Bull. Natl. Mus. Nat. Sci., Ser. A, 49(4), pp. 197–208, November 22, 2023
DOI: 10.50826/bnmnszool.49.4_197

The Nerve Bundle via the Median Nerve Innervating
the Ulnar Intrinsic Muscles of the Hand in a Gorilla Equivalent
to the Deep Branch of the Ulnar Nerve in the Human
Hidehiko Saito1, *, Tadasu K. Yamada2 and Yuko Tajima2
1

Retired hand surgeon, Seirei Hamamatsu Hospital, Hamamatsu, Japan2
Department of Zoology, National Museum of Nature and Science,
4–1–1 Amakubo, Tsukuba, Ibaraki 305–0005, Japan*E-mail: h-saito@grape.plala.or.jp(Received 20 May 2023; accepted 20 September 2023)

Abstract The ulnar antebrachial branch of the median nerve (UABM) was found in the rightforelimb of a female western gorilla (Gorilla Gorilla) estimated 40 years old. It branched oﬀ themedian nerve at the anterior aspect of the elbow and ran ulno-distally in the forearm toward thewrist. In the distal forearm, it bifurcates into the ulnar and radial bundles; the ulnar bundle is a sensory branch and the radial one is a motor branch. The radial one goes around the hook of thehamate deep into the palm and then radially as the deep branch of the ulnar nerve does in thehuman hand. The radial bundle innervates ulnar intrinsic muscles (all intrinsic muscles except forthenar muscles and radial two lumbricals). The proximal-ward intraneural dissection revealed thatmost nerve ﬁbers of this UABM come from the medial cord and some from the lateral cord, andthat further proximally one half of fasciculi coming from the medial cord originates from themedial division (Eisler) of the middle trunk·C7 root and the other half from the lower trunk·C8
root. We are not sure that this branch of the median nerve innervating the ulnar intrinsic muscles of
the hand is the common structure of the nervous system in the forelimb of the gorilla.
Key words: anatomy, ulnar antebrachial branch, median nerve, gorilla, ulnar intrinsic muscles, allmedian hand, Martin-Gruber anastomosis.

Introduction
The ulnar antebrachial branch of the median
nerve (UABM) was found in an ordinary anatomical dissection of the right forelimb of agorilla (Fig. 1) (Saito, 2020; Saito et al., 2021). It
innervates all intrinsic muscles except for thenar
muscles and radial two lumbricals of the hand.
We call those intrinsic muscles here ulnar intrinsic muscles (Fig. 2). All ulnar muscles are innervated by the deep branch of the ulnar nerve in thecommon type of the human anatomy. The main
purpose of this anatomical study is to ﬁnd out
from which nerve-root this UABM comes out© 2023 National Museum of Nature and Scienceand which trunk and cord it comes through inorder to consider its embryonic development.
The other purpose is to compare the connection
between this branch and the ulnar nerve with
Martin-Gruber anastomosis that is the one
between the median and the ulnar nerves occasionally seen in the human forearm.
Materials and Methods
The specimen used for this study was oﬀeredfrom the National Museum of Nature and Science, Tsukuba Japan, and the study itself wascarried out under the direction of co-authors
there.
The right forelimb of a female western gorilla

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Fig. 1. Ulnar antebrachial branch of the median nerve (UABM). It branched oﬀ the median nerve at the anterior
aspect of the elbow and ran ulno-distally in the forearm toward the wrist. The ulnar artery is seen to come out
from under the ulnar head of the pronator teres and to run distally along but about one cm apart from UABM.

