The sun., 1893-06-04

In COnneCIlOn WIN tHo stuuD VI NV
ism OF sensory perception. The phenomena
of hypnotism and telcpathy. for example. re.
ceiye only a passing referonce. The aim
IN to furnish those who hive not made
physiology q subject OF special atten-
tion, with suitable preparation for entering
upon tho moro recondite problems that under
lie physiological psychology The authors do
not assume to answer the interesting inquiry
whether or how far the sensory nerves.
viewed collectively. may have retained q part
Of tho power of telegraphng sensntions to the
brain, which has been specialized In the SOW
sral organs OF sense
l..
Touch 10 0 sensation OF pressure referredt to
the surface OF tho body. When we touch any
thing. there Is always certain amount OF
pressure between the sensitive surface and
tho body touched. What We call contact Is
Centle pressure: a greater amount OF force or
pressure makes the sensation OF touch moro
acute: by and by there is n feeling OF resist-
snoe, still referred to the skin: when n weight
a. supported on the palm OF the hand thore is
q sensation OF muscular resistance referred
not only to the skin. but also to the muscles.
whereby We become aware that the muscles arc
contracted: finally. tho pressure may bo 80
great as to cause o sensation OF pain. which.
however, may be confused with simultaneous
sensations OF contact and OF muscular re-
sistance. In its essence. therefore. touch Is
the appreciation OF mechanical force and In
this way It presents o strong resemblance to
hearing which is q moro delicate kind OF
touch. being due to variations Of pressure on
the auditory organs. In addition. however. to
sensations OF touch. contact with foreign
body may give rise to sensations OF heat or
cold. Thus. when We place something on the
palm OF the hand. tho resulting sensation may
be Of q complex character involving sensa-
tions Of contact, OF resistance. and OF temnera-
ture True tactie Impressions are absent from
internal mucous and Serious surfaces. rs has
been proved In mon having flstulous openings
into the stomach. intestines. bladder. or
pleural cavities. In such cases pressure does
not cause sensation OF touch. but one OF pain.
A consideration OF the structure OF tho ter
minal organs OF touch. which are minutoly de-
scribed in this manual shows that they must
serve for protecting the extremities OF tho
ssnsory nerves from direct pressure: for COM
municating slight vibrations OF pressure to the
nerve endings: and for so modifying extor-
nal pressures as to give them moro or less OF
rhythmic character. The direct contact OF
foreign body with q naked sensory nerve does
not produce sensation OF touch In tho proper
sense Of the word. but m ore or less disagree
able impression. 1t is q no less familiar OBJ
servation that oil parts OF the skin are not
equally sensitive. or. in other words. do not
communicate slight variations OF pressure
with equal facility. Among tho many parts OF
tho body enumerated in q graduated list. tho
tip of the tongue is the most sensitive.
next comes the under'suriace Of the third pha-
lanx OF finger. and then the red part OF the
lip. The least sensitive portions are the upper
arm. the thigh. and the centre OF the back Ac-
oording to tho method OF measuring tho sensi-
tivoness Of the skin suggested by Weber. those
parts are most sensitive which can discrimi-
nate the two points OF q pair OF compasses
held at the shortest distance from each other
The power OF disorim mating points may be in
creased by moistening tho skin. stretching it.
or taking baths containing common salt. An
anRmic condition.veno congestion cold. and
tho use Of solutions OF atropine. morphine.
stryohnine. alcohol. bromide OF potassium. and
hydrate Of chloral. blunt sensibilty. On the
other hand. tho sensitivsness OF tho skin Is said
to be stimulated by solution OF caffeine.
Not only is the skin sensitive. but one Is
able with great accuracy TO determine the
part that has been touched. This power is
termed the sense Of locality. Tho general law
Is that the greater the number OF ssnsory
nerves in given area Of skin. the greater Is
the degree OF precision in determining local
ity. From this point Of view tho tip Of
the finger is contrasted with the back Of
tn0 manu-. II iuleHJ 10 buppOseu tnaC inc
habitual use OF parts OF the skin would SO
educate the mind rs to enable US to identity
them. even although these parts might not
DO much moro sensitive than others. 1t Is
doubtful. however. II exercise increases sensi-
tiveness. Galton found that the performances
OF blind boys when examined by the Weber
method just referred to- were not superior to
those OF other boys: and he arrived at the con-
ciusion that the guidance Of tho blind depends
mainly on the multitude OF collateral indlca-
tions. to which they give much heed. and not
On their superiority in any Of them. The
power Of discriminating points on which we
have thus far besn dwelling. is not to be con-
founded with the absolute sensitiveness Of
any part OF the skin. Absolute as distin-
fuished from discriminative sensitivensss. is
determined by answering the questions: What
1s tho smallest pressure that can give riso to
o sensation: and what Is tho smallest differ
ence that can be observed between two ssnsa
tons2 The general outcome OF many and
delicate experiments is that the greatest
aoutoness is observable on tho forehead. tenn
ples. and back OF the hand and forearms.
Various theories have been propounded to
explain the phenomena OF tactile sensiblity.
but 1t is not pretended that any one is wholly
satisfactory. The Oldest. first put forth by
Weber and modified by later Investigators. is
TO the effect that while We refer every tactile
sensation to o certain position in the tactile
field. We do not refer 1t merely to point but
to Q minute area OF skin which has been
termed circle OF sensibility. 1t has also been
assumed that each circle hns its own nerve
fibre. but there is no proof that this 10 the
case. The suggestion OF Krause is pronounced
worthy OF special notice. namely. that
sansitiveness depends upon tho number OF
tactile corpuseles In given area. 1t 10
certain. indeed. that tactile corpuscles are
not absolutely essential to touch. for tho