (cash and non-cash) arising out of
the sale, exchange, collection or other disposition of all or any portion of the
Collateral  described in (a), (b), or (c) above,  including  without  limitation
proceeds in the form of stock, accounts, chattel paper, instruments,  documents,
goods, inventory and equipment.

The  security  interest in the  Collateral  hereby  granted by Debtor to Secured
Party may sometimes be referred to in this Agreement as the "Security Interest".

         1.2 Obligations.  This Agreement and the Security Interest shall secure
full and punctual payment and performance of the following indebtedness,  duties
and obligations (collectively, the "Obligations"):

                  (a) All liabilities and obligations of Debtor to Secured Party
(including, without limitation, any principal, interest, fees and other amounts,
and any  indemnity  obligations)  under and pursuant to the Purchase  Agreement,
this Agreement and/or any other contract or agreement  between Secured Party and
Debtor or any affiliate of Debtor (collectively, "Other Agreements"; and

                  (b) (i)  all  indebtedness,  obligations  and  liabilities  of
Debtor and/or any affiliate of Debtor to Secured Party of any kind or character,
now existing or hereafter arising, whether direct, indirect, related, unrelated,
fixed,  contingent,  liquidated,  unliquidated,  joint,  several  or  joint  and
several,  arising from,  connected with, or related to the Purchase Agreement or
any Other Agreement, or any other document, agreement, or instrument executed in
connection  either of them,  (ii) all accrued but unpaid  interest on any of the
indebtedness  described in (i) above, (iii) all obligations of Debtor and/or any
affiliate  of  Debtor  to  Secured  Party  under  any  documents  or  agreements
evidencing,  securing,  governing  and/or  pertaining  to all or any part of the
indebtedness  described  in (i) and (ii)  above,  (iv) all  costs  and  expenses
incurred by Secured Party in connection  with the collection and  administration
of all or any part of the  indebtedness  and obligations  described in (i), (ii)
and (iii) above or the protection or preservation  of, or realization  upon, the
collateral  securing  all or any  part of  such  indebtedness  and  obligations,
including  without  limitation  all  attorneys'  fees,  and  (v)  all  renewals,
extensions, modifications and rearrangements of the indebtedness and obligations
described in (i), (ii), (iii) and (iv) above.

                  (c) All sums now or  hereafter  loaned or  advanced by Secured
Party to  Debtor,  or  expended  by Secured  Party for the  account of Debtor or
otherwise owing by Debtor to Secured Party, in respect of the  Obligations,  and
all other sums  expended or advanced  by Secured  Party  pursuant to any term or
provision of this Agreement or any Other Agreement (i) to collect and/or enforce
the Obligations or (ii) to maintain, protect and preserve the Collateral.

                                   ARTICLE II

       DEBTOR'S REPRESENTATIONS AND WARRANTIES WITH RESPECT TO COLLATERAL

       Debtor hereby represents and warrants to Secured Party as follows:

         2.1 Ownership of Collateral.  Debtor has good and  marketable  title to
the Collateral  free and clear of any liens,  security  interests,  shareholders
agreement, calls, charge, or encumbrance,  except for this Security Interest. No
financing  statement or other  instrument  similar in effect covering all or any
part of the  Collateral is on file in any recording  office,  except as may have
been filed in favor of Secured Party relating to this Agreement.

         2.2  Power  &  Authority.  Debtor  has the  lawful  right,  power,  and
authority  to grant the Security  Interest in the  Collateral.  This  Agreement,
together  with all filings and other  actions  necessary or desirable to perfect
and protect such security interest,  which have been duly taken,  create a valid
and perfected first priority  security  interest in the Collateral  securing the
payment and performance of the Obligations.

         2.3  No  Agreements.  The  Shares  are  not  subject  to any  right  of
redemption,  or any  call or put  options,  voting  trust,  proxy,  shareholders
agreement,  right of first  refusal,  or any other  document or agreement  which
would in any way impair or adversely affect this Security Interest or the rights
of Secured Party under this Agreement.

         2.4  Securities.  Any  certificates  evidencing  securities  pledged as
Collateral  are valid and  genuine  and have not been  altered.  All  securities
pledged as Collateral have been duly  authorized and validly  issued,  are fully
paid and  non-assessable,  and were not issued in  violation  of the  preemptive
rights of any party or of any agreement by which Debtor or the issuer thereof is
bound.  No  restrictions  or  conditions  exist with  respect to the transfer or
voting of any securities pledged as Collateral.

                                   ARTICLE III

                  DEBTOR'S OTHER REPRESENTATIONS AND WARRANTIES

         3.1 Solvency of Debtor.  As of the date hereof,  (i) Debtor is solvent;
(ii) the fair saleable  value of Debtor's  assets exceeds  Debtor's  liabilities
(both fixed and contingent);  (iii) Debtor has sufficient capital to satisfy all
of Debtor's  obligations  as they  become due;  (iv) no  receiver,  trustee,  or
custodian has been appointed for, or taken  possession of, all or  substantially
all of the assets of Debtor,  either in a  proceeding  brought by Debtor or in a
proceeding  brought against Debtor;  (v) Debtor is not the subject of a petition
for relief under the United  States  Bankruptcy  Code or any similar  federal or
state insolvency law, including without limitation a petition filed by Debtor or
a petition filed by a third party seeking relief against Debtor; and (vi) Debtor
has no  intention  of filing a  petition  for  relief  under the  United  States
Bankruptcy  Code or any similar  federal or state  insolvency law, or of seeking
any other form of  creditor  relief,  within  the  two-year  period  immediately
following the date of this Agreement.

         3.2  Authority and  Compliance.  Debtor has full power and authority to
enter into this Agreement. Debtor has full power and authority to enter into and
perform its obligations  under the Purchase  Agreement and each Other Agreement.
No further  consent or approval is  required as a condition  to the  validity of
this  Agreement,  the Purchase  Agreement or any Other  Agreement.  Debtor is in
compliance with all applicable laws, ordinances,  statutes, orders, regulations,
judgments, writs, or decrees of any governmental entity to which it is subject.

         3.3 Binding Agreement.  This Agreement, the Purchase Agreement and each
Other Agreement  constitute valid and legally binding  obligations of Debtor, in
accordance  with  their  terms,  subject  to  (a)  the  applicable   bankruptcy,
insolvency,  reorganization,  moratorium,  and similar laws affecting creditors'
rights  generally  and  (b)  restrictions  imposed  by any  court  of  competent
jurisdiction on the enforcement of non-competition and exclusive use restrictive
covenants imposed under the Purchase Agreement or any Other Agreement.

         3.4 Litigation.  There are no proceedings  pending or, to the knowledge
of Debtor,  threatened before any court or  administrative  agency which will or
may have a material adverse effect on the financial  condition of Debtor or upon
Debtor's ability to perform its obligations  under this Agreement,  the Purchase
Agreement or any Other Agreement.

         3.5 No Conflicting Agreements.  There are no provisions of any existing
agreement,  mortgage,  indenture or contract  binding on Debtor or affecting its
property,  which  would  conflict  with or in any  way  prevent  the  execution,
delivery, or carrying out of the terms of this Agreement, the Purchase Agreement
or any Other Agreement.

         3.6  Ownership  of  Assets.  Debtor  has  good  and  full  title to the
Collateral,  and the  Collateral  is owned  free and  clear of  liens,  charges,
claims, security interests, and other encumbrances.

     3.7 Taxes. Debtor has filed all tax returns required to be filed by Debtor.

                                   ARTICLE IV

                  DEBTOR'S COVENANTS WITH RESPECT TO COLLATERAL

         Debtor  covenants  and agrees  that from the date  hereof and until the
payment  and  performance  in  full  of the  Obligations  unless  Secured  Party
otherwise consents in writing:

         4.1  Delivery of  Instruments  and/or  Certificates.  Contemporaneously
herewith,   Debtor  covenants  and  agrees  to  deliver  to  Secured  Party  any
certificates,   documents,   or  instruments   representing  or  evidencing  the
Collateral,  with Debtor's  endorsement  thereon and/or  accompanied by property
instruments of transfer and assignment duly executed in blank with, if requested
by Secured Party,  signatures  guaranteed by a member or member  organization in
good standing of an authorized Securities Transfer Agents Medallion Program, all
in form and substance satisfactory to Secured Party.

         4.2  Further  Assurances.   Debtor  will   contemporaneously  with  the
execution  hereof  and from  time to time  thereafter  at its  expense  promptly
execute and deliver all further  instruments  and documents and take all further
action  necessary or  appropriate or that Secured Party may request in order (i)
to perfect and protect the security  interest created or purported to be created
hereby and the first priority of such security interest,  (ii) to enable Secured
Party to exercise  and enforce its rights and  remedies  hereunder in respect of
the  Collateral,  and (iii) to otherwise  effect the purposes of this Agreement,
including  without  limitation:  (A)  executing  and  filing  any  financing  or
continuation  statements,  or any  amendments  thereto;  (B)  obtaining  written
confirmation  from the issuer of any  securities  pledged as  Collateral  of the
pledge of such securities,  in form and substance satisfactory to Secured Party;
(C)  cooperating  with Secured Party in registering the pledge of any securities
pledged as Collateral with the issuer of such securities;  (D) delivering notice
of Secured Party's security interest in any securities  pledged as Collateral to
any securities or financial  intermediary,  clearing  corporation or other party
required by Secured Party, in form and substance  satisfactory to Secured Party;
and  (E)  obtaining  written  confirmation  of  the  pledge  of  any  securities
constituting Collateral from any securities or financial intermediary,  clearing
corporation  or other party  required by Secured  Party,  in form and  substance
satisfactory to Secured Party.

         4.3 Additional Property. All Additional Property, as defined in Section
1.1(c)  above,  received by Debtor shall be received in trust for the benefit of
Secured Party.  All Additional  Property and all  certificates  or other written
instruments or documents  evidencing and/or representing the Additional Property
that is  received  by Debtor,  together  with such  instruments  of  transfer as
Secured Party may request,  shall  immediately be delivered to or deposited with
Secured Party and held by Secured  Party as  Collateral  under the terms of this
Agreement.  If the  Additional  Property  received  by Debtor and  delivered  to
Secured  Party  pursuant  to this  Section  shall  be  shares  of stock or other
securities,  such shares of stock or other  securities shall be duly endorsed in
blank or  accompanied  by proper  instruments  of transfer and  assignment  duly
executed in blank with, if requested by Secured Party,  signatures guaranteed by
a member or member  organization  in good standing of an  authorized  Securities
Transfer Agents  Medallion  Program,  all in form and substance  satisfactory to
Secured  Party.  Secured  Party  shall  be  deemed  to  have  possession  of any
Collateral in transit to Secured Party or its agent.

         4.4  Sale,  Transfer,  Encumbrance.  Debtor  will not  sell,  transfer,
mortgage,  or otherwise  encumber any  Collateral or impair the value thereof in
any manner without  Secured  Party's prior written  consent,  including  without
limitation by purchase, lease, barter, trade, payment deferral, or the creation,
assumption  or guarantee of  indebtedness  or other  lending of credit.  Secured
Party's written consent to any sale,  mortgage,  transfer,  or encumbrance shall
not be construed to be a waiver of this  provision in respect to any  subsequent
proposed sale, mortgage, transfer, or encumbrance.

         4.5 Liens. Neither Debtor nor any person acting on Debtor's behalf has,
or shall have any right, power, or authority to and shall not create,  incur, or
permit to be placed or  imposed,  upon the  Collateral,  any lien of any type or
nature whatsoever, other than the liens in favor of Secured Party.

         4.6 Matters or  Occurrences  Affecting  Collateral  or this  Agreement.
Debtor will promptly  notify Secured Party of any and all matters or occurrences
that may have a material adverse effect on the status or value of the Collateral
or this Agreement,  including  without  limitation the occurrence of an Event of
Default,  or an event  which,  with giving of notice or lapse of time,  or both,
would constitute an Event of Default.

     4.7  Agreements  Pertaining to  Collateral.  Debtor will not enter into any
type of contract or agreement  pertaining to any of the Collateral or in any way
transfer any voting rights pertaining to the Collateral to any person or entity.

         4.8 Dilution of Ownership.  As to any securities pledged as Collateral,
Debtor  will not  consent to or approve of the  issuance  of (i) any  additional
shares  of  any  class  of  securities  of  such  issuer,  (ii)  any  instrument
convertible  voluntarily  by  the  holder  thereof  or  automatically  upon  the
occurrence or  non-occurrence  of any event or condition  into, or  exchangeable
for, any such  securities,  or (iii) any warrants,  options,  contracts or other
commitments  entitling any third party to purchase or otherwise acquire any such
securities.  Notwithstanding  the  foregoing  or any  other  provision  of  this
Agreement to the contrary,  Debtor (i) shall comply with his  obligations  under
the  Purchase  Agreement  and each Other  Document,  and (ii) may consent to any
issuance of shares of Horizon if such  issuance has been  approved by a majority
of the Board of Directors of Horizon.

         4.9  Restrictions  on  Securities.  Debtor  will  not  enter  into  any
agreement  creating,  or otherwise permit to exist, any restriction or condition
upon the transfer,  voting or control of any  securities  pledged as Collateral,
except as consented to in writing by Secured Party. As to any securities pledged
as collateral, Debtor will not consent to or approve of any stock split, reverse
stock  split,  stock  dividend,  reclassification,   or  other  similar  act  or
transaction  regarding  the  Shares  unless  consented  to in writing by Secured
Party.

                                    ARTICLE V

                         DEBTOR'S AFFIRMATIVE COVENANTS

         Until payment and performance of all Obligations,  Debtor covenants and
agrees that it shall promptly  advise Secured Party in writing of any litigation
filed  against  Debtor  and of any  condition,  event or act which  comes to its
attention  that  would or might  have a  material  adverse  effect  on  Debtor's
financial condition or on Debtor's ability to perform the Obligations.

                                   ARTICLE VI

                               NEGATIVE COVENANTS

         Until payment and performance of all Obligations,  Debtor covenants and
agrees that Debtor will not, without the prior written consent of Secured Party:

     6.1 Liens. Grant, suffer, or permit liens on, or security interests in, the
Collateral.

         6.2  Violate  Other  Covenants.  Violate  or fail to  comply  with  any
covenants  or  agreements  regarding  other  debt  which  will or would with the
passage  of time or upon  demand  cause the  maturity  of any  other  debt to be
accelerated.

                                   ARTICLE VII

                              DEFAULT AND REMEDIES

     7.1 Events of Default.  An Event of Default  (herein so called) shall exist
if any one or more of the following events shall occur:

                  (a) The  failure  of Debtor to pay any amount  required  to be
paid under the Purchase Agreement (including,  without limitation, the indemnity
provisions  contained  therein),  or any other  amount  which  Debtor may now or
hereafter  owe to Secured Party under any Other  Agreement or otherwise,  within
(15) calendar days after such amount is due;

     (b) The failure of Debtor to pay any  Obligation  within (15) calendar days
after such amount is due; and

                  (c)  Debtor's  breach of a covenant in this  Agreement  or any
other  failure to perform  its  obligations  under this  Agreement  or any Other
Agreement.

     7.2 Secured Party's Remedies. Upon the occurrence of an Event of Default:

                  (a) Secured Party may declare the Obligations in whole or part
immediately due and may enforce payment and performance of the same and exercise
any rights under the Texas UCC,  rights and remedies of Secured Party under this
Agreement, or otherwise.

                  (b) Secured  Party may, at Secured  Party's  option and at the
expense of Debtor,  either in Secured Party's own right or in the name of Debtor
and in the same manner and to the same extent that Debtor  might  reasonably  so
act  if  this  Agreement  had  not  been  made:  (i) do  all  things  requisite,
convenient,  or  necessary  to enforce the  performance  and  observance  of all
rights,  remedies and privileges of Debtor arising from the  Collateral,  or any
part thereof,  including without limitation compromising,  waiving, excusing, or
in any manner releasing or discharging any obligation of any party to or arising
from the Collateral;  (ii) take possession of the books, papers,  chattel paper,
documents of title, and accounts of Debtor,  wherever  located,  relating to the
Collateral; (iii) sue or otherwise collect and receive money attributable to the
Collateral;  and (iv) exercise any other lawfully  available powers or remedies,
and do all other things  which  Secured  Party deems  requisite,  convenient  or
necessary  or which the  Secured  Party  deems  proper to protect  the  Security
Interest.

                  (b) Secured Party may foreclose  this  Agreement in the manner
now or  hereafter  provided  or  permitted  by law and may upon such  reasonable
notification  prior thereto as may be required by applicable  law (Debtor hereby
agreeing  that ten  days'  notice is  commercially  reasonable),  sell,  assign,
transfer,  or otherwise  dispose of the Collateral at public or private sale, in
whole  or in  part,  and  Secured  Party  may,  in its own  name or as  Debtor's
attorney-in-fact  effectively  assign and transfer the  Collateral,  or any part
thereof,   absolutely,  and  execute  and  deliver  all  necessary  assignments,
conveyances,  bills of sale, and other  instruments with power to substitute one
or more persons or  corporations  with like power.  Any such  foreclosure  sale,
assignment,  transfer,  or other  disposition  shall, to the extent permitted by
law,  be a  perpetual  bar,  both at law and in equity,  against  Debtor and all
persons and corporations  lawfully  claiming by or through or under Debtor.  Any
such foreclosure sale may be adjourned from time to time. Upon any sale, Secured
Party may bid for and purchase the  Collateral,  or any part  thereof,  and upon
compliance with the terms of sale may hold,  retain,  possess and dispose of the
Collateral, in its absolute right without further accountability.  Secured Party
shall have the right to be  credited  on the  amount of its bid a  corresponding
amount of the Obligations as of the date of such sale.

                  (c) If, in the opinion of Secured Party, there is any question
that a public sale or  distribution  of any Collateral will violate any state or
federal  securities  law,  Secured  Party  (i) may  offer  and  sell  securities
privately to purchasers who will agree to take them for investment  purposes and
not with a view to distribution  and who will agree to imposition of restrictive
legends on the  certificates  representing  the security,  or (ii) may sell such
securities in an intrastate  offering  under Section  3(a)(11) of the Securities
Act of 1933,  and no sale so made in good faith by Secured Party shall be deemed
to be not "commercially reasonable" because so made.

                  (d)  Not  in  limitation  of  any  other   provision  of  this
Agreement,  Secured  Party shall have all rights and remedies of a secured party
under the Texas UCC.

         7.3  Application  of Proceeds.  Secured Party may apply the proceeds of
any foreclosure  sale hereunder or from any other  permitted  disposition of the
Collateral  or any part  thereof as  follows:  (a) first,  to the payment of all
reasonable  costs and expenses of any foreclosure  and collection  hereunder and
all proceedings in connection  therewith,  including reasonable attorneys' fees;
(b) then, to the  reimbursement of Secured Party for all  disbursements  made by
Secured Party for taxes,  assessments or liens superior to the Security Interest
and  which  Secured  Party  shall  deem  expedient  to  pay;  (c)  then,  to the
reimbursement of Secured Party of any other  disbursements made by Secured Party
in accordance with the terms hereof or under the Purchase Agreement or any Other
Agreement;  (d) then,  to or among the amounts of fees,  interest and  principal
then owing and unpaid in respect of the Obligations, in such priority as Secured
Party may determine in its  discretion;  and (e) the remainder of such proceeds,
if any,  shall be paid to Debtor.  If such  proceeds  shall be  insufficient  to
discharge the entire  Obligations,  Secured Party shall have any other available
legal recourse  against Debtor under,  or for the  performance  of, the Purchase
Agreement and any Other  Agreement  between  Debtor and Secured  Party,  for the
deficiency,  together with interest  thereon at the maximum rate permitted under
applicable law.

         7.4  Enforcement  of  Obligations.  Nothing in this Agreement or in any
other  document  or  agreement  shall  affect or impair  the  unconditional  and
absolute right of Secured Party to enforce the  Obligations as and when the same
shall become due in accordance with the terms of the Purchase Agreement or Other
Agreement.

                                  ARTICLE VIII

                             RIGHTS OF SECURED PARTY

         8.1  Subrogation.  Upon the occurrence of an Event of Default,  Secured
Party,  at its  election,  may  subrogate  to all of the  interest,  rights  and
remedies  of the  Debtor,  in respect  to any of the  Collateral  or  agreements
pertaining thereto.

         8.2 Secured Party  Appointed  Attorney-in-Fact.  Debtor hereby appoints
Secured Party as  attorney-in-fact  of Debtor,  with full authority in the place
and stead of Debtor and in the name of Debtor, Secured Party or otherwise,  from
time to time on Secured  Party's  discretion and upon the occurrence of an Event
of Default, to take any action and to execute any instrument which Secured Party
may deem  necessary or advisable to accomplish  the purposes of this  Agreement,
including without  limitation:  (a) to ask, demand,  collect,  sue for, recover,
compound, receive and give acquittance and receipts for moneys due and to become
due under or in respect of any of the Collateral;  (b) to receive,  endorse, and
collect  any  drafts or other  instruments,  documents  and  chattel  paper,  in
connection  with clause (a) of this  Section 8.2; (c) to file any claims or take
any action or institute any proceeding which Secured Party may deem necessary or
desirable for the  collection  of any of the  Collateral or otherwise to enforce
the rights of Secured Party against any of the Collateral; and (d) to assign and
transfer the  Collateral,  or any part  thereof,  absolutely  and to execute and
deliver  endorsements,   assignments,  conveyances,  bills  of  sale  and  other
instruments  with power to substitute  one or more persons or  corporation  with
like power.

         8.3  Performance  by Secured  Party.  If Debtor  fails to  perform  any
agreement  contained  herein,  Secured  Party may itself  perform,  or cause the
performance  of, such  agreement,  and the reasonable  expenses of Secured Party
incurred in connection  therewith  shall be payable by Debtor under Section 8.8.
In no  event,  however,  shall  Secured  Party  have any  obligation  or  duties
whatsoever to perform any covenant or agreement of Debtor contained herein,  and
any such performance by Secured Party shall be wholly discretionary with Secured
Party.

         8.4 Duties of Secured  Party.  The powers  conferred upon Secured Party
hereunder  are solely to protect its  interest in the  Collateral  and shall not
impose any duty upon it to exercise any such powers. Except for the safe custody
of any  Collateral  in its  possession  and the  accounting  for money  actually
received by it hereunder,  Secured Party shall have no duty as to any Collateral
or as to the taking of any  necessary  steps to preserve  rights  against  prior
parties or any other rights  pertaining to any Collateral.  Without limiting the
generality of the foregoing,  Secured Party shall not have any obligation,  duty
or  responsibility  to do any of the  following:  (a) ascertain any  maturities,
calls,  conversions,  exchanges,  offers, tenders or similar matters relating to
the  Collateral or informing  Debtor with respect to any such matters;  (b) fix,
preserve  or  exercise  any  right,  privilege  or option  (whether  conversion,
redemption or otherwise) with respect to the Collateral; (c) collect any amounts
payable  in  respect  of the  Collateral;  (d)  sell all or any  portion  of the
Collateral,  for any reason;  or (e) hold the Collateral for or on behalf of any
party other than Debtor.

         8.5 No  Liability  of Secured  Party.  Neither the  acceptance  of this
Agreement by Secured Party,  nor the exercise of any rights hereunder by Secured
Party,  shall be construed in any way as an  assumption  by Secured Party of any
obligations,  responsibilities,  or duties of Debtor arising in connection  with
the  Collateral  assigned  hereunder  or  otherwise  bind  Secured  Party to the
performance of any  obligations  respecting the  Collateral,  it being expressly
understood  that Secured  Party shall not be obligated to perform,  observe,  or
discharge  any  obligation,  responsibility,  duty,  or  liability  of Debtor in
respect of any of the Collateral,  including without limitation  appearing in or
defending any action, expending any money or incurring any expense in connection
therewith.  TO THE FULLEST EXTENT  PERMITTED BY APPLICABLE LAW, DEBTOR SHALL AND
DOES AGREE TO INDEMNIFY,  PROTECT,  DEFEND AND HOLD HARMLESS  SECURED PARTY, ITS
OFFICERS, DIRECTORS, REPRESENTATIVES, AGENTS, EMPLOYEES, LENDERS, SUCCESSORS AND
ASSIGNS,  FROM AND AGAINST ALL  LIABILITIES,  CLAIMS,  DAMAGES,  LOSSES,  FINES,
PENALTIES,  CAUSES OF ACTIONS,  SUITS,  JUDGMENTS AND EXPENSES  (INCLUDING COURT
COSTS,  ATTORNEY'S  FEES  AND  COST OF  INVESTIGATION)  OF ANY  NATURE,  KIND OR
DESCRIPTION  OF ANY PERSON OR ENTITY,  DIRECTLY OR  INDIRECTLY,  ARISING OUT OF,
CAUSED BY OR  RESULTING  FROM (IN WHOLE OR IN PART),  ANY  UNINTENTIONAL  ACT OR
OMISSION (OR  INTENTIONAL  ACT OR OMISSION SO LONG AS SUCH ACT OR OMISSION  DOES
NOT  CONSTITUTE  NEGLIGENCE)  OF SECURED  PARTY (OR  ANYONE  ACTING ON BEHALF OF
SECURED PARTY) IN CONNECTION WITH THE COLLATERAL.  THE FOREGOING INDEMNITY SHALL
SURVIVE THE EXPIRATION OR EARLIER TERMINATION OF THIS AGREEMENT.

         8.6 Right of Secured Party to Defend Action Affecting Security. Secured
Party  may,  at the  expense  of  Debtor,  appear in and  defend  any  action or
proceeding at law or in equity  purporting to affect  Secured  Party's  Security
Interest under this Agreement.

         8.7 Right of  Secured  Party to Prevent  or Remedy  Default.  If Debtor
shall fail to perform any of the covenants,  conditions and agreements  required
to be performed  and  observed by Debtor  under the Purchase  Agreement or Other
Agreement,  or in respect of the Collateral  (subject to any applicable  default
cure  period),  Secured  Party (a) may but shall  not be  obligated  to take any
action Secured Party deems  necessary or desirable to prevent or remedy any such
default by Debtor or otherwise to protect the Security  Interest,  and (b) shall
have the absolute and immediate  right to take  possession of the  Collateral or
any  part  thereof  (to  the  extent  Secured  Party  has not  previously  taken
possession)  to such  extent  and as often  as the  Secured  Party,  in its sole
discretion, deems necessary or desirable in order to prevent or to cure any such
default by Debtor,  or  otherwise  to protect the  security  of this  Agreement.
Secured Party may advance or expend such sums of money for the account of Debtor
as Secured Party in its sole discretion deems necessary for any such purpose.

         8.8 Secured Party's Expenses. All reasonable advances, costs, expenses,
charges and attorneys' fees which Secured Party may make, pay or incur under any
provision  of this  Agreement  for the  protection  of its  security  or for the
enforcement of any of its rights hereunder,  including,  without limitation,  in
foreclosure  proceedings commenced and subsequently  abandoned or in any dispute
or litigation in which Secured Party or the holder of any of the Obligations may
become involved by reason of or arising out of the Purchase Agreement, any Other
Agreement or the  Collateral,  shall be a part of the  Obligations  and shall be
paid by Debtor to Secured Party, upon demand, and shall bear interest until paid
at the maximum  rate of interest  permitted  by  applicable  law,  from the date
incurred by Secured Party until repaid by Debtor.

         8.9. Convertible  Collateral.  Secured Party may present for conversion
any  Collateral  which is  convertible  into any other  instrument or investment
security or a  combination  thereof with cash,  but Secured Party shall not have
any duty to present for conversion any Collateral  unless it shall have received
from Debtor  detailed  written  instructions to that effect at a time reasonably
far in advance of the final conversion date to make such conversion possible.

         8.10 Secured Party's Right of Set-Off.  Upon the happening of any event
entitling  Secured  Party to pursue any remedy  provided  herein,  or if Secured
Party shall be served with garnishment process in which Debtor shall be named as
defendant,  whether or not Debtor  shall be in  default  hereunder  at the time,
Secured Party may, but shall not be required to, set-off any indebtedness  owing
by  Secured  Party  to  Debtor  against  any of the  Obligations  without  first
resorting to the security hereunder and without prejudice to any other rights or
remedies of Secured Party or its Security Interest.

         8.11 Remedies.  No right or remedy herein  reserved to Secured Party is
intended to be exclusive  of any other right or remedy,  but each and every such
remedy shall be cumulative,  not in lieu of, but in addition to any other rights
or remedies given under this Agreement and all other security documents. Any and
all of Secured  Party's  rights and remedies may be exercised  from time to time
and as often as such exercise as deemed necessary or desirable by Secured Party.

         8.12 Debtor's Waivers.  Debtor waives notice of the creation,  advance,
increase,  existence,  extension,  or  renewal  of, and of any  indulgence  with
respect to, the  Obligations;  waives notice of intent to accelerate,  notice of
acceleration,  notice  of  intent  to  demand,  presentment,  demand,  notice of
dishonor,  and  protest;   waives  notice  of  the  amount  of  the  Obligations
outstanding  at any time,  notice of any change in  financial  condition  of any
person liable for the  Obligations  or any part thereof,  notice of any Event of
Default,  and all other  notices  respecting  the  Obligations;  and agrees that
maturity of the Obligations  and any part thereof may be accelerated,  extended,
or renewed one or more times by Secured Party in its discretion,  without notice
to Debtor.

         8.13 Other Parties and Other Collateral.  No renewal or extension of or
any other  indulgence  with respect to the  Obligations or any part thereof,  no
release  of any  security,  no  release  of any  person  (including  any  maker,
endorser,  guarantor,  or  surety)  liable  on  the  Obligations,  no  delay  in
enforcement  of payment,  and no delay or admission or lack of diligence or care
in exercising any right or power with respect to the Obligations or any security
therefor  or  guaranty  thereof or under this  Agreement  shall in other  manner
impair  or affect  the  rights  of  Secured  Party  under  the law,  under  this
Agreement,  or under any other  document or  agreement  pertaining  to the other
security for the  Obligations,  before  foreclosing  upon the Collateral for the
purpose of paying the Obligations.  Debtor waives any right to the benefit of or
to require or control application of any other security or proceeds thereof, and
Debtor  agrees that Secured  Party shall have no duty or obligation to Debtor to
apply to the Obligations any such other security or proceeds thereof.

         8.14 Relationship Among Secured Party.  Either or both of Secured Party
are  entitled  to enforce  any and all rights  granted to Secured  Party in this
Agreement,  and to take any other  action  allowed to be taken by Secured  Party
under this  Agreement.  Neither  Secured  Party is under any  obligation  to act
jointly or in concert with the other Secured Party  pursuant to this  Agreement.
Any action  required  to be taken by Debtor,  or notice  required to be given by
Debtor, shall be taken or given with respect to both of Secured Party.

                                   ARTICLE IX

                                  MISCELLANEOUS

     9.1 Terms  Commercially  Reasonable.  The terms of this Agreement  shall be
deemed commercially reasonable within the meaning of the Texas UCC.

         9.2 Notices.  Any notices or demands  required or permitted to be given
hereunder  shall be  deemed  sufficiently  given if in  writing  and  personally
delivered or mailed (with all postage and charges prepaid), addressed to Secured
Party or to Debtor their respective  addresses set forth below, or at such other
address as the above parties may from time to time  designate by written  notice
to the other given in  accordance  with this Section  9.2.  Any such notice,  if
personally  delivered or  transmitted  by telex or telegram,  shall be deemed to
have been given on the date so delivered or transmitted or, if mailed, be deemed
to have been given on the day after such  notice is placed in the United  States
mail in accordance with this Section 9.2.

                  Secured Party:  1301 Capital of Texas Hwy., Suite C-300
                                  Austin, Travis County, Texas 78746
                                  Attn: President

                  with copy to:   Timothy L. LaFrey, Esq.
                                  Akin, Gump, Strauss, Hauer & Feld, L.L.P.
                                  1900 Frost Bank Plaza
                                  816 Congress Avenue
                                  Austin, Texas 78701

                  Debtor:         Bradley J. Sandler, M.D.
                                  403 Calle De Caballo
                                  Suisun City, California   94585-1501

         9.3 Parties Bound.  Secured Party's rights under this Agreement and the
Security Interest shall inure to the benefits of its successors and assigns, and
in the event of any  assignment  or  transfer of any of the  Obligations  or the
Collateral,  Secured  Party  thereafter  shall  be  fully  discharged  from  any
responsibility  with respect to the Collateral so assigned or  transferred,  but
Secured  Party shall  retain all rights and powers  hereby given with respect to
any of the  Obligations  or  Collateral  not so  assigned  or  transferred.  All
representations, warranties, and agreements of Debtor if more than one are joint
and several, and all shall be binding upon the personal representatives,  heirs,
successors, and assigns of Debtor.

         9.4 Waiver.  No delay of Secured Party in exercising any power or right
shall operate as a waiver thereof;  nor shall any single or partial  exercise of
any power or right preclude other or further exercise thereof or the exercise of
any other power or right.  No waiver by Secured Party of any right  hereunder of
any default by Debtor shall be binding upon Secured Party unless in writing, and
no failure by Secured  Party to exercise any power or right  hereunder or waiver
of any  default  by Debtor  shall  operate  as a waiver of any other or  further
exercise of such right or power of any further default.

         9.5 Agreement Continuing.  This Agreement shall constitute a continuing
agreement,  applying to all future as well as existing transactions,  whether or
not of the  character  contemplated  at the date of this  Agreement,  and if all
transactions between Secured Party and Debtor shall be closed at any time, shall
be equally  applicable to any new  transactions  thereafter.  Provisions of this
Agreement,  unless  by their  terms  exclusive,  shall be in  addition  to those
contained in any Other Agreement.

         9.6 Definitions. Unless the context indicated otherwise, definitions in
the Texas Business and Commerce Code ("Texas UCC") apply to words and phrases in
this Agreement; if Texas UCC definitions conflict, Chapter 9 definitions apply.

         9.7 Miscellaneous. In this Agreement, whenever the context so requires,
the neuter gender  includes the masculine and feminine,  and the singular number
includes  the plural and vice  versa.  The  headings  of  paragraphs  herein are
inserted only for convenience and shall in no way define,  describe or limit the
scope of intent of any  provisions  of this  Agreement.  No  change,  amendment,
modification,  cancellation,  or  discharge of any  provision of this  Agreement
shall be valid unless consented to in writing by Secured Party.

         9.8 Assignment of Secured  Party's  Interest.  Secured Party shall have
the right to assign all or any portion of its rights in this  Agreement  without
approval or consent.  Debtor may not assign this  Agreement or any of its rights
or obligations  hereunder  without the express prior written  consent of Secured
Party in each instance.

     9.9 Applicable  Laws.  THIS AGREEMENT SHALL BE GOVERNED BY AND CONSTRUED IN
ACCORDANCE  WITH THE LAWS OF THE STATE OF TEXAS AND THE  APPLICABLE  LAWS OF THE
UNITED STATES OF AMERICA.

     9.10 ENTIRE AGREEMENT.  THIS AGREEMENT AND THE PURCHASE AGREEMENT REPRESENT
THE FINAL AGREEMENT  BETWEEN THE PARTIES AND MAY NOT BE CONTRADICTED BY EVIDENCE
OF PRIOR,  CONTEMPORANEOUS,  OR SUBSEQUENT ORAL AGREEMENTS OF THE PARTIES. THERE
ARE NO UNWRITTEN ORAL AGREEMENTS BETWEEN THE PARTIES.

                            [Signature page follows]


<PAGE>


S-1



                                SIGNATURE PAGE TO

                                 ASSIGNMENT AND

                               SECURITY AGREEMENT

         EXECUTED this 1st day of September, 1999.



DEBTOR:                                   /s/ Bradley J. Sandler, M.D.

                                          Printed Name: Bradley J. Sandler, M.D.


SECURED PARTY:                            Prime Medical Operating, Inc.

                                          By: /s/ Cheryl Williams

                                          Name: Cheryl Williams

                                          Title: Vice President


                                          Prime/BDR Acquisition, L.L.C.

                                          By: /s/ Cheryl Williams

                                          Name: Cheryl Williams

                                          Title: Secretary and Manager

<PAGE>



                        ASSIGNMENT AND SECURITY AGREEMENT

     THIS  ASSIGNMENT  AND SECURITY  AGREEMENT  (this  "Agreement")  is made and
entered into as of the 1st day of September,  1999, by and between Prime Medical
Operating, Inc., a Delaware corporation ("PMOI"), Prime/BDR Acquisition, L.L.C.,
a Delaware limited  liability  company ("Prime") (PMOI and Prime are referred to
herein, jointly and severally,  as "Secured Party") and the Severin Family Trust
(the "Debtor").

                                    RECITALS:

         A. Debtor and Secured Party have  executed and  delivered  that certain
Stock  Purchase   Agreement  dated  as  of  September  1,  1999  (the  "Purchase
Agreement"),  pursuant to which  Secured  Party  purchased  from Debtor  certain
shares of the $0.01 par value  common  stock of Horizon  Vision  Center,  Inc, a
Nevada corporation ("Horizon").

         B. Secured Party has requested  that Debtor pledge the  Collateral  (as
defined below) to secure certain obligations and liabilities that Debtor may now
or hereafter have to Secured Party, including, without limitation, any indemnity
obligations arising under the Purchase Agreement.

     C. Debtor desires to enter into this Agreement as a material  inducement to
Secured  Party's  purchase  of  Debtor's  shares of Horizon  under the  Purchase
Agreement.

                                   AGREEMENT:

         NOW, THEREFORE, in consideration of the foregoing and the covenants and
agreements hereinafter set forth, and other good and valuable consideration, the
receipt and sufficiency of which Debtor  acknowledges,  Debtor and Secured Party
agree as follows:

                                    ARTICLE I

                       COLLATERAL AND SECURED OBLIGATIONS

         1.1 Grant of Security Interest.  Debtor hereby assigns,  transfers, and
pledges to Secured  Party,  and Debtor hereby grants to Secured Party a security
interest   in,   the   following   described   collateral   (collectively,   the
"Collateral"):

                  (a)  Shares  of  Horizon.  From  and  after  the  date of this
Agreement,  (i) all shares of the common  stock of Horizon  owned or acquired in
any manner by Debtor or Secured Party,  (collectively,  the "Shares"),  (ii) any
replacements,  substitutions,  or exchanges of the certificates representing the
Shares, and (iii) any and all options,  rescission rights,  registration rights,
conversion rights, subscription rights, contractual or quasi-contractual rights,
warrants,  redemption rights,  redemption proceeds, calls, preemptive rights and
all other rights and benefits pertaining to the Shares;

                  (b)  Accounts.  All  accounts  and  rights  now  or  hereafter
attributable to any of the Collateral  described in (a) above, and all rights of
Debtor now or hereafter arising under any agreement pertaining to the Collateral
described  in  (a)  above,   including  without  limitation  all  distributions,
proceeds, fees, dividends,  preferences,  payments or other benefits of whatever
nature  which  Debtor is now or may  hereafter  become  entitled to receive with
respect to any Collateral described in (a) above;

                  (c) Additional  Property.  "Collateral" shall also include the
following  property  (collectively,  the  "Additional  Property")  which  Debtor
becomes  entitled  to receive or shall  receive  in  connection  with any of the
Collateral  described in this Section 1.1: (i) any stock certificate,  including
without  limitation,  any  certificate  representing  a  stock  dividend  or any
certificate in connection with any recapitalization,  reclassification,  merger,
consolidation,  conversion,  sale of assets, combination of shares, stock split,
reverse  stock split or  spin-off;  (ii) any option,  warrant,  subscription  or
right,  whether as an addition to or in  substitution  of any of the  Collateral
described in this Section 1.1; (iii) any dividends or  distributions of any kind
whatsoever,  whether  distributable in cash,  stock or other property;  (iv) any
interest,  premium or principal  payments;  and (v) any conversion or redemption
proceeds; and

                  (d) Proceeds.  All proceeds (cash and non-cash) arising out of
the sale, exchange, collection or other disposition of all or any portion of the
Collateral  described in (a), (b), or (c) above,  including  without  limitation
proceeds in the form of stock, accounts, chattel paper, instruments,  documents,
goods, inventory and equipment.

The  security  interest in the  Collateral  hereby  granted by Debtor to Secured
Party may sometimes be referred to in this Agreement as the "Security Interest".

         1.2 Obligations.  This Agreement and the Security Interest shall secure
full and punctual payment and performance of the following indebtedness,  duties
and obligations (collectively, the "Obligations"):

                  (a) All liabilities and obligations of Debtor to Secured Party
(including, without limitation, any principal, interest, fees and other amounts,
and any  indemnity  obligations)  under and pursuant to the Purchase  Agreement,
this Agreement and/or any other contract or agreement  between Secured Party and
Debtor or any affiliate of Debtor (collectively, "Other Agreements"; and

                  (b) (i)  all  indebtedness,  obligations  and  liabilities  of
Debtor and/or any affiliate of Debtor to Secured Party of any kind or character,
now existing or hereafter arising, whether direct, indirect, related, unrelated,
fixed,  contingent,  liquidated,  unliquidated,  joint,  several  or  joint  and
several,  arising from,  connected with, or related to the Purchase Agreement or
any Other Agreement, or any other document, agreement, or instrument executed in
connection  either of them,  (ii) all accrued but unpaid  interest on any of the
indebtedness  described in (i) above, (iii) all obligations of Debtor and/or any
affiliate  of  Debtor  to  Secured  Party  under  any  documents  or  agreements
evidencing,  securing,  governing  and/or  pertaining  to all or any part of the
indebtedness  described  in (i) and (ii)  above,  (iv) all  costs  and  expenses
incurred by Secured Party in connection  with the collection and  administration
of all or any part of the  indebtedness  and obligations  described in (i), (ii)
and (iii) above or the protection or preservation  of, or realization  upon, the
collateral  securing  all or any  part of  such  indebtedness  and  obligations,
including  without  limitation  all  attorneys'  fees,  and  (v)  all  renewals,
extensions, modifications and rearrangements of the indebtedness and obligations
described in (i), (ii), (iii) and (iv) above.

                  (c) All sums now or  hereafter  loaned or  advanced by Secured
Party to  Debtor,  or  expended  by Secured  Party for the  account of Debtor or
otherwise owing by Debtor to Secured Party, in respect of the  Obligations,  and
all other sums  expended or advanced  by Secured  Party  pursuant to any term or
provision of this Agreement or any Other Agreement (i) to collect and/or enforce
the Obligations or (ii) to maintain, protect and preserve the Collateral.

                                   ARTICLE II

       DEBTOR'S REPRESENTATIONS AND WARRANTIES WITH RESPECT TO COLLATERAL

       Debtor hereby represents and warrants to Secured Party as follows:

         2.1 Ownership of Collateral.  Debtor has good and  marketable  title to
the Collateral  free and clear of any liens,  security  interests,  shareholders
agreement, calls, charge, or encumbrance,  except for this Security Interest. No
financing  statement or other  instrument  similar in effect covering all or any
part of the  Collateral is on file in any recording  office,  except as may have
been filed in favor of Secured Party relating to this Agreement.

         2.2  Power  &  Authority.  Debtor  has the  lawful  right,  power,  and
authority  to grant the Security  Interest in the  Collateral.  This  Agreement,
together  with all filings and other  actions  necessary or desirable to perfect
and protect such security interest,  which have been duly taken,  create a valid
and perfected first priority  security  interest in the Collateral  securing the
payment and performance of the Obligations.

         2.3  No  Agreements.  The  Shares  are  not  subject  to any  right  of
redemption,  or any  call or put  options,  voting  trust,  proxy,  shareholders
agreement,  right of first  refusal,  or any other  document or agreement  which
would in any way impair or adversely affect this Security Interest or the rights
of Secured Party under this Agreement.

         2.4  Securities.  Any  certificates  evidencing  securities  pledged as
Collateral  are valid and  genuine  and have not been  altered.  All  securities
pledged as Collateral have been duly  authorized and validly  issued,  are fully
paid and  non-assessable,  and were not issued in  violation  of the  preemptive
rights of any party or of any agreement by which Debtor or the issuer thereof is
bound.  No  restrictions  or  conditions  exist with  respect to the transfer or
voting of any securities pledged as Collateral.

                                   ARTICLE III

                  DEBTOR'S OTHER REPRESENTATIONS AND WARRANTIES

         3.1 Solvency of Debtor.  As of the date hereof,  (i) Debtor is solvent;
(ii) the fair saleable  value of Debtor's  assets exceeds  Debtor's  liabilities
(both fixed and contingent);  (iii) Debtor has sufficient capital to satisfy all
of Debtor's  obligations  as they  become due;  (iv) no  receiver,  trustee,  or
custodian has been appointed for, or taken  possession of, all or  substantially
all of the assets of Debtor,  either in a  proceeding  brought by Debtor or in a
proceeding  brought against Debtor;  (v) Debtor is not the subject of a petition
for relief under the United  States  Bankruptcy  Code or any similar  federal or
state insolvency law, including without limitation a petition filed by Debtor or
a petition filed by a third party seeking relief against Debtor; and (vi) Debtor
has no  intention  of filing a  petition  for  relief  under the  United  States
Bankruptcy  Code or any similar  federal or state  insolvency law, or of seeking
any other form of  creditor  relief,  within  the  two-year  period  immediately
following the date of this Agreement.

         3.2  Authority and  Compliance.  Debtor has full power and authority to
enter into this Agreement. Debtor has full power and authority to enter into and
perform its obligations  under the Purchase  Agreement and each Other Agreement.
No further  consent or approval is  required as a condition  to the  validity of
this  Agreement,  the Purchase  Agreement or any Other  Agreement.  Debtor is in
compliance with all applicable laws, ordinances,  statutes, orders, regulations,
judgments, writs, or decrees of any governmental entity to which it is subject.

         3.3 Binding Agreement.  This Agreement, the Purchase Agreement and each
Other Agreement  constitute valid and legally binding  obligations of Debtor, in
accordance  with  their  terms,  subject  to  (a)  the  applicable   bankruptcy,
insolvency,  reorganization,  moratorium,  and similar laws affecting creditors'
rights  generally  and  (b)  restrictions  imposed  by any  court  of  competent
jurisdiction on the enforcement of non-competition and exclusive use restrictive
covenants imposed under the Purchase Agreement or any Other Agreement.

         3.4 Litigation.  There are no proceedings  pending or, to the knowledge
of Debtor,  threatened before any court or  administrative  agency which will or
may have a material adverse effect on the financial  condition of Debtor or upon
Debtor's ability to perform its obligations  under this Agreement,  the Purchase
Agreement or any Other Agreement.

         3.5 No Conflicting Agreements.  There are no provisions of any existing
agreement,  mortgage,  indenture or contract  binding on Debtor or affecting its
property,  which  would  conflict  with or in any  way  prevent  the  execution,
delivery, or carrying out of the terms of this Agreement, the Purchase Agreement
or any Other Agreement.

         3.6  Ownership  of  Assets.  Debtor  has  good  and  full  title to the
Collateral,  and the  Collateral  is owned  free and  clear of  liens,  charges,
claims, security interests, and other encumbrances.

     3.7 Taxes. Debtor has filed all tax returns required to be filed by Debtor.

                                   ARTICLE IV

                  DEBTOR'S COVENANTS WITH RESPECT TO COLLATERAL

         Debtor  covenants  and agrees  that from the date  hereof and until the
payment  and  performance  in  full  of the  Obligations  unless  Secured  Party
otherwise consents in writing:

         4.1  Delivery of  Instruments  and/or  Certificates.  Contemporaneously
herewith,   Debtor  covenants  and  agrees  to  deliver  to  Secured  Party  any
certificates,   documents,   or  instruments   representing  or  evidencing  the
Collateral,  with Debtor's  endorsement  thereon and/or  accompanied by property
instruments of transfer and assignment duly executed in blank with, if requested
by Secured Party,  signatures  guaranteed by a member or member  organization in
good standing of an authorized Securities Transfer Agents Medallion Program, all
in form and substance satisfactory to Secured Party.

         4.2  Further  Assurances.   Debtor  will   contemporaneously  with  the
execution  hereof  and from  time to time  thereafter  at its  expense  promptly
execute and deliver all further  instruments  and documents and take all further
action  necessary or  appropriate or that Secured Party may request in order (i)
to perfect and protect the security  interest created or purported to be created
hereby and the first priority of such security interest,  (ii) to enable Secured
Party to exercise  and enforce its rights and  remedies  hereunder in respect of
the  Collateral,  and (iii) to otherwise  effect the purposes of this Agreement,
including  without  limitation:  (A)  executing  and  filing  any  financing  or
continuation  statements,  or any  amendments  thereto;  (B)  obtaining  written
confirmation  from the issuer of any  securities  pledged as  Collateral  of the
pledge of such securities,  in form and substance satisfactory to Secured Party;
(C)  cooperating  with Secured Party in registering the pledge of any securities
pledged as Collateral with the issuer of such securities;  (D) delivering notice
of Secured Party's security interest in any securities  pledged as Collateral to
any securities or financial  intermediary,  clearing  corporation or other party
required by Secured Party, in form and substance  satisfactory to Secured Party;
and  (E)  obtaining  written  confirmation  of  the  pledge  of  any  securities
constituting Collateral from any securities or financial intermediary,  clearing
corporation  or other party  required by Secured  Party,  in form and  substance
satisfactory to Secured Party.

         4.3 Additional Property. All Additional Property, as defined in Section
1.1(c)  above,  received by Debtor shall be received in trust for the benefit of
Secured Party.  All Additional  Property and all  certificates  or other written
instruments or documents  evidencing and/or representing the Additional Property
that is  received  by Debtor,  together  with such  instruments  of  transfer as
Secured Party may request,  shall  immediately be delivered to or deposited with
Secured Party and held by Secured  Party as  Collateral  under the terms of this
Agreement.  If the  Additional  Property  received  by Debtor and  delivered  to
Secured  Party  pursuant  to this  Section  shall  be  shares  of stock or other
securities,  such shares of stock or other  securities shall be duly endorsed in
blank or  accompanied  by proper  instruments  of transfer and  assignment  duly
executed in blank with, if requested by Secured Party,  signatures guaranteed by
a member or member  organization  in good standing of an  authorized  Securities
Transfer Agents  Medallion  Program,  all in form and substance  satisfactory to
Secured  Party.  Secured  Party  shall  be  deemed  to  have  possession  of any
Collateral in transit to Secured Party or its agent.

         4.4  Sale,  Transfer,  Encumbrance.  Debtor  will not  sell,  transfer,
mortgage,  or otherwise  encumber any  Collateral or impair the value thereof in
any manner without  Secured  Party's prior written  consent,  including  without
limitation by purchase, lease, barter, trade, payment deferral, or the creation,
assumption  or guarantee of  indebtedness  or other  lending of credit.  Secured
Party's written consent to any sale,  mortgage,  transfer,  or encumbrance shall
not be construed to be a waiver of this  provision in respect to any  subsequent
proposed sale, mortgage, transfer, or encumbrance.

         4.5 Liens. Neither Debtor nor any person acting on Debtor's behalf has,
or shall have any right, power, or authority to and shall not create,  incur, or
permit to be placed or  imposed,  upon the  Collateral,  any lien of any type or
nature whatsoever, other than the liens in favor of Secured Party.

         4.6 Matters or  Occurrences  Affecting  Collateral  or this  Agreement.
Debtor will promptly  notify Secured Party of any and all matters or occurrences
that may have a material adverse effect on the status or value of the Collateral
or this Agreement,  including  without  limitation the occurrence of an Event of
Default,  or an event  which,  with giving of notice or lapse of time,  or both,
would constitute an Event of Default.

     4.7  Agreements  Pertaining to  Collateral.  Debtor will not enter into any
type of contract or agreement  pertaining to any of the Collateral or in any way
transfer any voting rights pertaining to the Collateral to any person or entity.

         4.8 Dilution of Ownership.  As to any securities pledged as Collateral,
Debtor  will not  consent to or approve of the  issuance  of (i) any  additional
shares  of  any  class  of  securities  of  such  issuer,  (ii)  any  instrument
convertible  voluntarily  by  the  holder  thereof  or  automatically  upon  the
occurrence or  non-occurrence  of any event or condition  into, or  exchangeable
for, any such  securities,  or (iii) any warrants,  options,  contracts or other
commitments  entitling any third party to purchase or otherwise acquire any such
securities.  Notwithstanding  the  foregoing  or any  other  provision  of  this
Agreement to the contrary,  Debtor (i) shall comply with his  obligations  under
the  Purchase  Agreement  and each Other  Document,  and (ii) may consent to any
issuance of shares of Horizon if such  issuance has been  approved by a majority
of the Board of Directors of Horizon.

         4.9  Restrictions  on  Securities.  Debtor  will  not  enter  into  any
agreement  creating,  or otherwise permit to exist, any restriction or condition
upon the transfer,  voting or control of any  securities  pledged as Collateral,
except as consented to in writing by Secured Party. As to any securities pledged
as collateral, Debtor will not consent to or approve of any stock split, reverse
stock  split,  stock  dividend,  reclassification,   or  other  similar  act  or
transaction  regarding  the  Shares  unless  consented  to in writing by Secured
Party.

                                    ARTICLE V

                         DEBTOR'S AFFIRMATIVE COVENANTS

         Until payment and performance of all Obligations,  Debtor covenants and
agrees that it shall promptly  advise Secured Party in writing of any litigation
filed  against  Debtor  and of any  condition,  event or act which  comes to its
attention  that  would or might  have a  material  adverse  effect  on  Debtor's
financial condition or on Debtor's ability to perform the Obligations.

                                   ARTICLE VI

                               NEGATIVE COVENANTS

         Until payment and performance of all Obligations,  Debtor covenants and
agrees that Debtor will not, without the prior written consent of Secured Party:

     6.1 Liens. Grant, suffer, or permit liens on, or security interests in, the
Collateral.

         6.2  Violate  Other  Covenants.  Violate  or fail to  comply  with  any
covenants  or  agreements  regarding  other  debt  which  will or would with the
passage  of time or upon  demand  cause the  maturity  of any  other  debt to be
accelerated.

                                   ARTICLE VII

                              DEFAULT AND REMEDIES

     7.1 Events of Default.  An Event of Default  (herein so called) shall exist
if any one or more of the following events shall occur:

                  (a) The  failure  of Debtor to pay any amount  required  to be
paid under the Purchase Agreement (including,  without limitation, the indemnity
provisions  contained  therein),  or any other  amount  which  Debtor may now or
hereafter  owe to Secured Party under any Other  Agreement or otherwise,  within
(15) calendar days after such amount is due;

     (b) The failure of Debtor to pay any  Obligation  within (15) calendar days
after such amount is due; and

                  (c)  Debtor's  breach of a covenant in this  Agreement  or any
other  failure to perform  its  obligations  under this  Agreement  or any Other
Agreement.

     7.2 Secured Party's Remedies. Upon the occurrence of an Event of Default:

                  (a) Secured Party may declare the Obligations in whole or part
immediately due and may enforce payment and performance of the same and exercise
any rights under the Texas UCC,  rights and remedies of Secured Party under this
Agreement, or otherwise.

                  (b) Secured  Party may, at Secured  Party's  option and at the
expense of Debtor,  either in Secured Party's own right or in the name of Debtor
and in the same manner and to the same extent that Debtor  might  reasonably  so
act  if  this  Agreement  had  not  been  made:  (i) do  all  things  requisite,
convenient,  or  necessary  to enforce the  performance  and  observance  of all
rights,  remedies and privileges of Debtor arising from the  Collateral,  or any
part thereof,  including without limitation compromising,  waiving, excusing, or
in any manner releasing or discharging any obligation of any party to or arising
from the Collateral;  (ii) take possession of the books, papers,  chattel paper,
documents of title, and accounts of Debtor,  wherever  located,  relating to the
Collateral; (iii) sue or otherwise collect and receive money attributable to the
Collateral;  and (iv) exercise any other lawfully  available powers or remedies,
and do all other things  which  Secured  Party deems  requisite,  convenient  or
necessary  or which the  Secured  Party  deems  proper to protect  the  Security
Interest.

                  (b) Secured Party may foreclose  this  Agreement in the manner
now or  hereafter  provided  or  permitted  by law and may upon such  reasonable
notification  prior thereto as may be required by applicable  law (Debtor hereby
agreeing  that ten  days'  notice is  commercially  reasonable),  sell,  assign,
transfer,  or otherwise  dispose of the Collateral at public or private sale, in
whole  or in  part,  and  Secured  Party  may,  in its own  name or as  Debtor's
attorney-in-fact  effectively  assign and transfer the  Collateral,  or any part
thereof,   absolutely,  and  execute  and  deliver  all  necessary  assignments,
conveyances,  bills of sale, and other  instruments with power to substitute one
or more persons or  corporations  with like power.  Any such  foreclosure  sale,
assignment,  transfer,  or other  disposition  shall, to the extent permitted by
law,  be a  perpetual  bar,  both at law and in equity,  against  Debtor and all
persons and corporations  lawfully  claiming by or through or under Debtor.  Any
such foreclosure sale may be adjourned from time to time. Upon any sale, Secured
Party may bid for and purchase the  Collateral,  or any part  thereof,  and upon
compliance with the terms of sale may hold,  retain,  possess and dispose of the
Collateral, in its absolute right without further accountability.  Secured Party
shall have the right to be  credited  on the  amount of its bid a  corresponding
amount of the Obligations as of the date of such sale.

                  (c) If, in the opinion of Secured Party, there is any question
that a public sale or  distribution  of any Collateral will violate any state or
federal  securities  law,  Secured  Party  (i) may  offer  and  sell  securities
privately to purchasers who will agree to take them for investment  purposes and
not with a view to distribution  and who will agree to imposition of restrictive
legends on the  certificates  representing  the security,  or (ii) may sell such
securities in an intrastate  offering  under Section  3(a)(11) of the Securities
Act of 1933,  and no sale so made in good faith by Secured Party shall be deemed
to be not "commercially reasonable" because so made.

                  (d)  Not  in  limitation  of  any  other   provision  of  this
Agreement,  Secured  Party shall have all rights and remedies of a secured party
under the Texas UCC.

         7.3  Application  of Proceeds.  Secured Party may apply the proceeds of
any foreclosure  sale hereunder or from any other  permitted  disposition of the
Collateral  or any part  thereof as  follows:  (a) first,  to the payment of all
reasonable  costs and expenses of any foreclosure  and collection  hereunder and
all proceedings in connection  therewith,  including reasonable attorneys' fees;
(b) then, to the  reimbursement of Secured Party for all  disbursements  made by
Secured Party for taxes,  assessments or liens superior to the Security Interest
and  which  Secured  Party  shall  deem  expedient  to  pay;  (c)  then,  to the
reimbursement of Secured Party of any other  disbursements made by Secured Party
in accordance with the terms hereof or under the Purchase Agreement or any Other
Agreement;  (d) then,  to or among the amounts of fees,  interest and  principal
then owing and unpaid in respect of the Obligations, in such priority as Secured
Party may determine in its  discretion;  and (e) the remainder of such proceeds,
if any,  shall be paid to Debtor.  If such  proceeds  shall be  insufficient  to
discharge the entire  Obligations,  Secured Party shall have any other available
legal recourse  against Debtor under,  or for the  performance  of, the Purchase
Agreement and any Other  Agreement  between  Debtor and Secured  Party,  for the
deficiency,  together with interest  thereon at the maximum rate permitted under
applicable law.

         7.4  Enforcement  of  Obligations.  Nothing in this Agreement or in any
other  document  or  agreement  shall  affect or impair  the  unconditional  and
absolute right of Secured Party to enforce the  Obligations as and when the same
shall become due in accordance with the terms of the Purchase Agreement or Other
Agreement.

                                  ARTICLE VIII

                             RIGHTS OF SECURED PARTY

         8.1  Subrogation.  Upon the occurrence of an Event of Default,  Secured
Party,  at its  election,  may  subrogate  to all of the  interest,  rights  and
remedies  of the  Debtor,  in respect  to any of the  Collateral  or  agreements
pertaining thereto.

         8.2 Secured Party  Appointed  Attorney-in-Fact.  Debtor hereby appoints
Secured Party as  attorney-in-fact  of Debtor,  with full authority in the place
and stead of Debtor and in the name of Debtor, Secured Party or otherwise,  from
time to time on Secured  Party's  discretion and upon the occurrence of an Event
of Default, to take any action and to execute any instrument which Secured Party
may deem  necessary or advisable to accomplish  the purposes of this  Agreement,
including without  limitation:  (a) to ask, demand,  collect,  sue for, recover,
compound, receive and give acquittance and receipts for moneys due and to become
due under or in respect of any of the Collateral;  (b) to receive,  endorse, and
collect  any  drafts or other  instruments,  documents  and  chattel  paper,  in
connection  with clause (a) of this  Section 8.2; (c) to file any claims or take
any action or institute any proceeding which Secured Party may deem necessary or
desirable for the  collection  of any of the  Collateral or otherwise to enforce
the rights of Secured Party against any of the Collateral; and (d) to assign and
transfer the  Collateral,  or any part  thereof,  absolutely  and to execute and
deliver  endorsements,   assignments,  conveyances,  bills  of  sale  and  other
instruments  with power to substitute  one or more persons or  corporation  with
like power.

         8.3  Performance  by Secured  Party.  If Debtor  fails to  perform  any
agreement  contained  herein,  Secured  Party may itself  perform,  or cause the
performance  of, such  agreement,  and the reasonable  expenses of Secured Party
incurred in connection  therewith  shall be payable by Debtor under Section 8.8.
In no  event,  however,  shall  Secured  Party  have any  obligation  or  duties
whatsoever to perform any covenant or agreement of Debtor contained herein,  and
any such performance by Secured Party shall be wholly discretionary with Secured
Party.

         8.4 Duties of Secured  Party.  The powers  conferred upon Secured Party
hereunder  are solely to protect its  interest in the  Collateral  and shall not
impose any duty upon it to exercise any such powers. Except for the safe custody
of any  Collateral  in its  possession  and the  accounting  for money  actually
received by it hereunder,  Secured Party shall have no duty as to any Collateral
or as to the taking of any  necessary  steps to preserve  rights  against  prior
parties or any other rights  pertaining to any Collateral.  Without limiting the
generality of the foregoing,  Secured Party shall not have any obligation,  duty
or  responsibility  to do any of the  following:  (a) ascertain any  maturities,
calls,  conversions,  exchanges,  offers, tenders or similar matters relating to
the  Collateral or informing  Debtor with respect to any such matters;  (b) fix,
preserve  or  exercise  any  right,  privilege  or option  (whether  conversion,
redemption or otherwise) with respect to the Collateral; (c) collect any amounts
payable  in  respect  of the  Collateral;  (d)  sell all or any  portion  of the
Collateral,  for any reason;  or (e) hold the Collateral for or on behalf of any
party other than Debtor.

         8.5 No  Liability  of Secured  Party.  Neither the  acceptance  of this
Agreement by Secured Party,  nor the exercise of any rights hereunder by Secured
Party,  shall be construed in any way as an  assumption  by Secured Party of any
obligations,  responsibilities,  or duties of Debtor arising in connection  with
the  Collateral  assigned  hereunder  or  otherwise  bind  Secured  Party to the
performance of any  obligations  respecting the  Collateral,  it being expressly
understood  that Secured  Party shall not be obligated to perform,  observe,  or
discharge  any  obligation,  responsibility,  duty,  or  liability  of Debtor in
respect of any of the Collateral,  including without limitation  appearing in or
defending any action, expending any money or incurring any expense in connection
therewith.  TO THE FULLEST EXTENT  PERMITTED BY APPLICABLE LAW, DEBTOR SHALL AND
DOES AGREE TO INDEMNIFY,  PROTECT,  DEFEND AND HOLD HARMLESS  SECURED PARTY, ITS
OFFICERS, DIRECTORS, REPRESENTATIVES, AGENTS, EMPLOYEES, LENDERS, SUCCESSORS AND
ASSIGNS,  FROM AND AGAINST ALL  LIABILITIES,  CLAIMS,  DAMAGES,  LOSSES,  FINES,
PENALTIES,  CAUSES OF ACTIONS,  SUITS,  JUDGMENTS AND EXPENSES  (INCLUDING COURT
COSTS,  ATTORNEY'S  FEES  AND  COST OF  INVESTIGATION)  OF ANY  NATURE,  KIND OR
DESCRIPTION  OF ANY PERSON OR ENTITY,  DIRECTLY OR  INDIRECTLY,  ARISING OUT OF,
CAUSED BY OR  RESULTING  FROM (IN WHOLE OR IN PART),  ANY  UNINTENTIONAL  ACT OR
OMISSION (OR  INTENTIONAL  ACT OR OMISSION SO LONG AS SUCH ACT OR OMISSION  DOES
NOT  CONSTITUTE  NEGLIGENCE)  OF SECURED  PARTY (OR  ANYONE  ACTING ON BEHALF OF
SECURED PARTY) IN CONNECTION WITH THE COLLATERAL.  THE FOREGOING INDEMNITY SHALL
SURVIVE THE EXPIRATION OR EARLIER TERMINATION OF THIS AGREEMENT.

         8.6 Right of Secured Party to Defend Action Affecting Security. Secured
Party  may,  at the  expense  of  Debtor,  appear in and  defend  any  action or
proceeding at law or in equity  purporting to affect  Secured  Party's  Security
Interest under this Agreement.

         8.7 Right of  Secured  Party to Prevent  or Remedy  Default.  If Debtor
shall fail to perform any of the covenants,  conditions and agreements  required
to be performed  and  observed by Debtor  under the Purchase  Agreement or Other
Agreement,  or in respect of the Collateral  (subject to any applicable  default
cure  period),  Secured  Party (a) may but shall  not be  obligated  to take any
action Secured Party deems  necessary or desirable to prevent or remedy any such
default by Debtor or otherwise to protect the Security  Interest,  and (b) shall
have the absolute and immediate  right to take  possession of the  Collateral or
any  part  thereof  (to  the  extent  Secured  Party  has not  previously  taken
possession)  to such  extent  and as often  as the  Secured  Party,  in its sole
discretion, deems necessary or desirable in order to prevent or to cure any such
default by Debtor,  or  otherwise  to protect the  security  of this  Agreement.
Secured Party may advance or expend such sums of money for the account of Debtor
as Secured Party in its sole discretion deems necessary for any such purpose.

         8.8 Secured Party's Expenses. All reasonable advances, costs, expenses,
charges and attorneys' fees which Secured Party may make, pay or incur under any
provision  of this  Agreement  for the  protection  of its  security  or for the
enforcement of any of its rights hereunder,  including,  without limitation,  in
foreclosure  proceedings commenced and subsequently  abandoned or in any dispute
or litigation in which Secured Party or the holder of any of the Obligations may
become involved by reason of or arising out of the Purchase Agreement, any Other
Agreement or the  Collateral,  shall be a part of the  Obligations  and shall be
paid by Debtor to Secured Party, upon demand, and shall bear interest until paid
at the maximum  rate of interest  permitted  by  applicable  law,  from the date
incurred by Secured Party until repaid by Debtor.

         8.9. Convertible  Collateral.  Secured Party may present for conversion
any  Collateral  which is  convertible  into any other  instrument or investment
security or a  combination  thereof with cash,  but Secured Party shall not have
any duty to present for conversion any Collateral  unless it shall have received
from Debtor  detailed  written  instructions to that effect at a time reasonably
far in advance of the final conversion date to make such conversion possible.

         8.10 Secured Party's Right of Set-Off.  Upon the happening of any event
entitling  Secured  Party to pursue any remedy  provided  herein,  or if Secured
Party shall be served with garnishment process in which Debtor shall be named as
defendant,  whether or not Debtor  shall be in  default  hereunder  at the time,
Secured Party may, but shall not be required to, set-off any indebtedness  owing
by  Secured  Party  to  Debtor  against  any of the  Obligations  without  first
resorting to the security hereunder and without prejudice to any other rights or
remedies of Secured Party or its Security Interest.

         8.11 Remedies.  No right or remedy herein  reserved to Secured Party is
intended to be exclusive  of any other right or remedy,  but each and every such
remedy shall be cumulative,  not in lieu of, but in addition to any other rights
or remedies given under this Agreement and all other security documents. Any and
all of Secured  Party's  rights and remedies may be exercised  from time to time
and as often as such exercise as deemed necessary or desirable by Secured Party.

         8.12 Debtor's Waivers.  Debtor waives notice of the creation,  advance,
increase,  existence,  extension,  or  renewal  of, and of any  indulgence  with
respect to, the  Obligations;  waives notice of intent to accelerate,  notice of
acceleration,  notice  of  intent  to  demand,  presentment,  demand,  notice of
dishonor,  and  protest;   waives  notice  of  the  amount  of  the  Obligations
outstanding  at any time,  notice of any change in  financial  condition  of any
person liable for the  Obligations  or any part thereof,  notice of any Event of
Default,  and all other  notices  respecting  the  Obligations;  and agrees that
maturity of the Obligations  and any part thereof may be accelerated,  extended,
or renewed one or more times by Secured Party in its discretion,  without notice
to Debtor.

         8.13 Other Parties and Other Collateral.  No renewal or extension of or
any other  indulgence  with respect to the  Obligations or any part thereof,  no
release  of any  security,  no  release  of any  person  (including  any  maker,
endorser,  guarantor,  or  surety)  liable  on  the  Obligations,  no  delay  in
enforcement  of payment,  and no delay or admission or lack of diligence or care
in exercising any right or power with respect to the Obligations or any security
therefor  or  guaranty  thereof or under this  Agreement  shall in other  manner
impair  or affect  the  rights  of  Secured  Party  under  the law,  under  this
Agreement,  or under any other  document or  agreement  pertaining  to the other
security for the  Obligations,  before  foreclosing  upon the Collateral for the
purpose of paying the Obligations.  Debtor waives any right to the benefit of or
to require or control application of any other security or proceeds thereof, and
Debtor  agrees that Secured  Party shall have no duty or obligation to Debtor to
apply to the Obligations any such other security or proceeds thereof.

         8.14 Relationship Among Secured Party.  Either or both of Secured Party
are  entitled  to enforce  any and all rights  granted to Secured  Party in this
Agreement,  and to take any other  action  allowed to be taken by Secured  Party
under this  Agreement.  Neither  Secured  Party is under any  obligation  to act
jointly or in concert with the other Secured Party  pursuant to this  Agreement.
Any action  required  to be taken by Debtor,  or notice  required to be given by
Debtor, shall be taken or given with respect to both of Secured Party.

                                   ARTICLE IX

                                  MISCELLANEOUS

     9.1 Terms  Commercially  Reasonable.  The terms of this Agreement  shall be
deemed commercially reasonable within the meaning of the Texas UCC.

         9.2 Notices.  Any notices or demands  required or permitted to be given
hereunder  shall be  deemed  sufficiently  given if in  writing  and  personally
delivered or mailed (with all postage and charges prepaid), addressed to Secured
Party or to Debtor their respective  addresses set forth below, or at such other
address as the above parties may from time to time  designate by written  notice
to the other given in  accordance  with this Section  9.2.  Any such notice,  if
personally  delivered or  transmitted  by telex or telegram,  shall be deemed to
have been given on the date so delivered or transmitted or, if mailed, be deemed
to have been given on the day after such  notice is placed in the United  States
mail in accordance with this Section 9.2.

                  Secured Party:  1301 Capital of Texas Hwy., Suite C-300
                                  Austin, Travis County, Texas 78746
                                  Attn: President

                  with copy to:   Timothy L. LaFrey, Esq.
                                  Akin, Gump, Strauss, Hauer & Feld, L.L.P.
                                  1900 Frost Bank Plaza
                                  816 Congress Avenue
                                  Austin, Texas 78701

                  Debtor:         The Severin Family Trust
                                  Sanford L. Severin, Trustee
                                  1040 McCauley Road
                                  Danville, California   94526

         9.3 Parties Bound.  Secured Party's rights under this Agreement and the
Security Interest shall inure to the benefits of its successors and assigns, and
in the event of any  assignment  or  transfer of any of the  Obligations  or the
Collateral,  Secured  Party  thereafter  shall  be  fully  discharged  from  any
responsibility  with respect to the Collateral so assigned or  transferred,  but
Secured  Party shall  retain all rights and powers  hereby given with respect to
any of the  Obligations  or  Collateral  not so  assigned  or  transferred.  All
representations, warranties, and agreements of Debtor if more than one are joint
and several, and all shall be binding upon the personal representatives,  heirs,
successors, and assigns of Debtor.

         9.4 Waiver.  No delay of Secured Party in exercising any power or right
shall operate as a waiver thereof;  nor shall any single or partial  exercise of
any power or right preclude other or further exercise thereof or the exercise of
any other power or right.  No waiver by Secured Party of any right  hereunder of
any default by Debtor shall be binding upon Secured Party unless in writing, and
no failure by Secured  Party to exercise any power or right  hereunder or waiver
of any  default  by Debtor  shall  operate  as a waiver of any other or  further
exercise of such right or power of any further default.

         9.5 Agreement Continuing.  This Agreement shall constitute a continuing
agreement,  applying to all future as well as existing transactions,  whether or
not of the  character  contemplated  at the date of this  Agreement,  and if all
transactions between Secured Party and Debtor shall be closed at any time, shall
be equally  applicable to any new  transactions  thereafter.  Provisions of this
Agreement,  unless  by their  terms  exclusive,  shall be in  addition  to those
contained in any Other Agreement.

         9.6 Definitions. Unless the context indicated otherwise, definitions in
the Texas Business and Commerce Code ("Texas UCC") apply to words and phrases in
this Agreement; if Texas UCC definitions conflict, Chapter 9 definitions apply.

         9.7 Miscellaneous. In this Agreement, whenever the context so requires,
the neuter gender  includes the masculine and feminine,  and the singular number
includes  the plural and vice  versa.  The  headings  of  paragraphs  herein are
inserted only for convenience and shall in no way define,  describe or limit the
scope of intent of any  provisions  of this  Agreement.  No  change,  amendment,
modification,  cancellation,  or  discharge of any  provision of this  Agreement
shall be valid unless consented to in writing by Secured Party.

         9.8 Assignment of Secured  Party's  Interest.  Secured Party shall have
the right to assign all or any portion of its rights in this  Agreement  without
approval or consent.  Debtor may not assign this  Agreement or any of its rights
or obligations  hereunder  without the express prior written  consent of Secured
Party in each instance.

     9.9 Applicable  Laws.  THIS AGREEMENT SHALL BE GOVERNED BY AND CONSTRUED IN
ACCORDANCE  WITH THE LAWS OF THE STATE OF TEXAS AND THE  APPLICABLE  LAWS OF THE
UNITED STATES OF AMERICA.

     9.10 ENTIRE AGREEMENT.  THIS AGREEMENT AND THE PURCHASE AGREEMENT REPRESENT
THE FINAL AGREEMENT  BETWEEN THE PARTIES AND MAY NOT BE CONTRADICTED BY EVIDENCE
OF PRIOR,  CONTEMPORANEOUS,  OR SUBSEQUENT ORAL AGREEMENTS OF THE PARTIES. THERE
ARE NO UNWRITTEN ORAL AGREEMENTS BETWEEN THE PARTIES.

                            [Signature page follows]


<PAGE>


S-1



                                SIGNATURE PAGE TO

                                 ASSIGNMENT AND

                               SECURITY AGREEMENT

         EXECUTED this 1st day of September, 1999.



DEBTOR:                                   /s/ Sanford L. Severin, Trustee

                                          Printed Name: Sanford L. Severin,
                                          Trustee under the Severin Family Trust

SECURED PARTY:                            Prime Medical Operating, Inc.

                                          By: /s/ Cheryl Williams

                                          Name: Cheryl Williams

                                          Title: Vice President


                                          Prime/BDR Acquisition, L.L.C.

                                          By: /s/ Cheryl Williams

                                          Name: Cheryl Williams

                                          Title: Vice President

<PAGE>



                        ASSIGNMENT AND SECURITY AGREEMENT

     THIS  ASSIGNMENT  AND SECURITY  AGREEMENT  (this  "Agreement")  is made and
entered into as of the 1st day of September,  1999, by and between Prime Medical
Operating, Inc., a Delaware corporation ("PMOI"), Prime/BDR Acquisition, L.L.C.,
a Delaware limited  liability  company ("Prime") (PMOI and Prime are referred to
herein,  jointly and severally,  as "Secured  Party") and the Stephen and Andrea
Turner Family Trust (the "Debtor").

                                    RECITALS:

         A. Debtor and Secured Party have  executed and  delivered  that certain
Stock  Purchase   Agreement  dated  as  of  September  1,  1999  (the  "Purchase
Agreement"),  pursuant to which  Secured  Party  purchased  from Debtor  certain
shares of the $0.01 par value  common  stock of Horizon  Vision  Center,  Inc, a
Nevada corporation ("Horizon").

         B. Secured Party has requested  that Debtor pledge the  Collateral  (as
defined below) to secure certain obligations and liabilities that Debtor may now
or hereafter have to Secured Party, including, without limitation, any indemnity
obligations arising under the Purchase Agreement.

     C. Debtor desires to enter into this Agreement as a material  inducement to
Secured  Party's  purchase  of  Debtor's  shares of Horizon  under the  Purchase
Agreement.

                                   AGREEMENT:

         NOW, THEREFORE, in consideration of the foregoing and the covenants and
agreements hereinafter set forth, and other good and valuable consideration, the
receipt and sufficiency of which Debtor  acknowledges,  Debtor and Secured Party
agree as follows:

                                    ARTICLE I

                       COLLATERAL AND SECURED OBLIGATIONS

         1.1 Grant of Security Interest.  Debtor hereby assigns,  transfers, and
pledges to Secured  Party,  and Debtor hereby grants to Secured Party a security
interest   in,   the   following   described   collateral   (collectively,   the
"Collateral"):

                  (a)  Shares  of  Horizon.  From  and  after  the  date of this
Agreement,  (i) all shares of the common  stock of Horizon  owned or acquired in
any manner by Debtor or Secured Party,  (collectively,  the "Shares"),  (ii) any
replacements,  substitutions,  or exchanges of the certificates representing the
Shares, and (iii) any and all options,  rescission rights,  registration rights,
conversion rights, subscription rights, contractual or quasi-contractual rights,
warrants,  redemption rights,  redemption proceeds, calls, preemptive rights and
all other rights and benefits pertaining to the Shares;

                  (b)  Accounts.  All  accounts  and  rights  now  or  hereafter
attributable to any of the Collateral  described in (a) above, and all rights of
Debtor now or hereafter arising under any agreement pertaining to the Collateral
described  in  (a)  above,   including  without  limitation  all  distributions,
proceeds, fees, dividends,  preferences,  payments or other benefits of whatever
nature  which  Debtor is now or may  hereafter  become  entitled to receive with
respect to any Collateral described in (a) above;

                  (c) Additional  Property.  "Collateral" shall also include the
following  property  (collectively,  the  "Additional  Property")  which  Debtor
becomes  entitled  to receive or shall  receive  in  connection  with any of the
Collateral  described in this Section 1.1: (i) any stock certificate,  including
without  limitation,  any  certificate  representing  a  stock  dividend  or any
certificate in connection with any recapitalization,  reclassification,  merger,
consolidation,  conversion,  sale of assets, combination of shares, stock split,
reverse  stock split or  spin-off;  (ii) any option,  warrant,  subscription  or
right,  whether as an addition to or in  substitution  of any of the  Collateral
described in this Section 1.1; (iii) any dividends or  distributions of any kind
whatsoever,  whether  distributable in cash,  stock or other property;  (iv) any
interest,  premium or principal  payments;  and (v) any conversion or redemption
proceeds; and

                  (d) Proceeds.  All proceeds (cash and non-cash) arising out of
the sale, exchange, collection or other disposition of all or any portion of the
Collateral  described in (a), (b), or (c) above,  including  without  limitation
proceeds in the form of stock, accounts, chattel paper, instruments,  documents,
goods, inventory and equipment.

The  security  interest in the  Collateral  hereby  granted by Debtor to Secured
Party may sometimes be referred to in this Agreement as the "Security Interest".

         1.2 Obligations.  This Agreement and the Security Interest shall secure
full and punctual payment and performance of the following indebtedness,  duties
and obligations (collectively, the "Obligations"):

                  (a) All liabilities and obligations of Debtor to Secured Party
(including, without limitation, any principal, interest, fees and other amounts,
and any  indemnity  obligations)  under and pursuant to the Purchase  Agreement,
this Agreement and/or any other contract or agreement  between Secured Party and
Debtor or any affiliate of Debtor (collectively, "Other Agreements"; and

                  (b) (i)  all  indebtedness,  obligations  and  liabilities  of
Debtor and/or any affiliate of Debtor to Secured Party of any kind or character,
now existing or hereafter arising, whether direct, indirect, related, unrelated,
fixed,  contingent,  liquidated,  unliquidated,  joint,  several  or  joint  and
several,  arising from,  connected with, or related to the Purchase Agreement or
any Other Agreement, or any other document, agreement, or instrument executed in
connection  either of them,  (ii) all accrued but unpaid  interest on any of the
indebtedness  described in (i) above, (iii) all obligations of Debtor and/or any
affiliate  of  Debtor  to  Secured  Party  under  any  documents  or  agreements
evidencing,  securing,  governing  and/or  pertaining  to all or any part of the
indebtedness  described  in (i) and (ii)  above,  (iv) all  costs  and  expenses
incurred by Secured Party in connection  with the collection and  administration
of all or any part of the  indebtedness  and obligations  described in (i), (ii)
and (iii) above or the protection or preservation  of, or realization  upon, the
collateral  securing  all or any  part of  such  indebtedness  and  obligations,
including  without  limitation  all  attorneys'  fees,  and  (v)  all  renewals,
extensions, modifications and rearrangements of the indebtedness and obligations
described in (i), (ii), (iii) and (iv) above.

                  (c) All sums now or  hereafter  loaned or  advanced by Secured
Party to  Debtor,  or  expended  by Secured  Party for the  account of Debtor or
otherwise owing by Debtor to Secured Party, in respect of the  Obligations,  and
all other sums  expended or advanced  by Secured  Party  pursuant to any term or
provision of this Agreement or any Other Agreement (i) to collect and/or enforce
the Obligations or (ii) to maintain, protect and preserve the Collateral.

                                   ARTICLE II

       DEBTOR'S REPRESENTATIONS AND WARRANTIES WITH RESPECT TO COLLATERAL

       Debtor hereby represents and warrants to Secured Party as follows:

         2.1 Ownership of Collateral.  Debtor has good and  marketable  title to
the Collateral  free and clear of any liens,  security  interests,  shareholders
agreement, calls, charge, or encumbrance,  except for this Security Interest. No
financing  statement or other  instrument  similar in effect covering all or any
part of the  Collateral is on file in any recording  office,  except as may have
been filed in favor of Secured Party relating to this Agreement.

         2.2  Power  &  Authority.  Debtor  has the  lawful  right,  power,  and
authority  to grant the Security  Interest in the  Collateral.  This  Agreement,
together  with all filings and other  actions  necessary or desirable to perfect
and protect such security interest,  which have been duly taken,  create a valid
and perfected first priority  security  interest in the Collateral  securing the
payment and performance of the Obligations.

         2.3  No  Agreements.  The  Shares  are  not  subject  to any  right  of
redemption,  or any  call or put  options,  voting  trust,  proxy,  shareholders
agreement,  right of first  refusal,  or any other  document or agreement  which
would in any way impair or adversely affect this Security Interest or the rights
of Secured Party under this Agreement.

         2.4  Securities.  Any  certificates  evidencing  securities  pledged as
Collateral  are valid and  genuine  and have not been  altered.  All  securities
pledged as Collateral have been duly  authorized and validly  issued,  are fully
paid and  non-assessable,  and were not issued in  violation  of the  preemptive
rights of any party or of any agreement by which Debtor or the issuer thereof is
bound.  No  restrictions  or  conditions  exist with  respect to the transfer or
voting of any securities pledged as Collateral.

                                   ARTICLE III

                  DEBTOR'S OTHER REPRESENTATIONS AND WARRANTIES

         3.1 Solvency of Debtor.  As of the date hereof,  (i) Debtor is solvent;
(ii) the fair saleable  value of Debtor's  assets exceeds  Debtor's  liabilities
(both fixed and contingent);  (iii) Debtor has sufficient capital to satisfy all
of Debtor's  obligations  as they  become due;  (iv) no  receiver,  trustee,  or
custodian has been appointed for, or taken  possession of, all or  substantially
all of the assets of Debtor,  either in a  proceeding  brought by Debtor or in a
proceeding  brought against Debtor;  (v) Debtor is not the subject of a petition
for relief under the United  States  Bankruptcy  Code or any similar  federal or
state insolvency law, including without limitation a petition filed by Debtor or
a petition filed by a third party seeking relief against Debtor; and (vi) Debtor
has no  intention  of filing a  petition  for  relief  under the  United  States
Bankruptcy  Code or any similar  federal or state  insolvency law, or of seeking
any other form of  creditor  relief,  within  the  two-year  period  immediately
following the date of this Agreement.

         3.2  Authority and  Compliance.  Debtor has full power and authority to
enter into this Agreement. Debtor has full power and authority to enter into and
perform its obligations  under the Purchase  Agreement and each Other Agreement.
No further  consent or approval is  required as a condition  to the  validity of
this  Agreement,  the Purchase  Agreement or any Other  Agreement.  Debtor is in
compliance with all applicable laws, ordinances,  statutes, orders, regulations,
judgments, writs, or decrees of any governmental entity to which it is subject.

         3.3 Binding Agreement.  This Agreement, the Purchase Agreement and each
Other Agreement  constitute valid and legally binding  obligations of Debtor, in
accordance  with  their  terms,  subject  to  (a)  the  applicable   bankruptcy,
insolvency,  reorganization,  moratorium,  and similar laws affecting creditors'
rights  generally  and  (b)  restrictions  imposed  by any  court  of  competent
jurisdiction on the enforcement of non-competition and exclusive use restrictive
covenants imposed under the Purchase Agreement or any Other Agreement.

         3.4 Litigation.  There are no proceedings  pending or, to the knowledge
of Debtor,  threatened before any court or  administrative  agency which will or
may have a material adverse effect on the financial  condition of Debtor or upon
Debtor's ability to perform its obligations  under this Agreement,  the Purchase
Agreement or any Other Agreement.

         3.5 No Conflicting Agreements.  There are no provisions of any existing
agreement,  mortgage,  indenture or contract  binding on Debtor or affecting its
property,  which  would  conflict  with or in any  way  prevent  the  execution,
delivery, or carrying out of the terms of this Agreement, the Purchase Agreement
or any Other Agreement.

         3.6  Ownership  of  Assets.  Debtor  has  good  and  full  title to the
Collateral,  and the  Collateral  is owned  free and  clear of  liens,  charges,
claims, security interests, and other encumbrances.

     3.7 Taxes. Debtor has filed all tax returns required to be filed by Debtor.

                                   ARTICLE IV

                  DEBTOR'S COVENANTS WITH RESPECT TO COLLATERAL

         Debtor  covenants  and agrees  that from the date  hereof and until the
payment  and  performance  in  full  of the  Obligations  unless  Secured  Party
otherwise consents in writing:

         4.1  Delivery of  Instruments  and/or  Certificates.  Contemporaneously
herewith,   Debtor  covenants  and  agrees  to  deliver  to  Secured  Party  any
certificates,   documents,   or  instruments   representing  or  evidencing  the
Collateral,  with Debtor's  endorsement  thereon and/or  accompanied by property
instruments of transfer and assignment duly executed in blank with, if requested
by Secured Party,  signatures  guaranteed by a member or member  organization in
good standing of an authorized Securities Transfer Agents Medallion Program, all
in form and substance satisfactory to Secured Party.

         4.2  Further  Assurances.   Debtor  will   contemporaneously  with  the
execution  hereof  and from  time to time  thereafter  at its  expense  promptly
execute and deliver all further  instruments  and documents and take all further
action  necessary or  appropriate or that Secured Party may request in order (i)
to perfect and protect the security  interest created or purported to be created
hereby and the first priority of such security interest,  (ii) to enable Secured
Party to exercise  and enforce its rights and  remedies  hereunder in respect of
the  Collateral,  and (iii) to otherwise  effect the purposes of this Agreement,
including  without  limitation:  (A)  executing  and  filing  any  financing  or
continuation  statements,  or any  amendments  thereto;  (B)  obtaining  written
confirmation  from the issuer of any  securities  pledged as  Collateral  of the
pledge of such securities,  in form and substance satisfactory to Secured Party;
(C)  cooperating  with Secured Party in registering the pledge of any securities
pledged as Collateral with the issuer of such securities;  (D) delivering notice
of Secured Party's security interest in any securities  pledged as Collateral to
any securities or financial  intermediary,  clearing  corporation or other party
required by Secured Party, in form and substance  satisfactory to Secured Party;
and  (E)  obtaining  written  confirmation  of  the  pledge  of  any  securities
constituting Collateral from any securities or financial intermediary,  clearing
corporation  or other party  required by Secured  Party,  in form and  substance
satisfactory to Secured Party.

         4.3 Additional Property. All Additional Property, as defined in Section
1.1(c)  above,  received by Debtor shall be received in trust for the benefit of
Secured Party.  All Additional  Property and all  certificates  or other written
instruments or documents  evidencing and/or representing the Additional Property
that is  received  by Debtor,  together  with such  instruments  of  transfer as
Secured Party may request,  shall  immediately be delivered to or deposited with
Secured Party and held by Secured  Party as  Collateral  under the terms of this
Agreement.  If the  Additional  Property  received  by Debtor and  delivered  to
Secured  Party  pursuant  to this  Section  shall  be  shares  of stock or other
securities,  such shares of stock or other  securities shall be duly endorsed in
blank or  accompanied  by proper  instruments  of transfer and  assignment  duly
executed in blank with, if requested by Secured Party,  signatures guaranteed by
a member or member  organization  in good standing of an  authorized  Securities
Transfer Agents  Medallion  Program,  all in form and substance  satisfactory to
Secured  Party.  Secured  Party  shall  be  deemed  to  have  possession  of any
Collateral in transit to Secured Party or its agent.

         4.4  Sale,  Transfer,  Encumbrance.  Debtor  will not  sell,  transfer,
mortgage,  or otherwise  encumber any  Collateral or impair the value thereof in
any manner without  Secured  Party's prior written  consent,  including  without
limitation by purchase, lease, barter, trade, payment deferral, or the creation,
assumption  or guarantee of  indebtedness  or other  lending of credit.  Secured
Party's written consent to any sale,  mortgage,  transfer,  or encumbrance shall
not be construed to be a waiver of this  provision in respect to any  subsequent
proposed sale, mortgage, transfer, or encumbrance.

         4.5 Liens. Neither Debtor nor any person acting on Debtor's behalf has,
or shall have any right, power, or authority to and shall not create,  incur, or
permit to be placed or  imposed,  upon the  Collateral,  any lien of any type or
nature whatsoever, other than the liens in favor of Secured Party.

         4.6 Matters or  Occurrences  Affecting  Collateral  or this  Agreement.
Debtor will promptly  notify Secured Party of any and all matters or occurrences
that may have a material adverse effect on the status or value of the Collateral
or this Agreement,  including  without  limitation the occurrence of an Event of
Default,  or an event  which,  with giving of notice or lapse of time,  or both,
would constitute an Event of Default.

     4.7  Agreements  Pertaining to  Collateral.  Debtor will not enter into any
type of contract or agreement  pertaining to any of the Collateral or in any way
transfer any voting rights pertaining to the Collateral to any person or entity.

         4.8 Dilution of Ownership.  As to any securities pledged as Collateral,
Debtor  will not  consent to or approve of the  issuance  of (i) any  additional
shares  of  any  class  of  securities  of  such  issuer,  (ii)  any  instrument
convertible  voluntarily  by  the  holder  thereof  or  automatically  upon  the
occurrence or  non-occurrence  of any event or condition  into, or  exchangeable
for, any such  securities,  or (iii) any warrants,  options,  contracts or other
commitments  entitling any third party to purchase or otherwise acquire any such
securities.  Notwithstanding  the  foregoing  or any  other  provision  of  this
Agreement to the contrary,  Debtor (i) shall comply with his  obligations  under
the  Purchase  Agreement  and each Other  Document,  and (ii) may consent to any
issuance of shares of Horizon if such  issuance has been  approved by a majority
of the Board of Directors of Horizon.

         4.9  Restrictions  on  Securities.  Debtor  will  not  enter  into  any
agreement  creating,  or otherwise permit to exist, any restriction or condition
upon the transfer,  voting or control of any  securities  pledged as Collateral,
except as consented to in writing by Secured Party. As to any securities pledged
as collateral, Debtor will not consent to or approve of any stock split, reverse
stock  split,  stock  dividend,  reclassification,   or  other  similar  act  or
transaction  regarding  the  Shares  unless  consented  to in writing by Secured
Party.

                                    ARTICLE V

                         DEBTOR'S AFFIRMATIVE COVENANTS

         Until payment and performance of all Obligations,  Debtor covenants and
agrees that it shall promptly  advise Secured Party in writing of any litigation
filed  against  Debtor  and of any  condition,  event or act which  comes to its
attention  that  would or might  have a  material  adverse  effect  on  Debtor's
financial condition or on Debtor's ability to perform the Obligations.

                                   ARTICLE VI

                               NEGATIVE COVENANTS

         Until payment and performance of all Obligations,  Debtor covenants and
agrees that Debtor will not, without the prior written consent of Secured Party:

     6.1 Liens. Grant, suffer, or permit liens on, or security interests in, the
Collateral.

         6.2  Violate  Other  Covenants.  Violate  or fail to  comply  with  any
covenants  or  agreements  regarding  other  debt  which  will or would with the
passage  of time or upon  demand  cause the  maturity  of any  other  debt to be
accelerated.

                                   ARTICLE VII

                              DEFAULT AND REMEDIES

     7.1 Events of Default.  An Event of Default  (herein so called) shall exist
if any one or more of the following events shall occur:

                  (a) The  failure  of Debtor to pay any amount  required  to be
paid under the Purchase Agreement (including,  without limitation, the indemnity
provisions  contained  therein),  or any other  amount  which  Debtor may now or
hereafter  owe to Secured Party under any Other  Agreement or otherwise,  within
(15) calendar days after such amount is due;

     (b) The failure of Debtor to pay any  Obligation  within (15) calendar days
after such amount is due; and

                  (c)  Debtor's  breach of a covenant in this  Agreement  or any
other  failure to perform  its  obligations  under this  Agreement  or any Other
Agreement.

     7.2 Secured Party's Remedies. Upon the occurrence of an Event of Default:

                  (a) Secured Party may declare the Obligations in whole or part
immediately due and may enforce payment and performance of the same and exercise
any rights under the Texas UCC,  rights and remedies of Secured Party under this
Agreement, or otherwise.

                  (b) Secured  Party may, at Secured  Party's  option and at the
expense of Debtor,  either in Secured Party's own right or in the name of Debtor
and in the same manner and to the same extent that Debtor  might  reasonably  so
act  if  this  Agreement  had  not  been  made:  (i) do  all  things  requisite,
convenient,  or  necessary  to enforce the  performance  and  observance  of all
rights,  remedies and privileges of Debtor arising from the  Collateral,  or any
part thereof,  including without limitation compromising,  waiving, excusing, or
in any manner releasing or discharging any obligation of any party to or arising
from the Collateral;  (ii) take possession of the books, papers,  chattel paper,
documents of title, and accounts of Debtor,  wherever  located,  relating to the
Collateral; (iii) sue or otherwise collect and receive money attributable to the
Collateral;  and (iv) exercise any other lawfully  available powers or remedies,
and do all other things  which  Secured  Party deems  requisite,  convenient  or
necessary  or which the  Secured  Party  deems  proper to protect  the  Security
Interest.

                  (b) Secured Party may foreclose  this  Agreement in the manner
now or  hereafter  provided  or  permitted  by law and may upon such  reasonable
notification  prior thereto as may be required by applicable  law (Debtor hereby
agreeing  that ten  days'  notice is  commercially  reasonable),  sell,  assign,
transfer,  or otherwise  dispose of the Collateral at public or private sale, in
whole  or in  part,  and  Secured  Party  may,  in its own  name or as  Debtor's
attorney-in-fact  effectively  assign and transfer the  Collateral,  or any part
thereof,   absolutely,  and  execute  and  deliver  all  necessary  assignments,
conveyances,  bills of sale, and other  instruments with power to substitute one
or more persons or  corporations  with like power.  Any such  foreclosure  sale,
assignment,  transfer,  or other  disposition  shall, to the extent permitted by
law,  be a  perpetual  bar,  both at law and in equity,  against  Debtor and all
persons and corporations  lawfully  claiming by or through or under Debtor.  Any
such foreclosure sale may be adjourned from time to time. Upon any sale, Secured
Party may bid for and purchase the  Collateral,  or any part  thereof,  and upon
compliance with the terms of sale may hold,  retain,  possess and dispose of the
Collateral, in its absolute right without further accountability.  Secured Party
shall have the right to be  credited  on the  amount of its bid a  corresponding
amount of the Obligations as of the date of such sale.

                  (c) If, in the opinion of Secured Party, there is any question
that a public sale or  distribution  of any Collateral will violate any state or
federal  securities  law,  Secured  Party  (i) may  offer  and  sell  securities
privately to purchasers who will agree to take them for investment  purposes and
not with a view to distribution  and who will agree to imposition of restrictive
legends on the  certificates  representing  the security,  or (ii) may sell such
securities in an intrastate  offering  under Section  3(a)(11) of the Securities
Act of 1933,  and no sale so made in good faith by Secured Party shall be deemed
to be not "commercially reasonable" because so made.

                  (d)  Not  in  limitation  of  any  other   provision  of  this
Agreement,  Secured  Party shall have all rights and remedies of a secured party
under the Texas UCC.

         7.3  Application  of Proceeds.  Secured Party may apply the proceeds of
any foreclosure  sale hereunder or from any other  permitted  disposition of the
Collateral  or any part  thereof as  follows:  (a) first,  to the payment of all
reasonable  costs and expenses of any foreclosure  and collection  hereunder and
all proceedings in connection  therewith,  including reasonable attorneys' fees;
(b) then, to the  reimbursement of Secured Party for all  disbursements  made by
Secured Party for taxes,  assessments or liens superior to the Security Interest
and  which  Secured  Party  shall  deem  expedient  to  pay;  (c)  then,  to the
reimbursement of Secured Party of any other  disbursements made by Secured Party
in accordance with the terms hereof or under the Purchase Agreement or any Other
Agreement;  (d) then,  to or among the amounts of fees,  interest and  principal
then owing and unpaid in respect of the Obligations, in such priority as Secured
Party may determine in its  discretion;  and (e) the remainder of such proceeds,
if any,  shall be paid to Debtor.  If such  proceeds  shall be  insufficient  to
discharge the entire  Obligations,  Secured Party shall have any other available
legal recourse  against Debtor under,  or for the  performance  of, the Purchase
Agreement and any Other  Agreement  between  Debtor and Secured  Party,  for the
deficiency,  together with interest  thereon at the maximum rate permitted under
applicable law.

         7.4  Enforcement  of  Obligations.  Nothing in this Agreement or in any
other  document  or  agreement  shall  affect or impair  the  unconditional  and
absolute right of Secured Party to enforce the  Obligations as and when the same
shall become due in accordance with the terms of the Purchase Agreement or Other
Agreement.

                                  ARTICLE VIII

                             RIGHTS OF SECURED PARTY

         8.1  Subrogation.  Upon the occurrence of an Event of Default,  Secured
Party,  at its  election,  may  subrogate  to all of the  interest,  rights  and
remedies  of the  Debtor,  in respect  to any of the  Collateral  or  agreements
pertaining thereto.

         8.2 Secured Party  Appointed  Attorney-in-Fact.  Debtor hereby appoints
Secured Party as  attorney-in-fact  of Debtor,  with full authority in the place
and stead of Debtor and in the name of Debtor, Secured Party or otherwise,  from
time to time on Secured  Party's  discretion and upon the occurrence of an Event
of Default, to take any action and to execute any instrument which Secured Party
may deem  necessary or advisable to accomplish  the purposes of this  Agreement,
including without  limitation:  (a) to ask, demand,  collect,  sue for, recover,
compound, receive and give acquittance and receipts for moneys due and to become
due under or in respect of any of the Collateral;  (b) to receive,  endorse, and
collect  any  drafts or other  instruments,  documents  and  chattel  paper,  in
connection  with clause (a) of this  Section 8.2; (c) to file any claims or take
any action or institute any proceeding which Secured Party may deem necessary or
desirable for the  collection  of any of the  Collateral or otherwise to enforce
the rights of Secured Party against any of the Collateral; and (d) to assign and
transfer the  Collateral,  or any part  thereof,  absolutely  and to execute and
deliver  endorsements,   assignments,  conveyances,  bills  of  sale  and  other
instruments  with power to substitute  one or more persons or  corporation  with
like power.

         8.3  Performance  by Secured  Party.  If Debtor  fails to  perform  any
agreement  contained  herein,  Secured  Party may itself  perform,  or cause the
performance  of, such  agreement,  and the reasonable  expenses of Secured Party
incurred in connection  therewith  shall be payable by Debtor under Section 8.8.
In no  event,  however,  shall  Secured  Party  have any  obligation  or  duties
whatsoever to perform any covenant or agreement of Debtor contained herein,  and
any such performance by Secured Party shall be wholly discretionary with Secured
Party.

         8.4 Duties of Secured  Party.  The powers  conferred upon Secured Party
hereunder  are solely to protect its  interest in the  Collateral  and shall not
impose any duty upon it to exercise any such powers. Except for the safe custody
of any  Collateral  in its  possession  and the  accounting  for money  actually
received by it hereunder,  Secured Party shall have no duty as to any Collateral
or as to the taking of any  necessary  steps to preserve  rights  against  prior
parties or any other rights  pertaining to any Collateral.  Without limiting the
generality of the foregoing,  Secured Party shall not have any obligation,  duty
or  responsibility  to do any of the  following:  (a) ascertain any  maturities,
calls,  conversions,  exchanges,  offers, tenders or similar matters relating to
the  Collateral or informing  Debtor with respect to any such matters;  (b) fix,
preserve  or  exercise  any  right,  privilege  or option  (whether  conversion,
redemption or otherwise) with respect to the Collateral; (c) collect any amounts
payable  in  respect  of the  Collateral;  (d)  sell all or any  portion  of the
Collateral,  for any reason;  or (e) hold the Collateral for or on behalf of any
party other than Debtor.

         8.5 No  Liability  of Secured  Party.  Neither the  acceptance  of this
Agreement by Secured Party,  nor the exercise of any rights hereunder by Secured
Party,  shall be construed in any way as an  assumption  by Secured Party of any
obligations,  responsibilities,  or duties of Debtor arising in connection  with
the  Collateral  assigned  hereunder  or  otherwise  bind  Secured  Party to the
performance of any  obligations  respecting the  Collateral,  it being expressly
understood  that Secured  Party shall not be obligated to perform,  observe,  or
discharge  any  obligation,  responsibility,  duty,  or  liability  of Debtor in
respect of any of the Collateral,  including without limitation  appearing in or
defending any action, expending any money or incurring any expense in connection
therewith.  TO THE FULLEST EXTENT  PERMITTED BY APPLICABLE LAW, DEBTOR SHALL AND
DOES AGREE TO INDEMNIFY,  PROTECT,  DEFEND AND HOLD HARMLESS  SECURED PARTY, ITS
OFFICERS, DIRECTORS, REPRESENTATIVES, AGENTS, EMPLOYEES, LENDERS, SUCCESSORS AND
ASSIGNS,  FROM AND AGAINST ALL  LIABILITIES,  CLAIMS,  DAMAGES,  LOSSES,  FINES,
PENALTIES,  CAUSES OF ACTIONS,  SUITS,  JUDGMENTS AND EXPENSES  (INCLUDING COURT
COSTS,  ATTORNEY'S  FEES  AND  COST OF  INVESTIGATION)  OF ANY  NATURE,  KIND OR
DESCRIPTION  OF ANY PERSON OR ENTITY,  DIRECTLY OR  INDIRECTLY,  ARISING OUT OF,
CAUSED BY OR  RESULTING  FROM (IN WHOLE OR IN PART),  ANY  UNINTENTIONAL  ACT OR
OMISSION (OR  INTENTIONAL  ACT OR OMISSION SO LONG AS SUCH ACT OR OMISSION  DOES
NOT  CONSTITUTE  NEGLIGENCE)  OF SECURED  PARTY (OR  ANYONE  ACTING ON BEHALF OF
SECURED PARTY) IN CONNECTION WITH THE COLLATERAL.  THE FOREGOING INDEMNITY SHALL
SURVIVE THE EXPIRATION OR EARLIER TERMINATION OF THIS AGREEMENT.

         8.6 Right of Secured Party to Defend Action Affecting Security. Secured
Party  may,  at the  expense  of  Debtor,  appear in and  defend  any  action or
proceeding at law or in equity  purporting to affect  Secured  Party's  Security
Interest under this Agreement.

         8.7 Right of  Secured  Party to Prevent  or Remedy  Default.  If Debtor
shall fail to perform any of the covenants,  conditions and agreements  required
to be performed  and  observed by Debtor  under the Purchase  Agreement or Other
Agreement,  or in respect of the Collateral  (subject to any applicable  default
cure  period),  Secured  Party (a) may but shall  not be  obligated  to take any
action Secured Party deems  necessary or desirable to prevent or remedy any such
default by Debtor or otherwise to protect the Security  Interest,  and (b) shall
have the absolute and immediate  right to take  possession of the  Collateral or
any  part  thereof  (to  the  extent  Secured  Party  has not  previously  taken
possession)  to such  extent  and as often  as the  Secured  Party,  in its sole
discretion, deems necessary or desirable in order to prevent or to cure any such
default by Debtor,  or  otherwise  to protect the  security  of this  Agreement.
Secured Party may advance or expend such sums of money for the account of Debtor
as Secured Party in its sole discretion deems necessary for any such purpose.

         8.8 Secured Party's Expenses. All reasonable advances, costs, expenses,
charges and attorneys' fees which Secured Party may make, pay or incur under any
provision  of this  Agreement  for the  protection  of its  security  or for the
enforcement of any of its rights hereunder,  including,  without limitation,  in
foreclosure  proceedings commenced and subsequently  abandoned or in any dispute
or litigation in which Secured Party or the holder of any of the Obligations may
become involved by reason of or arising out of the Purchase Agreement, any Other
Agreement or the  Collateral,  shall be a part of the  Obligations  and shall be
paid by Debtor to Secured Party, upon demand, and shall bear interest until paid
at the maximum  rate of interest  permitted  by  applicable  law,  from the date
incurred by Secured Party until repaid by Debtor.

         8.9. Convertible  Collateral.  Secured Party may present for conversion
any  Collateral  which is  convertible  into any other  instrument or investment
security or a  combination  thereof with cash,  but Secured Party shall not have
any duty to present for conversion any Collateral  unless it shall have received
from Debtor  detailed  written  instructions to that effect at a time reasonably
far in advance of the final conversion date to make such conversion possible.

         8.10 Secured Party's Right of Set-Off.  Upon the happening of any event
entitling  Secured  Party to pursue any remedy  provided  herein,  or if Secured
Party shall be served with garnishment process in which Debtor shall be named as
defendant,  whether or not Debtor  shall be in  default  hereunder  at the time,
Secured Party may, but shall not be required to, set-off any indebtedness  owing
by  Secured  Party  to  Debtor  against  any of the  Obligations  without  first
resorting to the security hereunder and without prejudice to any other rights or
remedies of Secured Party or its Security Interest.

         8.11 Remedies.  No right or remedy herein  reserved to Secured Party is
intended to be exclusive  of any other right or remedy,  but each and every such
remedy shall be cumulative,  not in lieu of, but in addition to any other rights
or remedies given under this Agreement and all other security documents. Any and
all of Secured  Party's  rights and remedies may be exercised  from time to time
and as often as such exercise as deemed necessary or desirable by Secured Party.

         8.12 Debtor's Waivers.  Debtor waives notice of the creation,  advance,
increase,  existence,  extension,  or  renewal  of, and of any  indulgence  with
respect to, the  Obligations;  waives notice of intent to accelerate,  notice of
acceleration,  notice  of  intent  to  demand,  presentment,  demand,  notice of
dishonor,  and  protest;   waives  notice  of  the  amount  of  the  Obligations
outstanding  at any time,  notice of any change in  financial  condition  of any
person liable for the  Obligations  or any part thereof,  notice of any Event of
Default,  and all other  notices  respecting  the  Obligations;  and agrees that
maturity of the Obligations  and any part thereof may be accelerated,  extended,
or renewed one or more times by Secured Party in its discretion,  without notice
to Debtor.

         8.13 Other Parties and Other Collateral.  No renewal or extension of or
any other  indulgence  with respect to the  Obligations or any part thereof,  no
release  of any  security,  no  release  of any  person  (including  any  maker,
endorser,  guarantor,  or  surety)  liable  on  the  Obligations,  no  delay  in
enforcement  of payment,  and no delay or admission or lack of diligence or care
in exercising any right or power with respect to the Obligations or any security
therefor  or  guaranty  thereof or under this  Agreement  shall in other  manner
impair  or affect  the  rights  of  Secured  Party  under  the law,  under  this
Agreement,  or under any other  document or  agreement  pertaining  to the other
security for the  Obligations,  before  foreclosing  upon the Collateral for the
purpose of paying the Obligations.  Debtor waives any right to the benefit of or
to require or control application of any other security or proceeds thereof, and
Debtor  agrees that Secured  Party shall have no duty or obligation to Debtor to
apply to the Obligations any such other security or proceeds thereof.

         8.14 Relationship Among Secured Party.  Either or both of Secured Party
are  entitled  to enforce  any and all rights  granted to Secured  Party in this
Agreement,  and to take any other  action  allowed to be taken by Secured  Party
under this  Agreement.  Neither  Secured  Party is under any  obligation  to act
jointly or in concert with the other Secured Party  pursuant to this  Agreement.
Any action  required  to be taken by Debtor,  or notice  required to be given by
Debtor, shall be taken or given with respect to both of Secured Party.

                                   ARTICLE IX

                                  MISCELLANEOUS

     9.1 Terms  Commercially  Reasonable.  The terms of this Agreement  shall be
deemed commercially reasonable within the meaning of the Texas UCC.

         9.2 Notices.  Any notices or demands  required or permitted to be given
hereunder  shall be  deemed  sufficiently  given if in  writing  and  personally
delivered or mailed (with all postage and charges prepaid), addressed to Secured
Party or to Debtor their respective  addresses set forth below, or at such other
address as the above parties may from time to time  designate by written  notice
to the other given in  accordance  with this Section  9.2.  Any such notice,  if
personally  delivered or  transmitted  by telex or telegram,  shall be deemed to
have been given on the date so delivered or transmitted or, if mailed, be deemed
to have been given on the day after such  notice is placed in the United  States
mail in accordance with this Section 9.2.

                  Secured Party:  1301 Capital of Texas Hwy., Suite C-300
                                  Austin, Travis County, Texas 78746
                                  Attn: President

                  with copy to:   Timothy L. LaFrey, Esq.
                                  Akin, Gump, Strauss, Hauer & Feld, L.L.P.
                                  1900 Frost Bank Plaza
                                  816 Congress Avenue
                                  Austin, Texas 78701

                  Debtor:         Stephen and Andrea Turner Family Trust
                                  250 Stonewall Road
                                  Berkeley, California   94705

         9.3 Parties Bound.  Secured Party's rights under this Agreement and the
Security Interest shall inure to the benefits of its successors and assigns, and
in the event of any  assignment  or  transfer of any of the  Obligations  or the
Collateral,  Secured  Party  thereafter  shall  be  fully  discharged  from  any
responsibility  with respect to the Collateral so assigned or  transferred,  but
Secured  Party shall  retain all rights and powers  hereby given with respect to
any of the  Obligations  or  Collateral  not so  assigned  or  transferred.  All
representations, warranties, and agreements of Debtor if more than one are joint
and several, and all shall be binding upon the personal representatives,  heirs,
successors, and assigns of Debtor.

         9.4 Waiver.  No delay of Secured Party in exercising any power or right
shall operate as a waiver thereof;  nor shall any single or partial  exercise of
any power or right preclude other or further exercise thereof or the exercise of
any other power or right.  No waiver by Secured Party of any right  hereunder of
any default by Debtor shall be binding upon Secured Party unless in writing, and
no failure by Secured  Party to exercise any power or right  hereunder or waiver
of any  default  by Debtor  shall  operate  as a waiver of any other or  further
exercise of such right or power of any further default.

         9.5 Agreement Continuing.  This Agreement shall constitute a continuing
agreement,  applying to all future as well as existing transactions,  whether or
not of the  character  contemplated  at the date of this  Agreement,  and if all
transactions between Secured Party and Debtor shall be closed at any time, shall
be equally  applicable to any new  transactions  thereafter.  Provisions of this
Agreement,  unless  by their  terms  exclusive,  shall be in  addition  to those
contained in any Other Agreement.

         9.6 Definitions. Unless the context indicated otherwise, definitions in
the Texas Business and Commerce Code ("Texas UCC") apply to words and phrases in
this Agreement; if Texas UCC definitions conflict, Chapter 9 definitions apply.

         9.7 Miscellaneous. In this Agreement, whenever the context so requires,
the neuter gender  includes the masculine and feminine,  and the singular number
includes  the plural and vice  versa.  The  headings  of  paragraphs  herein are
inserted only for convenience and shall in no way define,  describe or limit the
scope of intent of any  provisions  of this  Agreement.  No  change,  amendment,
modification,  cancellation,  or  discharge of any  provision of this  Agreement
shall be valid unless consented to in writing by Secured Party.

         9.8 Assignment of Secured  Party's  Interest.  Secured Party shall have
the right to assign all or any portion of its rights in this  Agreement  without
approval or consent.  Debtor may not assign this  Agreement or any of its rights
or obligations  hereunder  without the express prior written  consent of Secured
Party in each instance.

     9.9 Applicable  Laws.  THIS AGREEMENT SHALL BE GOVERNED BY AND CONSTRUED IN
ACCORDANCE  WITH THE LAWS OF THE STATE OF TEXAS AND THE  APPLICABLE  LAWS OF THE
UNITED STATES OF AMERICA.

     9.10 ENTIRE AGREEMENT.  THIS AGREEMENT AND THE PURCHASE AGREEMENT REPRESENT
THE FINAL AGREEMENT  BETWEEN THE PARTIES AND MAY NOT BE CONTRADICTED BY EVIDENCE
OF PRIOR,  CONTEMPORANEOUS,  OR SUBSEQUENT ORAL AGREEMENTS OF THE PARTIES. THERE
ARE NO UNWRITTEN ORAL AGREEMENTS BETWEEN THE PARTIES.

                            [Signature page follows]


<PAGE>


S-1



                                SIGNATURE PAGE TO

                                 ASSIGNMENT AND

                               SECURITY AGREEMENT

         EXECUTED this 1st day of September, 1999.



DEBTOR:                                   /s/ Stephen G. Turner, M.D.

                                          Printed Name: Stephen G. Turner, M.D.
                                          Trustee under the Stephen and Andrea
                                          Turner Family Trust

                                          /s/ Andrea J. Turner

                                          Printed Name: Andrea J. Turner
                                          Trustee under the Stephen and Andrea
                                          Turner Family Trust


SECURED PARTY:                            Prime Medical Operating, Inc.

                                          By: /s/ Cheryl Williams

                                          Name: Cheryl Williams

                                          Title: Vice President


                                          Prime/BDR Acquisition, L.L.C.

                                          By: /s/ Cheryl Williams

                                          Name: Cheryl Williams

                                          Title: Vice President

<PAGE>



                        ASSIGNMENT AND SECURITY AGREEMENT

     THIS  ASSIGNMENT  AND SECURITY  AGREEMENT  (this  "Agreement")  is made and
entered into as of the 1st day of September,  1999, by and between Prime Medical
Operating, Inc., a Delaware corporation ("PMOI"), Prime/BDR Acquisition, L.L.C.,
a Delaware limited  liability  company ("Prime") (PMOI and Prime are referred to
herein,  jointly and severally,  as "Secured  Party") and Stephen Wilmarth, M.D.
(the "Debtor").

                                    RECITALS:

         A. Debtor and Secured Party have  executed and  delivered  that certain
Stock  Purchase   Agreement  dated  as  of  September  1,  1999  (the  "Purchase
Agreement"),  pursuant to which  Secured  Party  purchased  from Debtor  certain
shares of the $0.01 par value  common  stock of Horizon  Vision  Center,  Inc, a
Nevada corporation ("Horizon").

         B. Secured Party has requested  that Debtor pledge the  Collateral  (as
defined below) to secure certain obligations and liabilities that Debtor may now
or hereafter have to Secured Party, including, without limitation, any indemnity
obligations arising under the Purchase Agreement.

     C. Debtor desires to enter into this Agreement as a material  inducement to
Secured  Party's  purchase  of  Debtor's  shares of Horizon  under the  Purchase
Agreement.

                                   AGREEMENT:

         NOW, THEREFORE, in consideration of the foregoing and the covenants and
agreements hereinafter set forth, and other good and valuable consideration, the
receipt and sufficiency of which Debtor  acknowledges,  Debtor and Secured Party
agree as follows:

                                    ARTICLE I

                       COLLATERAL AND SECURED OBLIGATIONS

         1.1 Grant of Security Interest.  Debtor hereby assigns,  transfers, and
pledges to Secured  Party,  and Debtor hereby grants to Secured Party a security
interest   in,   the   following   described   collateral   (collectively,   the
"Collateral"):

                  (a)  Shares  of  Horizon.  From  and  after  the  date of this
Agreement,  (i) all shares of the common  stock of Horizon  owned or acquired in
any manner by Debtor or Secured Party,  (collectively,  the "Shares"),  (ii) any
replacements,  substitutions,  or exchanges of the certificates representing the
Shares, and (iii) any and all options,  rescission rights,  registration rights,
conversion rights, subscription rights, contractual or quasi-contractual rights,
warrants,  redemption rights,  redemption proceeds, calls, preemptive rights and
all other rights and benefits pertaining to the Shares;

                  (b)  Accounts.  All  accounts  and  rights  now  or  hereafter
attributable to any of the Collateral  described in (a) above, and all rights of
Debtor now or hereafter arising under any agreement pertaining to the Collateral
described  in  (a)  above,   including  without  limitation  all  distributions,
proceeds, fees, dividends,  preferences,  payments or other benefits of whatever
nature  which  Debtor is now or may  hereafter  become  entitled to receive with
respect to any Collateral described in (a) above;

                  (c) Additional  Property.  "Collateral" shall also include the
following  property  (collectively,  the  "Additional  Property")  which  Debtor
becomes  entitled  to receive or shall  receive  in  connection  with any of the
Collateral  described in this Section 1.1: (i) any stock certificate,  including
without  limitation,  any  certificate  representing  a  stock  dividend  or any
certificate in connection with any recapitalization,  reclassification,  merger,
consolidation,  conversion,  sale of assets, combination of shares, stock split,
reverse  stock split or  spin-off;  (ii) any option,  warrant,  subscription  or
right,  whether as an addition to or in  substitution  of any of the  Collateral
described in this Section 1.1; (iii) any dividends or  distributions of any kind
whatsoever,  whether  distributable in cash,  stock or other property;  (iv) any
interest,  premium or principal  payments;  and (v) any conversion or redemption
proceeds; and

                  (d) Proceeds.  All proceeds (cash and non-cash) arising out of
the sale, exchange, collection or other disposition of all or any portion of the
Collateral  described in (a), (b), or (c) above,  including  without  limitation
proceeds in the form of stock, accounts, chattel paper, instruments,  documents,
goods, inventory and equipment.

The  security  interest in the  Collateral  hereby  granted by Debtor to Secured
Party may sometimes be referred to in this Agreement as the "Security Interest".

         1.2 Obligations.  This Agreement and the Security Interest shall secure
full and punctual payment and performance of the following indebtedness,  duties
and obligations (collectively, the "Obligations"):

                  (a) All liabilities and obligations of Debtor to Secured Party
(including, without limitation, any principal, interest, fees and other amounts,
and any  indemnity  obligations)  under and pursuant to the Purchase  Agreement,
this Agreement and/or any other contract or agreement  between Secured Party and
Debtor or any affiliate of Debtor (collectively, "Other Agreements"; and

                  (b) (i)  all  indebtedness,  obligations  and  liabilities  of
Debtor and/or any affiliate of Debtor to Secured Party of any kind or character,
now existing or hereafter arising, whether direct, indirect, related, unrelated,
fixed,  contingent,  liquidated,  unliquidated,  joint,  several  or  joint  and
several,  arising from,  connected with, or related to the Purchase Agreement or
any Other Agreement, or any other document, agreement, or instrument executed in
connection  either of them,  (ii) all accrued but unpaid  interest on any of the
indebtedness  described in (i) above, (iii) all obligations of Debtor and/or any
affiliate  of  Debtor  to  Secured  Party  under  any  documents  or  agreements
evidencing,  securing,  governing  and/or  pertaining  to all or any part of the
indebtedness  described  in (i) and (ii)  above,  (iv) all  costs  and  expenses
incurred by Secured Party in connection  with the collection and  administration
of all or any part of the  indebtedness  and obligations  described in (i), (ii)
and (iii) above or the protection or preservation  of, or realization  upon, the
collateral  securing  all or any  part of  such  indebtedness  and  obligations,
including  without  limitation  all  attorneys'  fees,  and  (v)  all  renewals,
extensions, modifications and rearrangements of the indebtedness and obligations
described in (i), (ii), (iii) and (iv) above.

                  (c) All sums now or  hereafter  loaned or  advanced by Secured
Party to  Debtor,  or  expended  by Secured  Party for the  account of Debtor or
otherwise owing by Debtor to Secured Party, in respect of the  Obligations,  and
all other sums  expended or advanced  by Secured  Party  pursuant to any term or
provision of this Agreement or any Other Agreement (i) to collect and/or enforce
the Obligations or (ii) to maintain, protect and preserve the Collateral.

                                   ARTICLE II

       DEBTOR'S REPRESENTATIONS AND WARRANTIES WITH RESPECT TO COLLATERAL

       Debtor hereby represents and warrants to Secured Party as follows:

         2.1 Ownership of Collateral.  Debtor has good and  marketable  title to
the Collateral  free and clear of any liens,  security  interests,  shareholders
agreement, calls, charge, or encumbrance,  except for this Security Interest. No
financing  statement or other  instrument  similar in effect covering all or any
part of the  Collateral is on file in any recording  office,  except as may have
been filed in favor of Secured Party relating to this Agreement.

         2.2  Power  &  Authority.  Debtor  has the  lawful  right,  power,  and
authority  to grant the Security  Interest in the  Collateral.  This  Agreement,
together  with all filings and other  actions  necessary or desirable to perfect
and protect such security interest,  which have been duly taken,  create a valid
and perfected first priority  security  interest in the Collateral  securing the
payment and performance of the Obligations.

         2.3  No  Agreements.  The  Shares  are  not  subject  to any  right  of
redemption,  or any  call or put  options,  voting  trust,  proxy,  shareholders
agreement,  right of first  refusal,  or any other  document or agreement  which
would in any way impair or adversely affect this Security Interest or the rights
of Secured Party under this Agreement.

         2.4  Securities.  Any  certificates  evidencing  securities  pledged as
Collateral  are valid and  genuine  and have not been  altered.  All  securities
pledged as Collateral have been duly  authorized and validly  issued,  are fully
paid and  non-assessable,  and were not issued in  violation  of the  preemptive
rights of any party or of any agreement by which Debtor or the issuer thereof is
bound.  No  restrictions  or  conditions  exist with  respect to the transfer or
voting of any securities pledged as Collateral.

                                   ARTICLE III

                  DEBTOR'S OTHER REPRESENTATIONS AND WARRANTIES

         3.1 Solvency of Debtor.  As of the date hereof,  (i) Debtor is solvent;
(ii) the fair saleable  value of Debtor's  assets exceeds  Debtor's  liabilities
(both fixed and contingent);  (iii) Debtor has sufficient capital to satisfy all
of Debtor's  obligations  as they  become due;  (iv) no  receiver,  trustee,  or
custodian has been appointed for, or taken  possession of, all or  substantially
all of the assets of Debtor,  either in a  proceeding  brought by Debtor or in a
proceeding  brought against Debtor;  (v) Debtor is not the subject of a petition
for relief under the United  States  Bankruptcy  Code or any similar  federal or
state insolvency law, including without limitation a petition filed by Debtor or
a petition filed by a third party seeking relief against Debtor; and (vi) Debtor
has no  intention  of filing a  petition  for  relief  under the  United  States
Bankruptcy  Code or any similar  federal or state  insolvency law, or of seeking
any other form of  creditor  relief,  within  the  two-year  period  immediately
following the date of this Agreement.

         3.2  Authority and  Compliance.  Debtor has full power and authority to
enter into this Agreement. Debtor has full power and authority to enter into and
perform its obligations  under the Purchase  Agreement and each Other Agreement.
No further  consent or approval is  required as a condition  to the  validity of
this  Agreement,  the Purchase  Agreement or any Other  Agreement.  Debtor is in
compliance with all applicable laws, ordinances,  statutes, orders, regulations,
judgments, writs, or decrees of any governmental entity to which it is subject.

         3.3 Binding Agreement.  This Agreement, the Purchase Agreement and each
Other Agreement  constitute valid and legally binding  obligations of Debtor, in
accordance  with  their  terms,  subject  to  (a)  the  applicable   bankruptcy,
insolvency,  reorganization,  moratorium,  and similar laws affecting creditors'
rights  generally  and  (b)  restrictions  imposed  by any  court  of  competent
jurisdiction on the enforcement of non-competition and exclusive use restrictive
covenants imposed under the Purchase Agreement or any Other Agreement.

         3.4 Litigation.  There are no proceedings  pending or, to the knowledge
of Debtor,  threatened before any court or  administrative  agency which will or
may have a material adverse effect on the financial  condition of Debtor or upon
Debtor's ability to perform its obligations  under this Agreement,  the Purchase
Agreement or any Other Agreement.

         3.5 No Conflicting Agreements.  There are no provisions of any existing
agreement,  mortgage,  indenture or contract  binding on Debtor or affecting its
property,  which  would  conflict  with or in any  way  prevent  the  execution,
delivery, or carrying out of the terms of this Agreement, the Purchase Agreement
or any Other Agreement.

         3.6  Ownership  of  Assets.  Debtor  has  good  and  full  title to the
Collateral,  and the  Collateral  is owned  free and  clear of  liens,  charges,
claims, security interests, and other encumbrances.

     3.7 Taxes. Debtor has filed all tax returns required to be filed by Debtor.

                                   ARTICLE IV

                  DEBTOR'S COVENANTS WITH RESPECT TO COLLATERAL

         Debtor  covenants  and agrees  that from the date  hereof and until the
payment  and  performance  in  full  of the  Obligations  unless  Secured  Party
otherwise consents in writing:

         4.1  Delivery of  Instruments  and/or  Certificates.  Contemporaneously
herewith,   Debtor  covenants  and  agrees  to  deliver  to  Secured  Party  any
certificates,   documents,   or  instruments   representing  or  evidencing  the
Collateral,  with Debtor's  endorsement  thereon and/or  accompanied by property
instruments of transfer and assignment duly executed in blank with, if requested
by Secured Party,  signatures  guaranteed by a member or member  organization in
good standing of an authorized Securities Transfer Agents Medallion Program, all
in form and substance satisfactory to Secured Party.

         4.2  Further  Assurances.   Debtor  will   contemporaneously  with  the
execution  hereof  and from  time to time  thereafter  at its  expense  promptly
execute and deliver all further  instruments  and documents and take all further
action  necessary or  appropriate or that Secured Party may request in order (i)
to perfect and protect the security  interest created or purported to be created
hereby and the first priority of such security interest,  (ii) to enable Secured
Party to exercise  and enforce its rights and  remedies  hereunder in respect of
the  Collateral,  and (iii) to otherwise  effect the purposes of this Agreement,
including  without  limitation:  (A)  executing  and  filing  any  financing  or
continuation  statements,  or any  amendments  thereto;  (B)  obtaining  written
confirmation  from the issuer of any  securities  pledged as  Collateral  of the
pledge of such securities,  in form and substance satisfactory to Secured Party;
(C)  cooperating  with Secured Party in registering the pledge of any securities
pledged as Collateral with the issuer of such securities;  (D) delivering notice
of Secured Party's security interest in any securities  pledged as Collateral to
any securities or financial  intermediary,  clearing  corporation or other party
required by Secured Party, in form and substance  satisfactory to Secured Party;
and  (E)  obtaining  written  confirmation  of  the  pledge  of  any  securities
constituting Collateral from any securities or financial intermediary,  clearing
corporation  or other party  required by Secured  Party,  in form and  substance
satisfactory to Secured Party.

         4.3 Additional Property. All Additional Property, as defined in Section
1.1(c)  above,  received by Debtor shall be received in trust for the benefit of
Secured Party.  All Additional  Property and all  certificates  or other written
instruments or documents  evidencing and/or representing the Additional Property
that is  received  by Debtor,  together  with such  instruments  of  transfer as
Secured Party may request,  shall  immediately be delivered to or deposited with
Secured Party and held by Secured  Party as  Collateral  under the terms of this
Agreement.  If the  Additional  Property  received  by Debtor and  delivered  to
Secured  Party  pursuant  to this  Section  shall  be  shares  of stock or other
securities,  such shares of stock or other  securities shall be duly endorsed in
blank or  accompanied  by proper  instruments  of transfer and  assignment  duly
executed in blank with, if requested by Secured Party,  signatures guaranteed by
a member or member  organization  in good standing of an  authorized  Securities
Transfer Agents  Medallion  Program,  all in form and substance  satisfactory to
Secured  Party.  Secured  Party  shall  be  deemed  to  have  possession  of any
Collateral in transit to Secured Party or its agent.

         4.4  Sale,  Transfer,  Encumbrance.  Debtor  will not  sell,  transfer,
mortgage,  or otherwise  encumber any  Collateral or impair the value thereof in
any manner without  Secured  Party's prior written  consent,  including  without
limitation by purchase, lease, barter, trade, payment deferral, or the creation,
assumption  or guarantee of  indebtedness  or other  lending of credit.  Secured
Party's written consent to any sale,  mortgage,  transfer,  or encumbrance shall
not be construed to be a waiver of this  provision in respect to any  subsequent
proposed sale, mortgage, transfer, or encumbrance.

         4.5 Liens. Neither Debtor nor any person acting on Debtor's behalf has,
or shall have any right, power, or authority to and shall not create,  incur, or
permit to be placed or  imposed,  upon the  Collateral,  any lien of any type or
nature whatsoever, other than the liens in favor of Secured Party.

         4.6 Matters or  Occurrences  Affecting  Collateral  or this  Agreement.
Debtor will promptly  notify Secured Party of any and all matters or occurrences
that may have a material adverse effect on the status or value of the Collateral
or this Agreement,  including  without  limitation the occurrence of an Event of
Default,  or an event  which,  with giving of notice or lapse of time,  or both,
would constitute an Event of Default.

     4.7  Agreements  Pertaining to  Collateral.  Debtor will not enter into any
type of contract or agreement  pertaining to any of the Collateral or in any way
transfer any voting rights pertaining to the Collateral to any person or entity.

         4.8 Dilution of Ownership.  As to any securities pledged as Collateral,
Debtor  will not  consent to or approve of the  issuance  of (i) any  additional
shares  of  any  class  of  securities  of  such  issuer,  (ii)  any  instrument
convertible  voluntarily  by  the  holder  thereof  or  automatically  upon  the
occurrence or  non-occurrence  of any event or condition  into, or  exchangeable
for, any such  securities,  or (iii) any warrants,  options,  contracts or other
commitments  entitling any third party to purchase or otherwise acquire any such
securities.  Notwithstanding  the  foregoing  or any  other  provision  of  this
Agreement to the contrary,  Debtor (i) shall comply with his  obligations  under
the  Purchase  Agreement  and each Other  Document,  and (ii) may consent to any
issuance of shares of Horizon if such  issuance has been  approved by a majority
of the Board of Directors of Horizon.

         4.9  Restrictions  on  Securities.  Debtor  will  not  enter  into  any
agreement  creating,  or otherwise permit to exist, any restriction or condition
upon the transfer,  voting or control of any  securities  pledged as Collateral,
except as consented to in writing by Secured Party. As to any securities pledged
as collateral, Debtor will not consent to or approve of any stock split, reverse
stock  split,  stock  dividend,  reclassification,   or  other  similar  act  or
transaction  regarding  the  Shares  unless  consented  to in writing by Secured
Party.

                                    ARTICLE V

                         DEBTOR'S AFFIRMATIVE COVENANTS

         Until payment and performance of all Obligations,  Debtor covenants and
agrees that it shall promptly  advise Secured Party in writing of any litigation
filed  against  Debtor  and of any  condition,  event or act which  comes to its
attention  that  would or might  have a  material  adverse  effect  on  Debtor's
financial condition or on Debtor's ability to perform the Obligations.

                                   ARTICLE VI

                               NEGATIVE COVENANTS

         Until payment and performance of all Obligations,  Debtor covenants and
agrees that Debtor will not, without the prior written consent of Secured Party:

     6.1 Liens. Grant, suffer, or permit liens on, or security interests in, the
Collateral.

         6.2  Violate  Other  Covenants.  Violate  or fail to  comply  with  any
covenants  or  agreements  regarding  other  debt  which  will or would with the
passage  of time or upon  demand  cause the  maturity  of any  other  debt to be
accelerated.

                                   ARTICLE VII

                              DEFAULT AND REMEDIES

     7.1 Events of Default.  An Event of Default  (herein so called) shall exist
if any one or more of the following events shall occur:

                  (a) The  failure  of Debtor to pay any amount  required  to be
paid under the Purchase Agreement (including,  without limitation, the indemnity
provisions  contained  therein),  or any other  amount  which  Debtor may now or
hereafter  owe to Secured Party under any Other  Agreement or otherwise,  within
(15) calendar days after such amount is due;

     (b) The failure of Debtor to pay any  Obligation  within (15) calendar days
after such amount is due; and

                  (c)  Debtor's  breach of a covenant in this  Agreement  or any
other  failure to perform  its  obligations  under this  Agreement  or any Other
Agreement.

     7.2 Secured Party's Remedies. Upon the occurrence of an Event of Default:

                  (a) Secured Party may declare the Obligations in whole or part
immediately due and may enforce payment and performance of the same and exercise
any rights under the Texas UCC,  rights and remedies of Secured Party under this
Agreement, or otherwise.

                  (b) Secured  Party may, at Secured  Party's  option and at the
expense of Debtor,  either in Secured Party's own right or in the name of Debtor
and in the same manner and to the same extent that Debtor  might  reasonably  so
act  if  this  Agreement  had  not  been  made:  (i) do  all  things  requisite,
convenient,  or  necessary  to enforce the  performance  and  observance  of all
rights,  remedies and privileges of Debtor arising from the  Collateral,  or any
part thereof,  including without limitation compromising,  waiving, excusing, or
in any manner releasing or discharging any obligation of any party to or arising
from the Collateral;  (ii) take possession of the books, papers,  chattel paper,
documents of title, and accounts of Debtor,  wherever  located,  relating to the
Collateral; (iii) sue or otherwise collect and receive money attributable to the
Collateral;  and (iv) exercise any other lawfully  available powers or remedies,
and do all other things  which  Secured  Party deems  requisite,  convenient  or
necessary  or which the  Secured  Party  deems  proper to protect  the  Security
Interest.

                  (b) Secured Party may foreclose  this  Agreement in the manner
now or  hereafter  provided  or  permitted  by law and may upon such  reasonable
notification  prior thereto as may be required by applicable  law (Debtor hereby
agreeing  that ten  days'  notice is  commercially  reasonable),  sell,  assign,
transfer,  or otherwise  dispose of the Collateral at public or private sale, in
whole  or in  part,  and  Secured  Party  may,  in its own  name or as  Debtor's
attorney-in-fact  effectively  assign and transfer the  Collateral,  or any part
thereof,   absolutely,  and  execute  and  deliver  all  necessary  assignments,
conveyances,  bills of sale, and other  instruments with power to substitute one
or more persons or  corporations  with like power.  Any such  foreclosure  sale,
assignment,  transfer,  or other  disposition  shall, to the extent permitted by
law,  be a  perpetual  bar,  both at law and in equity,  against  Debtor and all
persons and corporations  lawfully  claiming by or through or under Debtor.  Any
such foreclosure sale may be adjourned from time to time. Upon any sale, Secured
Party may bid for and purchase the  Collateral,  or any part  thereof,  and upon
compliance with the terms of sale may hold,  retain,  possess and dispose of the
Collateral, in its absolute right without further accountability.  Secured Party
shall have the right to be  credited  on the  amount of its bid a  corresponding
amount of the Obligations as of the date of such sale.

                  (c) If, in the opinion of Secured Party, there is any question
that a public sale or  distribution  of any Collateral will violate any state or
federal  securities  law,  Secured  Party  (i) may  offer  and  sell  securities
privately to purchasers who will agree to take them for investment  purposes and
not with a view to distribution  and who will agree to imposition of restrictive
legends on the  certificates  representing  the security,  or (ii) may sell such
securities in an intrastate  offering  under Section  3(a)(11) of the Securities
Act of 1933,  and no sale so made in good faith by Secured Party shall be deemed
to be not "commercially reasonable" because so made.

                  (d)  Not  in  limitation  of  any  other   provision  of  this
Agreement,  Secured  Party shall have all rights and remedies of a secured party
under the Texas UCC.

         7.3  Application  of Proceeds.  Secured Party may apply the proceeds of
any foreclosure  sale hereunder or from any other  permitted  disposition of the
Collateral  or any part  thereof as  follows:  (a) first,  to the payment of all
reasonable  costs and expenses of any foreclosure  and collection  hereunder and
all proceedings in connection  therewith,  including reasonable attorneys' fees;
(b) then, to the  reimbursement of Secured Party for all  disbursements  made by
Secured Party for taxes,  assessments or liens superior to the Security Interest
and  which  Secured  Party  shall  deem  expedient  to  pay;  (c)  then,  to the
reimbursement of Secured Party of any other  disbursements made by Secured Party
in accordance with the terms hereof or under the Purchase Agreement or any Other
Agreement;  (d) then,  to or among the amounts of fees,  interest and  principal
then owing and unpaid in respect of the Obligations, in such priority as Secured
Party may determine in its  discretion;  and (e) the remainder of such proceeds,
if any,  shall be paid to Debtor.  If such  proceeds  shall be  insufficient  to
discharge the entire  Obligations,  Secured Party shall have any other available
legal recourse  against Debtor under,  or for the  performance  of, the Purchase
Agreement and any Other  Agreement  between  Debtor and Secured  Party,  for the
deficiency,  together with interest  thereon at the maximum rate permitted under
applicable law.

         7.4  Enforcement  of  Obligations.  Nothing in this Agreement or in any
other  document  or  agreement  shall  affect or impair  the  unconditional  and
absolute right of Secured Party to enforce the  Obligations as and when the same
shall become due in accordance with the terms of the Purchase Agreement or Other
Agreement.

                                  ARTICLE VIII

                             RIGHTS OF SECURED PARTY

         8.1  Subrogation.  Upon the occurrence of an Event of Default,  Secured
Party,  at its  election,  may  subrogate  to all of the  interest,  rights  and
remedies  of the  Debtor,  in respect  to any of the  Collateral  or  agreements
pertaining thereto.

         8.2 Secured Party  Appointed  Attorney-in-Fact.  Debtor hereby appoints
Secured Party as  attorney-in-fact  of Debtor,  with full authority in the place
and stead of Debtor and in the name of Debtor, Secured Party or otherwise,  from
time to time on Secured  Party's  discretion and upon the occurrence of an Event
of Default, to take any action and to execute any instrument which Secured Party
may deem  necessary or advisable to accomplish  the purposes of this  Agreement,
including without  limitation:  (a) to ask, demand,  collect,  sue for, recover,
compound, receive and give acquittance and receipts for moneys due and to become
due under or in respect of any of the Collateral;  (b) to receive,  endorse, and
collect  any  drafts or other  instruments,  documents  and  chattel  paper,  in
connection  with clause (a) of this  Section 8.2; (c) to file any claims or take
any action or institute any proceeding which Secured Party may deem necessary or
desirable for the  collection  of any of the  Collateral or otherwise to enforce
the rights of Secured Party against any of the Collateral; and (d) to assign and
transfer the  Collateral,  or any part  thereof,  absolutely  and to execute and
deliver  endorsements,   assignments,  conveyances,  bills  of  sale  and  other
instruments  with power to substitute  one or more persons or  corporation  with
like power.

         8.3  Performance  by Secured  Party.  If Debtor  fails to  perform  any
agreement  contained  herein,  Secured  Party may itself  perform,  or cause the
performance  of, such  agreement,  and the reasonable  expenses of Secured Party
incurred in connection  therewith  shall be payable by Debtor under Section 8.8.
In no  event,  however,  shall  Secured  Party  have any  obligation  or  duties
whatsoever to perform any covenant or agreement of Debtor contained herein,  and
any such performance by Secured Party shall be wholly discretionary with Secured
Party.

         8.4 Duties of Secured  Party.  The powers  conferred upon Secured Party
hereunder  are solely to protect its  interest in the  Collateral  and shall not
impose any duty upon it to exercise any such powers. Except for the safe custody
of any  Collateral  in its  possession  and the  accounting  for money  actually
received by it hereunder,  Secured Party shall have no duty as to any Collateral
or as to the taking of any  necessary  steps to preserve  rights  against  prior
parties or any other rights  pertaining to any Collateral.  Without limiting the
generality of the foregoing,  Secured Party shall not have any obligation,  duty
or  responsibility  to do any of the  following:  (a) ascertain any  maturities,
calls,  conversions,  exchanges,  offers, tenders or similar matters relating to
the  Collateral or informing  Debtor with respect to any such matters;  (b) fix,
preserve  or  exercise  any  right,  privilege  or option  (whether  conversion,
redemption or otherwise) with respect to the Collateral; (c) collect any amounts
payable  in  respect  of the  Collateral;  (d)  sell all or any  portion  of the
Collateral,  for any reason;  or (e) hold the Collateral for or on behalf of any
party other than Debtor.

         8.5 No  Liability  of Secured  Party.  Neither the  acceptance  of this
Agreement by Secured Party,  nor the exercise of any rights hereunder by Secured
Party,  shall be construed in any way as an  assumption  by Secured Party of any
obligations,  responsibilities,  or duties of Debtor arising in connection  with
the  Collateral  assigned  hereunder  or  otherwise  bind  Secured  Party to the
performance of any  obligations  respecting the  Collateral,  it being expressly
understood  that Secured  Party shall not be obligated to perform,  observe,  or
discharge  any  obligation,  responsibility,  duty,  or  liability  of Debtor in
respect of any of the Collateral,  including without limitation  appearing in or
defending any action, expending any money or incurring any expense in connection
therewith.  TO THE FULLEST EXTENT  PERMITTED BY APPLICABLE LAW, DEBTOR SHALL AND
DOES AGREE TO INDEMNIFY,  PROTECT,  DEFEND AND HOLD HARMLESS  SECURED PARTY, ITS
OFFICERS, DIRECTORS, REPRESENTATIVES, AGENTS, EMPLOYEES, LENDERS, SUCCESSORS AND
ASSIGNS,  FROM AND AGAINST ALL  LIABILITIES,  CLAIMS,  DAMAGES,  LOSSES,  FINES,
PENALTIES,  CAUSES OF ACTIONS,  SUITS,  JUDGMENTS AND EXPENSES  (INCLUDING COURT
COSTS,  ATTORNEY'S  FEES  AND  COST OF  INVESTIGATION)  OF ANY  NATURE,  KIND OR
DESCRIPTION  OF ANY PERSON OR ENTITY,  DIRECTLY OR  INDIRECTLY,  ARISING OUT OF,
CAUSED BY OR  RESULTING  FROM (IN WHOLE OR IN PART),  ANY  UNINTENTIONAL  ACT OR
OMISSION (OR  INTENTIONAL  ACT OR OMISSION SO LONG AS SUCH ACT OR OMISSION  DOES
NOT  CONSTITUTE  NEGLIGENCE)  OF SECURED  PARTY (OR  ANYONE  ACTING ON BEHALF OF
SECURED PARTY) IN CONNECTION WITH THE COLLATERAL.  THE FOREGOING INDEMNITY SHALL
SURVIVE THE EXPIRATION OR EARLIER TERMINATION OF THIS AGREEMENT.

         8.6 Right of Secured Party to Defend Action Affecting Security. Secured
Party  may,  at the  expense  of  Debtor,  appear in and  defend  any  action or
proceeding at law or in equity  purporting to affect  Secured  Party's  Security
Interest under this Agreement.

         8.7 Right of  Secured  Party to Prevent  or Remedy  Default.  If Debtor
shall fail to perform any of the covenants,  conditions and agreements  required
to be performed  and  observed by Debtor  under the Purchase  Agreement or Other
Agreement,  or in respect of the Collateral  (subject to any applicable  default
cure  period),  Secured  Party (a) may but shall  not be  obligated  to take any
action Secured Party deems  necessary or desirable to prevent or remedy any such
default by Debtor or otherwise to protect the Security  Interest,  and (b) shall
have the absolute and immediate  right to take  possession of the  Collateral or
any  part  thereof  (to  the  extent  Secured  Party  has not  previously  taken
possession)  to such  extent  and as often  as the  Secured  Party,  in its sole
discretion, deems necessary or desirable in order to prevent or to cure any such
default by Debtor,  or  otherwise  to protect the  security  of this  Agreement.
Secured Party may advance or expend such sums of money for the account of Debtor
as Secured Party in its sole discretion deems necessary for any such purpose.

         8.8 Secured Party's Expenses. All reasonable advances, costs, expenses,
charges and attorneys' fees which Secured Party may make, pay or incur under any
provision  of this  Agreement  for the  protection  of its  security  or for the
enforcement of any of its rights hereunder,  including,  without limitation,  in
foreclosure  proceedings commenced and subsequently  abandoned or in any dispute
or litigation in which Secured Party or the holder of any of the Obligations may
become involved by reason of or arising out of the Purchase Agreement, any Other
Agreement or the  Collateral,  shall be a part of the  Obligations  and shall be
paid by Debtor to Secured Party, upon demand, and shall bear interest until paid
at the maximum  rate of interest  permitted  by  applicable  law,  from the date
incurred by Secured Party until repaid by Debtor.

         8.9. Convertible  Collateral.  Secured Party may present for conversion
any  Collateral  which is  convertible  into any other  instrument or investment
security or a  combination  thereof with cash,  but Secured Party shall not have
any duty to present for conversion any Collateral  unless it shall have received
from Debtor  detailed  written  instructions to that effect at a time reasonably
far in advance of the final conversion date to make such conversion possible.

         8.10 Secured Party's Right of Set-Off.  Upon the happening of any event
entitling  Secured  Party to pursue any remedy  provided  herein,  or if Secured
Party shall be served with garnishment process in which Debtor shall be named as
defendant,  whether or not Debtor  shall be in  default  hereunder  at the time,
Secured Party may, but shall not be required to, set-off any indebtedness  owing
by  Secured  Party  to  Debtor  against  any of the  Obligations  without  first
resorting to the security hereunder and without prejudice to any other rights or
remedies of Secured Party or its Security Interest.

         8.11 Remedies.  No right or remedy herein  reserved to Secured Party is
intended to be exclusive  of any other right or remedy,  but each and every such
remedy shall be cumulative,  not in lieu of, but in addition to any other rights
or remedies given under this Agreement and all other security documents. Any and
all of Secured  Party's  rights and remedies may be exercised  from time to time
and as often as such exercise as deemed necessary or desirable by Secured Party.

         8.12 Debtor's Waivers.  Debtor waives notice of the creation,  advance,
increase,  existence,  extension,  or  renewal  of, and of any  indulgence  with
respect to, the  Obligations;  waives notice of intent to accelerate,  notice of
acceleration,  notice  of  intent  to  demand,  presentment,  demand,  notice of
dishonor,  and  protest;   waives  notice  of  the  amount  of  the  Obligations
outstanding  at any time,  notice of any change in  financial  condition  of any
person liable for the  Obligations  or any part thereof,  notice of any Event of
Default,  and all other  notices  respecting  the  Obligations;  and agrees that
maturity of the Obligations  and any part thereof may be accelerated,  extended,
or renewed one or more times by Secured Party in its discretion,  without notice
to Debtor.

         8.13 Other Parties and Other Collateral.  No renewal or extension of or
any other  indulgence  with respect to the  Obligations or any part thereof,  no
release  of any  security,  no  release  of any  person  (including  any  maker,
endorser,  guarantor,  or  surety)  liable  on  the  Obligations,  no  delay  in
enforcement  of payment,  and no delay or admission or lack of diligence or care
in exercising any right or power with respect to the Obligations or any security
therefor  or  guaranty  thereof or under this  Agreement  shall in other  manner
impair  or affect  the  rights  of  Secured  Party  under  the law,  under  this
Agreement,  or under any other  document or  agreement  pertaining  to the other
security for the  Obligations,  before  foreclosing  upon the Collateral for the
purpose of paying the Obligations.  Debtor waives any right to the benefit of or
to require or control application of any other security or proceeds thereof, and
Debtor  agrees that Secured  Party shall have no duty or obligation to Debtor to
apply to the Obligations any such other security or proceeds thereof.

         8.14 Relationship Among Secured Party.  Either or both of Secured Party
are  entitled  to enforce  any and all rights  granted to Secured  Party in this
Agreement,  and to take any other  action  allowed to be taken by Secured  Party
under this  Agreement.  Neither  Secured  Party is under any  obligation  to act
jointly or in concert with the other Secured Party  pursuant to this  Agreement.
Any action  required  to be taken by Debtor,  or notice  required to be given by
Debtor, shall be taken or given with respect to both of Secured Party.

                                   ARTICLE IX

                                  MISCELLANEOUS

     9.1 Terms  Commercially  Reasonable.  The terms of this Agreement  shall be
deemed commercially reasonable within the meaning of the Texas UCC.

         9.2 Notices.  Any notices or demands  required or permitted to be given
hereunder  shall be  deemed  sufficiently  given if in  writing  and  personally
delivered or mailed (with all postage and charges prepaid), addressed to Secured
Party or to Debtor their respective  addresses set forth below, or at such other
address as the above parties may from time to time  designate by written  notice
to the other given in  accordance  with this Section  9.2.  Any such notice,  if
personally  delivered or  transmitted  by telex or telegram,  shall be deemed to
have been given on the date so delivered or transmitted or, if mailed, be deemed
to have been given on the day after such  notice is placed in the United  States
mail in accordance with this Section 9.2.

                  Secured Party:  1301 Capital of Texas Hwy., Suite C-300
                                  Austin, Travis County, Texas 78746
                                  Attn: President

                  with copy to:   Timothy L. LaFrey, Esq.
                                  Akin, Gump, Strauss, Hauer & Feld, L.L.P.
                                  1900 Frost Bank Plaza
                                  816 Congress Avenue
                                  Austin, Texas 78701

                  Debtor:         Medical Vision Technology Profit Sharing Plan
                                  FBO Stephen Wilmarth, M.D.
                                  9824 Carlton Court
                                  Granite Bay, California   95746

         9.3 Parties Bound.  Secured Party's rights under this Agreement and the
Security Interest shall inure to the benefits of its successors and assigns, and
in the event of any  assignment  or  transfer of any of the  Obligations  or the
Collateral,  Secured  Party  thereafter  shall  be  fully  discharged  from  any
responsibility  with respect to the Collateral so assigned or  transferred,  but
Secured  Party shall  retain all rights and powers  hereby given with respect to
any of the  Obligations  or  Collateral  not so  assigned  or  transferred.  All
representations, warranties, and agreements of Debtor if more than one are joint
and several, and all shall be binding upon the personal representatives,  heirs,
successors, and assigns of Debtor.

         9.4 Waiver.  No delay of Secured Party in exercising any power or right
shall operate as a waiver thereof;  nor shall any single or partial  exercise of
any power or right preclude other or further exercise thereof or the exercise of
any other power or right.  No waiver by Secured Party of any right  hereunder of
any default by Debtor shall be binding upon Secured Party unless in writing, and
no failure by Secured  Party to exercise any power or right  hereunder or waiver
of any  default  by Debtor  shall  operate  as a waiver of any other or  further
exercise of such right or power of any further default.

         9.5 Agreement Continuing.  This Agreement shall constitute a continuing
agreement,  applying to all future as well as existing transactions,  whether or
not of the  character  contemplated  at the date of this  Agreement,  and if all
transactions between Secured Party and Debtor shall be closed at any time, shall
be equally  applicable to any new  transactions  thereafter.  Provisions of this
Agreement,  unless  by their  terms  exclusive,  shall be in  addition  to those
contained in any Other Agreement.

         9.6 Definitions. Unless the context indicated otherwise, definitions in
the Texas Business and Commerce Code ("Texas UCC") apply to words and phrases in
this Agreement; if Texas UCC definitions conflict, Chapter 9 definitions apply.

         9.7 Miscellaneous. In this Agreement, whenever the context so requires,
the neuter gender  includes the masculine and feminine,  and the singular number
includes  the plural and vice  versa.  The  headings  of  paragraphs  herein are
inserted only for convenience and shall in no way define,  describe or limit the
scope of intent of any  provisions  of this  Agreement.  No  change,  amendment,
modification,  cancellation,  or  discharge of any  provision of this  Agreement
shall be valid unless consented to in writing by Secured Party.

         9.8 Assignment of Secured  Party's  Interest.  Secured Party shall have
the right to assign all or any portion of its rights in this  Agreement  without
approval or consent.  Debtor may not assign this  Agreement or any of its rights
or obligations  hereunder  without the express prior written  consent of Secured
Party in each instance.

     9.9 Applicable  Laws.  THIS AGREEMENT SHALL BE GOVERNED BY AND CONSTRUED IN
ACCORDANCE  WITH THE LAWS OF THE STATE OF TEXAS AND THE  APPLICABLE  LAWS OF THE
UNITED STATES OF AMERICA.

     9.10 ENTIRE AGREEMENT.  THIS AGREEMENT AND THE PURCHASE AGREEMENT REPRESENT
THE FINAL AGREEMENT  BETWEEN THE PARTIES AND MAY NOT BE CONTRADICTED BY EVIDENCE
OF PRIOR,  CONTEMPORANEOUS,  OR SUBSEQUENT ORAL AGREEMENTS OF THE PARTIES. THERE
ARE NO UNWRITTEN ORAL AGREEMENTS BETWEEN THE PARTIES.

                            [Signature page follows]


<PAGE>


S-1



                                SIGNATURE PAGE TO

                                 ASSIGNMENT AND

                               SECURITY AGREEMENT

         EXECUTED this 1st day of September, 1999.



DEBTOR:                                   /s/ Stephen Wilmarth, M.D.

                                          Printed Name: Stephen Wilmarth, M.D.


SECURED PARTY:                            Prime Medical Operating, Inc.

                                          By: /s/ Cheryl Williams

                                          Name: Cheryl Williams

                                          Title: Vice President


                                          Prime/BDR Acquisition, L.L.C.

                                          By: /s/ Cheryl Williams

                                          Name: Cheryl Williams

                                          Title: Vice President

<PAGE>



                        ASSIGNMENT AND SECURITY AGREEMENT

     THIS  ASSIGNMENT  AND SECURITY  AGREEMENT  (this  "Agreement")  is made and
entered into as of the 1st day of September,  1999, by and between Prime Medical
Operating, Inc., a Delaware corporation ("PMOI"), Prime/BDR Acquisition, L.L.C.,
a Delaware limited  liability  company ("Prime") (PMOI and Prime are referred to
herein, jointly and severally, as "Secured Party") and Medical Vision Technology
Profit Sharing Plan for the benefit of Stephen Wilmarth, M.D. (the "Debtor").

                                    RECITALS:

         A. Debtor and Secured Party have  executed and  delivered  that certain
Stock  Purchase   Agreement  dated  as  of  September  1,  1999  (the  "Purchase
Agreement"),  pursuant to which  Secured  Party  purchased  from Debtor  certain
shares of the $0.01 par value  common  stock of Horizon  Vision  Center,  Inc, a
Nevada corporation ("Horizon").

         B. Secured Party has requested  that Debtor pledge the  Collateral  (as
defined below) to secure certain obligations and liabilities that Debtor may now
or hereafter have to Secured Party, including, without limitation, any indemnity
obligations arising under the Purchase Agreement.

     C. Debtor desires to enter into this Agreement as a material  inducement to
Secured  Party's  purchase  of  Debtor's  shares of Horizon  under the  Purchase
Agreement.

                                   AGREEMENT:

         NOW, THEREFORE, in consideration of the foregoing and the covenants and
agreements hereinafter set forth, and other good and valuable consideration, the
receipt and sufficiency of which Debtor  acknowledges,  Debtor and Secured Party
agree as follows:

                                    ARTICLE I

                       COLLATERAL AND SECURED OBLIGATIONS

         1.1 Grant of Security Interest.  Debtor hereby assigns,  transfers, and
pledges to Secured  Party,  and Debtor hereby grants to Secured Party a security
interest   in,   the   following   described   collateral   (collectively,   the
"Collateral"):

                  (a)  Shares  of  Horizon.  From  and  after  the  date of this
Agreement,  (i) all shares of the common  stock of Horizon  owned or acquired in
any manner by Debtor or Secured Party,  (collectively,  the "Shares"),  (ii) any
replacements,  substitutions,  or exchanges of the certificates representing the
Shares, and (iii) any and all options,  rescission rights,  registration rights,
conversion rights, subscription rights, contractual or quasi-contractual rights,
warrants,  redemption rights,  redemption proceeds, calls, preemptive rights and
all other rights and benefits pertaining to the Shares;

                  (b)  Accounts.  All  accounts  and  rights  now  or  hereafter
attributable to any of the Collateral  described in (a) above, and all rights of
Debtor now or hereafter arising under any agreement pertaining to the Collateral
described  in  (a)  above,   including  without  limitation  all  distributions,
proceeds, fees, dividends,  preferences,  payments or other benefits of whatever
nature  which  Debtor is now or may  hereafter  become  entitled to receive with
respect to any Collateral described in (a) above;

                  (c) Additional  Property.  "Collateral" shall also include the
following  property  (collectively,  the  "Additional  Property")  which  Debtor
becomes  entitled  to receive or shall  receive  in  connection  with any of the
Collateral  described in this Section 1.1: (i) any stock certificate,  including
without  limitation,  any  certificate  representing  a  stock  dividend  or any
certificate in connection with any recapitalization,  reclassification,  merger,
consolidation,  conversion,  sale of assets, combination of shares, stock split,
reverse  stock split or  spin-off;  (ii) any option,  warrant,  subscription  or
right,  whether as an addition to or in  substitution  of any of the  Collateral
described in this Section 1.1; (iii) any dividends or  distributions of any kind
whatsoever,  whether  distributable in cash,  stock or other property;  (iv) any
interest,  premium or principal  payments;  and (v) any conversion or redemption
proceeds; and

                  (d) Proceeds.  All proceeds (cash and non-cash) arising out of
the sale, exchange, collection or other disposition of all or any portion of the
Collateral  described in (a), (b), or (c) above,  including  without  limitation
proceeds in the form of stock, accounts, chattel paper, instruments,  documents,
goods, inventory and equipment.

The  security  interest in the  Collateral  hereby  granted by Debtor to Secured
Party may sometimes be referred to in this Agreement as the "Security Interest".

         1.2 Obligations.  This Agreement and the Security Interest shall secure
full and punctual payment and performance of the following indebtedness,  duties
and obligations (collectively, the "Obligations"):

                  (a) All liabilities and obligations of Debtor to Secured Party
(including, without limitation, any principal, interest, fees and other amounts,
and any  indemnity  obligations)  under and pursuant to the Purchase  Agreement,
this Agreement and/or any other contract or agreement  between Secured Party and
Debtor or any affiliate of Debtor (collectively, "Other Agreements"; and

                  (b) (i)  all  indebtedness,  obligations  and  liabilities  of
Debtor and/or any affiliate of Debtor to Secured Party of any kind or character,
now existing or hereafter arising, whether direct, indirect, related, unrelated,
fixed,  contingent,  liquidated,  unliquidated,  joint,  several  or  joint  and
several,  arising from,  connected with, or related to the Purchase Agreement or
any Other Agreement, or any other document, agreement, or instrument executed in
connection  either of them,  (ii) all accrued but unpaid  interest on any of the
indebtedness  described in (i) above, (iii) all obligations of Debtor and/or any
affiliate  of  Debtor  to  Secured  Party  under  any  documents  or  agreements
evidencing,  securing,  governing  and/or  pertaining  to all or any part of the
indebtedness  described  in (i) and (ii)  above,  (iv) all  costs  and  expenses
incurred by Secured Party in connection  with the collection and  administration
of all or any part of the  indebtedness  and obligations  described in (i), (ii)
and (iii) above or the protection or preservation  of, or realization  upon, the
collateral  securing  all or any  part of  such  indebtedness  and  obligations,
including  without  limitation  all  attorneys'  fees,  and  (v)  all  renewals,
extensions, modifications and rearrangements of the indebtedness and obligations
described in (i), (ii), (iii) and (iv) above.

                  (c) All sums now or  hereafter  loaned or  advanced by Secured
Party to  Debtor,  or  expended  by Secured  Party for the  account of Debtor or
otherwise owing by Debtor to Secured Party, in respect of the  Obligations,  and
all other sums  expended or advanced  by Secured  Party  pursuant to any term or
provision of this Agreement or any Other Agreement (i) to collect and/or enforce
the Obligations or (ii) to maintain, protect and preserve the Collateral.

                                   ARTICLE II

       DEBTOR'S REPRESENTATIONS AND WARRANTIES WITH RESPECT TO COLLATERAL

       Debtor hereby represents and warrants to Secured Party as follows:

         2.1 Ownership of Collateral.  Debtor has good and  marketable  title to
the Collateral  free and clear of any liens,  security  interests,  shareholders
agreement, calls, charge, or encumbrance,  except for this Security Interest. No
financing  statement or other  instrument  similar in effect covering all or any
part of the  Collateral is on file in any recording  office,  except as may have
been filed in favor of Secured Party relating to this Agreement.

         2.2  Power  &  Authority.  Debtor  has the  lawful  right,  power,  and
authority  to grant the Security  Interest in the  Collateral.  This  Agreement,
together  with all filings and other  actions  necessary or desirable to perfect
and protect such security interest,  which have been duly taken,  create a valid
and perfected first priority  security  interest in the Collateral  securing the
payment and performance of the Obligations.

         2.3  No  Agreements.  The  Shares  are  not  subject  to any  right  of
redemption,  or any  call or put  options,  voting  trust,  proxy,  shareholders
agreement,  right of first  refusal,  or any other  document or agreement  which
would in any way impair or adversely affect this Security Interest or the rights
of Secured Party under this Agreement.

         2.4  Securities.  Any  certificates  evidencing  securities  pledged as
Collateral  are valid and  genuine  and have not been  altered.  All  securities
pledged as Collateral have been duly  authorized and validly  issued,  are fully
paid and  non-assessable,  and were not issued in  violation  of the  preemptive
rights of any party or of any agreement by which Debtor or the issuer thereof is
bound.  No  restrictions  or  conditions  exist with  respect to the transfer or
voting of any securities pledged as Collateral.

                                   ARTICLE III

                  DEBTOR'S OTHER REPRESENTATIONS AND WARRANTIES

         3.1 Solvency of Debtor.  As of the date hereof,  (i) Debtor is solvent;
(ii) the fair saleable  value of Debtor's  assets exceeds  Debtor's  liabilities
(both fixed and contingent);  (iii) Debtor has sufficient capital to satisfy all
of Debtor's  obligations  as they  become due;  (iv) no  receiver,  trustee,  or
custodian has been appointed for, or taken  possession of, all or  substantially
all of the assets of Debtor,  either in a  proceeding  brought by Debtor or in a
proceeding  brought against Debtor;  (v) Debtor is not the subject of a petition
for relief under the United  States  Bankruptcy  Code or any similar  federal or
state insolvency law, including without limitation a petition filed by Debtor or
a petition filed by a third party seeking relief against Debtor; and (vi) Debtor
has no  intention  of filing a  petition  for  relief  under the  United  States
Bankruptcy  Code or any similar  federal or state  insolvency law, or of seeking
any other form of  creditor  relief,  within  the  two-year  period  immediately
following the date of this Agreement.

         3.2  Authority and  Compliance.  Debtor has full power and authority to
enter into this Agreement. Debtor has full power and authority to enter into and
perform its obligations  under the Purchase  Agreement and each Other Agreement.
No further  consent or approval is  required as a condition  to the  validity of
this  Agreement,  the Purchase  Agreement or any Other  Agreement.  Debtor is in
compliance with all applicable laws, ordinances,  statutes, orders, regulations,
judgments, writs, or decrees of any governmental entity to which it is subject.

         3.3 Binding Agreement.  This Agreement, the Purchase Agreement and each
Other Agreement  constitute valid and legally binding  obligations of Debtor, in
accordance  with  their  terms,  subject  to  (a)  the  applicable   bankruptcy,
insolvency,  reorganization,  moratorium,  and similar laws affecting creditors'
rights  generally  and  (b)  restrictions  imposed  by any  court  of  competent
jurisdiction on the enforcement of non-competition and exclusive use restrictive
covenants imposed under the Purchase Agreement or any Other Agreement.

         3.4 Litigation.  There are no proceedings  pending or, to the knowledge
of Debtor,  threatened before any court or  administrative  agency which will or
may have a material adverse effect on the financial  condition of Debtor or upon
Debtor's ability to perform its obligations  under this Agreement,  the Purchase
Agreement or any Other Agreement.

         3.5 No Conflicting Agreements.  There are no provisions of any existing
agreement,  mortgage,  indenture or contract  binding on Debtor or affecting its
property,  which  would  conflict  with or in any  way  prevent  the  execution,
delivery, or carrying out of the terms of this Agreement, the Purchase Agreement
or any Other Agreement.

         3.6  Ownership  of  Assets.  Debtor  has  good  and  full  title to the
Collateral,  and the  Collateral  is owned  free and  clear of  liens,  charges,
claims, security interests, and other encumbrances.

     3.7 Taxes. Debtor has filed all tax returns required to be filed by Debtor.

                                   ARTICLE IV

                  DEBTOR'S COVENANTS WITH RESPECT TO COLLATERAL

         Debtor  covenants  and agrees  that from the date  hereof and until the
payment  and  performance  in  full  of the  Obligations  unless  Secured  Party
otherwise consents in writing:

         4.1  Delivery of  Instruments  and/or  Certificates.  Contemporaneously
herewith,   Debtor  covenants  and  agrees  to  deliver  to  Secured  Party  any
certificates,   documents,   or  instruments   representing  or  evidencing  the
Collateral,  with Debtor's  endorsement  thereon and/or  accompanied by property
instruments of transfer and assignment duly executed in blank with, if requested
by Secured Party,  signatures  guaranteed by a member or member  organization in
good standing of an authorized Securities Transfer Agents Medallion Program, all
in form and substance satisfactory to Secured Party.

         4.2  Further  Assurances.   Debtor  will   contemporaneously  with  the
execution  hereof  and from  time to time  thereafter  at its  expense  promptly
execute and deliver all further  instruments  and documents and take all further
action  necessary or  appropriate or that Secured Party may request in order (i)
to perfect and protect the security  interest created or purported to be created
hereby and the first priority of such security interest,  (ii) to enable Secured
Party to exercise  and enforce its rights and  remedies  hereunder in respect of
the  Collateral,  and (iii) to otherwise  effect the purposes of this Agreement,
including  without  limitation:  (A)  executing  and  filing  any  financing  or
continuation  statements,  or any  amendments  thereto;  (B)  obtaining  written
confirmation  from the issuer of any  securities  pledged as  Collateral  of the
pledge of such securities,  in form and substance satisfactory to Secured Party;
(C)  cooperating  with Secured Party in registering the pledge of any securities
pledged as Collateral with the issuer of such securities;  (D) delivering notice
of Secured Party's security interest in any securities  pledged as Collateral to
any securities or financial  intermediary,  clearing  corporation or other party
required by Secured Party, in form and substance  satisfactory to Secured Party;
and  (E)  obtaining  written  confirmation  of  the  pledge  of  any  securities
constituting Collateral from any securities or financial intermediary,  clearing
corporation  or other party  required by Secured  Party,  in form and  substance
satisfactory to Secured Party.

         4.3 Additional Property. All Additional Property, as defined in Section
1.1(c)  above,  received by Debtor shall be received in trust for the benefit of
Secured Party.  All Additional  Property and all  certificates  or other written
instruments or documents  evidencing and/or representing the Additional Property
that is  received  by Debtor,  together  with such  instruments  of  transfer as
Secured Party may request,  shall  immediately be delivered to or deposited with
Secured Party and held by Secured  Party as  Collateral  under the terms of this
Agreement.  If the  Additional  Property  received  by Debtor and  delivered  to
Secured  Party  pursuant  to this  Section  shall  be  shares  of stock or other
securities,  such shares of stock or other  securities shall be duly endorsed in
blank or  accompanied  by proper  instruments  of transfer and  assignment  duly
executed in blank with, if requested by Secured Party,  signatures guaranteed by
a member or member  organization  in good standing of an  authorized  Securities
Transfer Agents  Medallion  Program,  all in form and substance  satisfactory to
Secured  Party.  Secured  Party  shall  be  deemed  to  have  possession  of any
Collateral in transit to Secured Party or its agent.

         4.4  Sale,  Transfer,  Encumbrance.  Debtor  will not  sell,  transfer,
mortgage,  or otherwise  encumber any  Collateral or impair the value thereof in
any manner without  Secured  Party's prior written  consent,  including  without
limitation by purchase, lease, barter, trade, payment deferral, or the creation,
assumption  or guarantee of  indebtedness  or other  lending of credit.  Secured
Party's written consent to any sale,  mortgage,  transfer,  or encumbrance shall
not be construed to be a waiver of this  provision in respect to any  subsequent
proposed sale, mortgage, transfer, or encumbrance.

         4.5 Liens. Neither Debtor nor any person acting on Debtor's behalf has,
or shall have any right, power, or authority to and shall not create,  incur, or
permit to be placed or  imposed,  upon the  Collateral,  any lien of any type or
nature whatsoever, other than the liens in favor of Secured Party.

         4.6 Matters or  Occurrences  Affecting  Collateral  or this  Agreement.
Debtor will promptly  notify Secured Party of any and all matters or occurrences
that may have a material adverse effect on the status or value of the Collateral
or this Agreement,  including  without  limitation the occurrence of an Event of
Default,  or an event  which,  with giving of notice or lapse of time,  or both,
would constitute an Event of Default.

     4.7  Agreements  Pertaining to  Collateral.  Debtor will not enter into any
type of contract or agreement  pertaining to any of the Collateral or in any way
transfer any voting rights pertaining to the Collateral to any person or entity.

         4.8 Dilution of Ownership.  As to any securities pledged as Collateral,
Debtor  will not  consent to or approve of the  issuance  of (i) any  additional
shares  of  any  class  of  securities  of  such  issuer,  (ii)  any  instrument
convertible  voluntarily  by  the  holder  thereof  or  automatically  upon  the
occurrence or  non-occurrence  of any event or condition  into, or  exchangeable
for, any such  securities,  or (iii) any warrants,  options,  contracts or other
commitments  entitling any third party to purchase or otherwise acquire any such
securities.  Notwithstanding  the  foregoing  or any  other  provision  of  this
Agreement to the contrary,  Debtor (i) shall comply with his  obligations  under
the  Purchase  Agreement  and each Other  Document,  and (ii) may consent to any
issuance of shares of Horizon if such  issuance has been  approved by a majority
of the Board of Directors of Horizon.

         4.9  Restrictions  on  Securities.  Debtor  will  not  enter  into  any
agreement  creating,  or otherwise permit to exist, any restriction or condition
upon the transfer,  voting or control of any  securities  pledged as Collateral,
except as consented to in writing by Secured Party. As to any securities pledged
as collateral, Debtor will not consent to or approve of any stock split, reverse
stock  split,  stock  dividend,  reclassification,   or  other  similar  act  or
transaction  regarding  the  Shares  unless  consented  to in writing by Secured
Party.

                                    ARTICLE V

                         DEBTOR'S AFFIRMATIVE COVENANTS

         Until payment and performance of all Obligations,  Debtor covenants and
agrees that it shall promptly  advise Secured Party in writing of any litigation
filed  against  Debtor  and of any  condition,  event or act which  comes to its
attention  that  would or might  have a  material  adverse  effect  on  Debtor's
financial condition or on Debtor's ability to perform the Obligations.

                                   ARTICLE VI

                               NEGATIVE COVENANTS

         Until payment and performance of all Obligations,  Debtor covenants and
agrees that Debtor will not, without the prior written consent of Secured Party:

     6.1 Liens. Grant, suffer, or permit liens on, or security interests in, the
Collateral.

         6.2  Violate  Other  Covenants.  Violate  or fail to  comply  with  any
covenants  or  agreements  regarding  other  debt  which  will or would with the
passage  of time or upon  demand  cause the  maturity  of any  other  debt to be
accelerated.

                                   ARTICLE VII

                              DEFAULT AND REMEDIES

     7.1 Events of Default.  An Event of Default  (herein so called) shall exist
if any one or more of the following events shall occur:

                  (a) The  failure  of Debtor to pay any amount  required  to be
paid under the Purchase Agreement (including,  without limitation, the indemnity
provisions  contained  therein),  or any other  amount  which  Debtor may now or
hereafter  owe to Secured Party under any Other  Agreement or otherwise,  within
(15) calendar days after such amount is due;

     (b) The failure of Debtor to pay any  Obligation  within (15) calendar days
after such amount is due; and

                  (c)  Debtor's  breach of a covenant in this  Agreement  or any
other  failure to perform  its  obligations  under this  Agreement  or any Other
Agreement.

     7.2 Secured Party's Remedies. Upon the occurrence of an Event of Default:

                  (a) Secured Party may declare the Obligations in whole or part
immediately due and may enforce payment and performance of the same and exercise
any rights under the Texas UCC,  rights and remedies of Secured Party under this
Agreement, or otherwise.

                  (b) Secured  Party may, at Secured  Party's  option and at the
expense of Debtor,  either in Secured Party's own right or in the name of Debtor
and in the same manner and to the same extent that Debtor  might  reasonably  so
act  if  this  Agreement  had  not  been  made:  (i) do  all  things  requisite,
convenient,  or  necessary  to enforce the  performance  and  observance  of all
rights,  remedies and privileges of Debtor arising from the  Collateral,  or any
part thereof,  including without limitation compromising,  waiving, excusing, or
in any manner releasing or discharging any obligation of any party to or arising
from the Collateral;  (ii) take possession of the books, papers,  chattel paper,
documents of title, and accounts of Debtor,  wherever  located,  relating to the
Collateral; (iii) sue or otherwise collect and receive money attributable to the
Collateral;  and (iv) exercise any other lawfully  available powers or remedies,
and do all other things  which  Secured  Party deems  requisite,  convenient  or
necessary  or which the  Secured  Party  deems  proper to protect  the  Security
Interest.

                  (b) Secured Party may foreclose  this  Agreement in the manner
now or  hereafter  provided  or  permitted  by law and may upon such  reasonable
notification  prior thereto as may be required by applicable  law (Debtor hereby
agreeing  that ten  days'  notice is  commercially  reasonable),  sell,  assign,
transfer,  or otherwise  dispose of the Collateral at public or private sale, in
whole  or in  part,  and  Secured  Party  may,  in its own  name or as  Debtor's
attorney-in-fact  effectively  assign and transfer the  Collateral,  or any part
thereof,   absolutely,  and  execute  and  deliver  all  necessary  assignments,
conveyances,  bills of sale, and other  instruments with power to substitute one
or more persons or  corporations  with like power.  Any such  foreclosure  sale,
assignment,  transfer,  or other  disposition  shall, to the extent permitted by
law,  be a  perpetual  bar,  both at law and in equity,  against  Debtor and all
persons and corporations  lawfully  claiming by or through or under Debtor.  Any
such foreclosure sale may be adjourned from time to time. Upon any sale, Secured
Party may bid for and purchase the  Collateral,  or any part  thereof,  and upon
compliance with the terms of sale may hold,  retain,  possess and dispose of the
Collateral, in its absolute right without further accountability.  Secured Party
shall have the right to be  credited  on the  amount of its bid a  corresponding
amount of the Obligations as of the date of such sale.

                  (c) If, in the opinion of Secured Party, there is any question
that a public sale or  distribution  of any Collateral will violate any state or
federal  securities  law,  Secured  Party  (i) may  offer  and  sell  securities
privately to purchasers who will agree to take them for investment  purposes and
not with a view to distribution  and who will agree to imposition of restrictive
legends on the  certificates  representing  the security,  or (ii) may sell such
securities in an intrastate  offering  under Section  3(a)(11) of the Securities
Act of 1933,  and no sale so made in good faith by Secured Party shall be deemed
to be not "commercially reasonable" because so made.

                  (d)  Not  in  limitation  of  any  other   provision  of  this
Agreement,  Secured  Party shall have all rights and remedies of a secured party
under the Texas UCC.

         7.3  Application  of Proceeds.  Secured Party may apply the proceeds of
any foreclosure  sale hereunder or from any other  permitted  disposition of the
Collateral  or any part  thereof as  follows:  (a) first,  to the payment of all
reasonable  costs and expenses of any foreclosure  and collection  hereunder and
all proceedings in connection  therewith,  including reasonable attorneys' fees;
(b) then, to the  reimbursement of Secured Party for all  disbursements  made by
Secured Party for taxes,  assessments or liens superior to the Security Interest
and  which  Secured  Party  shall  deem  expedient  to  pay;  (c)  then,  to the
reimbursement of Secured Party of any other  disbursements made by Secured Party
in accordance with the terms hereof or under the Purchase Agreement or any Other
Agreement;  (d) then,  to or among the amounts of fees,  interest and  principal
then owing and unpaid in respect of the Obligations, in such priority as Secured
Party may determine in its  discretion;  and (e) the remainder of such proceeds,
if any,  shall be paid to Debtor.  If such  proceeds  shall be  insufficient  to
discharge the entire  Obligations,  Secured Party shall have any other available
legal recourse  against Debtor under,  or for the  performance  of, the Purchase
Agreement and any Other  Agreement  between  Debtor and Secured  Party,  for the
deficiency,  together with interest  thereon at the maximum rate permitted under
applicable law.

         7.4  Enforcement  of  Obligations.  Nothing in this Agreement or in any
other  document  or  agreement  shall  affect or impair  the  unconditional  and
absolute right of Secured Party to enforce the  Obligations as and when the same
shall become due in accordance with the terms of the Purchase Agreement or Other
Agreement.

                                  ARTICLE VIII

                             RIGHTS OF SECURED PARTY

         8.1  Subrogation.  Upon the occurrence of an Event of Default,  Secured
Party,  at its  election,  may  subrogate  to all of the  interest,  rights  and
remedies  of the  Debtor,  in respect  to any of the  Collateral  or  agreements
pertaining thereto.

         8.2 Secured Party  Appointed  Attorney-in-Fact.  Debtor hereby appoints
Secured Party as  attorney-in-fact  of Debtor,  with full authority in the place
and stead of Debtor and in the name of Debtor, Secured Party or otherwise,  from
time to time on Secured  Party's  discretion and upon the occurrence of an Event
of Default, to take any action and to execute any instrument which Secured Party
may deem  necessary or advisable to accomplish  the purposes of this  Agreement,
including without  limitation:  (a) to ask, demand,  collect,  sue for, recover,
compound, receive and give acquittance and receipts for moneys due and to become
due under or in respect of any of the Collateral;  (b) to receive,  endorse, and
collect  any  drafts or other  instruments,  documents  and  chattel  paper,  in
connection  with clause (a) of this  Section 8.2; (c) to file any claims or take
any action or institute any proceeding which Secured Party may deem necessary or
desirable for the  collection  of any of the  Collateral or otherwise to enforce
the rights of Secured Party against any of the Collateral; and (d) to assign and
transfer the  Collateral,  or any part  thereof,  absolutely  and to execute and
deliver  endorsements,   assignments,  conveyances,  bills  of  sale  and  other
instruments  with power to substitute  one or more persons or  corporation  with
like power.

         8.3  Performance  by Secured  Party.  If Debtor  fails to  perform  any
agreement  contained  herein,  Secured  Party may itself  perform,  or cause the
performance  of, such  agreement,  and the reasonable  expenses of Secured Party
incurred in connection  therewith  shall be payable by Debtor under Section 8.8.
In no  event,  however,  shall  Secured  Party  have any  obligation  or  duties
whatsoever to perform any covenant or agreement of Debtor contained herein,  and
any such performance by Secured Party shall be wholly discretionary with Secured
Party.

         8.4 Duties of Secured  Party.  The powers  conferred upon Secured Party
hereunder  are solely to protect its  interest in the  Collateral  and shall not
impose any duty upon it to exercise any such powers. Except for the safe custody
of any  Collateral  in its  possession  and the  accounting  for money  actually
received by it hereunder,  Secured Party shall have no duty as to any Collateral
or as to the taking of any  necessary  steps to preserve  rights  against  prior
parties or any other rights  pertaining to any Collateral.  Without limiting the
generality of the foregoing,  Secured Party shall not have any obligation,  duty
or  responsibility  to do any of the  following:  (a) ascertain any  maturities,
calls,  conversions,  exchanges,  offers, tenders or similar matters relating to
the  Collateral or informing  Debtor with respect to any such matters;  (b) fix,
preserve  or  exercise  any  right,  privilege  or option  (whether  conversion,
redemption or otherwise) with respect to the Collateral; (c) collect any amounts
payable  in  respect  of the  Collateral;  (d)  sell all or any  portion  of the
Collateral,  for any reason;  or (e) hold the Collateral for or on behalf of any
party other than Debtor.

         8.5 No  Liability  of Secured  Party.  Neither the  acceptance  of this
Agreement by Secured Party,  nor the exercise of any rights hereunder by Secured
Party,  shall be construed in any way as an  assumption  by Secured Party of any
obligations,  responsibilities,  or duties of Debtor arising in connection  with
the  Collateral  assigned  hereunder  or  otherwise  bind  Secured  Party to the
performance of any  obligations  respecting the  Collateral,  it being expressly
understood  that Secured  Party shall not be obligated to perform,  observe,  or
discharge  any  obligation,  responsibility,  duty,  or  liability  of Debtor in
respect of any of the Collateral,  including without limitation  appearing in or
defending any action, expending any money or incurring any expense in connection
therewith.  TO THE FULLEST EXTENT  PERMITTED BY APPLICABLE LAW, DEBTOR SHALL AND
DOES AGREE TO INDEMNIFY,  PROTECT,  DEFEND AND HOLD HARMLESS  SECURED PARTY, ITS
OFFICERS, DIRECTORS, REPRESENTATIVES, AGENTS, EMPLOYEES, LENDERS, SUCCESSORS AND
ASSIGNS,  FROM AND AGAINST ALL  LIABILITIES,  CLAIMS,  DAMAGES,  LOSSES,  FINES,
PENALTIES,  CAUSES OF ACTIONS,  SUITS,  JUDGMENTS AND EXPENSES  (INCLUDING COURT
COSTS,  ATTORNEY'S  FEES  AND  COST OF  INVESTIGATION)  OF ANY  NATURE,  KIND OR
DESCRIPTION  OF ANY PERSON OR ENTITY,  DIRECTLY OR  INDIRECTLY,  ARISING OUT OF,
CAUSED BY OR  RESULTING  FROM (IN WHOLE OR IN PART),  ANY  UNINTENTIONAL  ACT OR
OMISSION (OR  INTENTIONAL  ACT OR OMISSION SO LONG AS SUCH ACT OR OMISSION  DOES
NOT  CONSTITUTE  NEGLIGENCE)  OF SECURED  PARTY (OR  ANYONE  ACTING ON BEHALF OF
SECURED PARTY) IN CONNECTION WITH THE COLLATERAL.  THE FOREGOING INDEMNITY SHALL
SURVIVE THE EXPIRATION OR EARLIER TERMINATION OF THIS AGREEMENT.

         8.6 Right of Secured Party to Defend Action Affecting Security. Secured
Party  may,  at the  expense  of  Debtor,  appear in and  defend  any  action or
proceeding at law or in equity  purporting to affect  Secured  Party's  Security
Interest under this Agreement.

         8.7 Right of  Secured  Party to Prevent  or Remedy  Default.  If Debtor
shall fail to perform any of the covenants,  conditions and agreements  required
to be performed  and  observed by Debtor  under the Purchase  Agreement or Other
Agreement,  or in respect of the Collateral  (subject to any applicable  default
cure  period),  Secured  Party (a) may but shall  not be  obligated  to take any
action Secured Party deems  necessary or desirable to prevent or remedy any such
default by Debtor or otherwise to protect the Security  Interest,  and (b) shall
have the absolute and immediate  right to take  possession of the  Collateral or
any  part  thereof  (to  the  extent  Secured  Party  has not  previously  taken
possession)  to such  extent  and as often  as the  Secured  Party,  in its sole
discretion, deems necessary or desirable in order to prevent or to cure any such
default by Debtor,  or  otherwise  to protect the  security  of this  Agreement.
Secured Party may advance or expend such sums of money for the account of Debtor
as Secured Party in its sole discretion deems necessary for any such purpose.

         8.8 Secured Party's Expenses. All reasonable advances, costs, expenses,
charges and attorneys' fees which Secured Party may make, pay or incur under any
provision  of this  Agreement  for the  protection  of its  security  or for the
enforcement of any of its rights hereunder,  including,  without limitation,  in
foreclosure  proceedings commenced and subsequently  abandoned or in any dispute
or litigation in which Secured Party or the holder of any of the Obligations may
become involved by reason of or arising out of the Purchase Agreement, any Other
Agreement or the  Collateral,  shall be a part of the  Obligations  and shall be
paid by Debtor to Secured Party, upon demand, and shall bear interest until paid
at the maximum  rate of interest  permitted  by  applicable  law,  from the date
incurred by Secured Party until repaid by Debtor.

         8.9. Convertible  Collateral.  Secured Party may present for conversion
any  Collateral  which is  convertible  into any other  instrument or investment
security or a  combination  thereof with cash,  but Secured Party shall not have
any duty to present for conversion any Collateral  unless it shall have received
from Debtor  detailed  written  instructions to that effect at a time reasonably
far in advance of the final conversion date to make such conversion possible.

         8.10 Secured Party's Right of Set-Off.  Upon the happening of any event
entitling  Secured  Party to pursue any remedy  provided  herein,  or if Secured
Party shall be served with garnishment process in which Debtor shall be named as
defendant,  whether or not Debtor  shall be in  default  hereunder  at the time,
Secured Party may, but shall not be required to, set-off any indebtedness  owing
by  Secured  Party  to  Debtor  against  any of the  Obligations  without  first
resorting to the security hereunder and without prejudice to any other rights or
remedies of Secured Party or its Security Interest.

         8.11 Remedies.  No right or remedy herein  reserved to Secured Party is
intended to be exclusive  of any other right or remedy,  but each and every such
remedy shall be cumulative,  not in lieu of, but in addition to any other rights
or remedies given under this Agreement and all other security documents. Any and
all of Secured  Party's  rights and remedies may be exercised  from time to time
and as often as such exercise as deemed necessary or desirable by Secured Party.

         8.12 Debtor's Waivers.  Debtor waives notice of the creation,  advance,
increase,  existence,  extension,  or  renewal  of, and of any  indulgence  with
respect to, the  Obligations;  waives notice of intent to accelerate,  notice of
acceleration,  notice  of  intent  to  demand,  presentment,  demand,  notice of
dishonor,  and  protest;   waives  notice  of  the  amount  of  the  Obligations
outstanding  at any time,  notice of any change in  financial  condition  of any
person liable for the  Obligations  or any part thereof,  notice of any Event of
Default,  and all other  notices  respecting  the  Obligations;  and agrees that
maturity of the Obligations  and any part thereof may be accelerated,  extended,
or renewed one or more times by Secured Party in its discretion,  without notice
to Debtor.

         8.13 Other Parties and Other Collateral.  No renewal or extension of or
any other  indulgence  with respect to the  Obligations or any part thereof,  no
release  of any  security,  no  release  of any  person  (including  any  maker,
endorser,  guarantor,  or  surety)  liable  on  the  Obligations,  no  delay  in
enforcement  of payment,  and no delay or admission or lack of diligence or care
in exercising any right or power with respect to the Obligations or any security
therefor  or  guaranty  thereof or under this  Agreement  shall in other  manner
impair  or affect  the  rights  of  Secured  Party  under  the law,  under  this
Agreement,  or under any other  document or  agreement  pertaining  to the other
security for the  Obligations,  before  foreclosing  upon the Collateral for the
purpose of paying the Obligations.  Debtor waives any right to the benefit of or
to require or control application of any other security or proceeds thereof, and
Debtor  agrees that Secured  Party shall have no duty or obligation to Debtor to
apply to the Obligations any such other security or proceeds thereof.

         8.14 Relationship Among Secured Party.  Either or both of Secured Party
are  entitled  to enforce  any and all rights  granted to Secured  Party in this
Agreement,  and to take any other  action  allowed to be taken by Secured  Party
under this  Agreement.  Neither  Secured  Party is under any  obligation  to act
jointly or in concert with the other Secured Party  pursuant to this  Agreement.
Any action  required  to be taken by Debtor,  or notice  required to be given by
Debtor, shall be taken or given with respect to both of Secured Party.

                                   ARTICLE IX

                                  MISCELLANEOUS

     9.1 Terms  Commercially  Reasonable.  The terms of this Agreement  shall be
deemed commercially reasonable within the meaning of the Texas UCC.

         9.2 Notices.  Any notices or demands  required or permitted to be given
hereunder  shall be  deemed  sufficiently  given if in  writing  and  personally
delivered or mailed (with all postage and charges prepaid), addressed to Secured
Party or to Debtor their respective  addresses set forth below, or at such other
address as the above parties may from time to time  designate by written  notice
to the other given in  accordance  with this Section  9.2.  Any such notice,  if
personally  delivered or  transmitted  by telex or telegram,  shall be deemed to
have been given on the date so delivered or transmitted or, if mailed, be deemed
to have been given on the day after such  notice is placed in the United  States
mail in accordance with this Section 9.2.

                  Secured Party:  1301 Capital of Texas Hwy., Suite C-300
                                  Austin, Travis County, Texas 78746
                                  Attn: President

                  with copy to:   Timothy L. LaFrey, Esq.
                                  Akin, Gump, Strauss, Hauer & Feld, L.L.P.
                                  1900 Frost Bank Plaza
                                  816 Congress Avenue
                                  Austin, Texas 78701

                  Debtor:         Stephen Wilmarth, M.D.
                                  9824 Carlton Court
                                  Granite Bay, California   95746

         9.3 Parties Bound.  Secured Party's rights under this Agreement and the
Security Interest shall inure to the benefits of its successors and assigns, and
in the event of any  assignment  or  transfer of any of the  Obligations  or the
Collateral,  Secured  Party  thereafter  shall  be  fully  discharged  from  any
responsibility  with respect to the Collateral so assigned or  transferred,  but
Secured  Party shall  retain all rights and powers  hereby given with respect to
any of the  Obligations  or  Collateral  not so  assigned  or  transferred.  All
representations, warranties, and agreements of Debtor if more than one are joint
and several, and all shall be binding upon the personal representatives,  heirs,
successors, and assigns of Debtor.

         9.4 Waiver.  No delay of Secured Party in exercising any power or right
shall operate as a waiver thereof;  nor shall any single or partial  exercise of
any power or right preclude other or further exercise thereof or the exercise of
any other power or right.  No waiver by Secured Party of any right  hereunder of
any default by Debtor shall be binding upon Secured Party unless in writing, and
no failure by Secured  Party to exercise any power or right  hereunder or waiver
of any  default  by Debtor  shall  operate  as a waiver of any other or  further
exercise of such right or power of any further default.

         9.5 Agreement Continuing.  This Agreement shall constitute a continuing
agreement,  applying to all future as well as existing transactions,  whether or
not of the  character  contemplated  at the date of this  Agreement,  and if all
transactions between Secured Party and Debtor shall be closed at any time, shall
be equally  applicable to any new  transactions  thereafter.  Provisions of this
Agreement,  unless  by their  terms  exclusive,  shall be in  addition  to those
contained in any Other Agreement.

         9.6 Definitions. Unless the context indicated otherwise, definitions in
the Texas Business and Commerce Code ("Texas UCC") apply to words and phrases in
this Agreement; if Texas UCC definitions conflict, Chapter 9 definitions apply.

         9.7 Miscellaneous. In this Agreement, whenever the context so requires,
the neuter gender  includes the masculine and feminine,  and the singular number
includes  the plural and vice  versa.  The  headings  of  paragraphs  herein are
inserted only for convenience and shall in no way define,  describe or limit the
scope of intent of any  provisions  of this  Agreement.  No  change,  amendment,
modification,  cancellation,  or  discharge of any  provision of this  Agreement
shall be valid unless consented to in writing by Secured Party.

         9.8 Assignment of Secured  Party's  Interest.  Secured Party shall have
the right to assign all or any portion of its rights in this  Agreement  without
approval or consent.  Debtor may not assign this  Agreement or any of its rights
or obligations  hereunder  without the express prior written  consent of Secured
Party in each instance.

     9.9 Applicable  Laws.  THIS AGREEMENT SHALL BE GOVERNED BY AND CONSTRUED IN
ACCORDANCE  WITH THE LAWS OF THE STATE OF TEXAS AND THE  APPLICABLE  LAWS OF THE
UNITED STATES OF AMERICA.

     9.10 ENTIRE AGREEMENT.  THIS AGREEMENT AND THE PURCHASE AGREEMENT REPRESENT
THE FINAL AGREEMENT  BETWEEN THE PARTIES AND MAY NOT BE CONTRADICTED BY EVIDENCE
OF PRIOR,  CONTEMPORANEOUS,  OR SUBSEQUENT ORAL AGREEMENTS OF THE PARTIES. THERE
ARE NO UNWRITTEN ORAL AGREEMENTS BETWEEN THE PARTIES.

                            [Signature page follows]


<PAGE>


S-1



                                SIGNATURE PAGE TO

                                 ASSIGNMENT AND

                               SECURITY AGREEMENT

         EXECUTED this 1st day of September, 1999.



DEBTOR:                                 /s/ Stephen Wilmarth, M.D.

                                        Printed Name: Stephen Wilmarth, M.D.,
                                        Trustee under Medical Vision Technology
                                        Profit Sharing Plan for the benefit of
                                        Stephen Wilmarth, M.D.

SECURED PARTY:                          Prime Medical Operating, Inc.

                                        By: /s/ Cheryl Williams

                                        Name: Cheryl Williams

                                        Title: Vice President


                                        Prime/BDR Acquisition, L.L.C.

                                        By: /s/ Cheryl Williams

                                        Name: Cheryl Williams

                                        Title: Vice President

<PAGE>