(Gorilla Gorilla) estimated 40years old that diedat Ueno Zoo in Tokyo was used for this anatomical dissection. The forelimb had been amputatedat the level of the lateral margin of the cervicalspines without identifying each cervical root.
Anatomical dissection was carried out under
3.5 times magniﬁcation of surgical loupes, andalso 4.0–16 times magniﬁcation of an operating
microscope especially for intraneural funiculardissection. After peeling oﬀ the skin and subcutaneous tissue, the anatomical dissection was carried out to identify the individual muscles andthe nerves in the routine manner. The scapula andclavicle were ﬁrst dissected oﬀ the muscles andthen removed out as one piece by disarticulating
the shoulder joint. The bones and joints below
the shoulder joint were dissected oﬀ the soft tissues through the extended ulnar approach andextracted out en-masse while the muscles being
attached to the innervating nerves in order tomake the muscle-nerve specimen of the entire
forelimb from the level of the nerve-root to theﬁngertip.
The specimen was kept in formalin solution
for a few weeks and then spread in water-formalin solution in a large tray. Individual muscles
and nerves are identiﬁed. Arteries and veins running along nerve trunks and branches were separated oﬀ the latter and discarded to clarify individual nerves and innervating muscles.
The intraneural dissection of the individual
nerves was carried out by removing the external
epineurium in the solution under the magniﬁcation using an operating microscope. Each fasciculus (nerve ﬁber bundle covered with the perineurium) was isolated by removing theinterfascicular epineurium, and then followed as
proximally as possible from the neuromuscularjunction (the end point of each motor branch inthe muscle visible with the naked eyes) to thenerve-root.
The UABM was ﬁrst dissected proximally andseparated from the anterior interosseous nerve
and the main trunk of the median nerve. It wasMedian nerve branch innervating ulnar intrinsics in a gorilla

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Fig. 2. Innervation of all ulnar intrinsic muscles by the radial bundle of UABM. The radial bundle (motor
branch) ﬁrst gives oﬀ branches to the hypothenar muscles and to the lumbrical muscles of the little and ringﬁngers (marked with asterisks), and then to interossei (4th DI, 3rd PI and 3rd DI, 2nd PI). It goes further radially to give branches to both heads of ADP.

then dissected proximally toward the level of theroots by using the intraneural dissection technique in order to clarify from which nerve-roots
the nerve ﬁbers of this branch come out andwhich trunk and cord they come through.
The ﬁndings were then compared ﬁrst withthose described in the Ravenʼs book ʻAnatomy of
the gorillaʼ (Gregory, 1950) and then withdescriptions in the established textbooks of thehuman anatomy (Lampe and Netter, 1951; Chusid and McDonald, 1964; Boileau Grant and Basmajian, 1965; Kaplan, 1965; Hollinshead, 1969;
Lampe, 1974; Anderson, 1983; Ueba, 1985;
Tountas and Bergman, 1993) and also the pertinent papers including the anatomy of the brachial
plexus in other species of anthropoids (Kusakabe
et al., 1965; Nishimura et al., 1965; Kusakabe,1976) and ﬁnally analyzed from the standpoint ofa hand surgeon. For simplicity, the name of eachﬁnger was substituted by that of the corresponding ﬁnger of the human hand to facilitate understanding the anatomical diﬀerences between thegorillaʼs forelimb and the human upper extremity.
Results
1) Brachial plexus
The problem of this specimen is that the forelimb had been amputated without identifying
each cervical root in advance of this anatomical
study. So, the number of each root was judged
from the whole structure of the plexus; the upper
trunk composed of the ﬁfth cervical (C5) and thesixth cervical (C6) roots, the middle trunk composed of the seventh cervical (C7) root and thelower trunk composed of the eighth cervical (C8)root (Fig. 3A). A thin nerve bundle was judged tobe the fourth cervical (C4) root sitting just cra-

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A

B

Fig. 3. Brachial plexus. A. The brachial plexus iscomposed of the upper trunk·C4, C5 and C6
roots, the middle trunk·C7 root and the lower
trunk·C8 root. B. The subclavian artery was
found to split the middle trunk into two bundles: upper and lower. The upper bundle consists of the anterior and posterior divisions, andthe lower bundle is the medial division (Eisler)
going to the medial cord.

nial to the upper trunk. There was not any contribution from the ﬁrst or the second thoracic(Th1/2) root to the lower trunk. The brachial
plexus of this specimen was judged to be composed of mainly C5 to C8 roots and thin nerve
root of C4. It is considered to be such a type as
the preﬁxed brachial plexus in the human.
In this specimen with the large vessels and surrounding soft tissue attached to the brachial
plexus, the subclavian artery was found to split the
middle trunk·C7 root into two bundles: upper andlower (Fig. 3B). The upper bundle consists of theanterior and posterior divisions; the anterior goesto the lateral cord and the posterior does to theposterior cord. The lower bundle is the medial

division (Eisler) going to the medial cord.
2) Ulnar antebrachial branch of the median
nerve (UABM)a) Its route and branches
The median nerve trifurcates at the level of the
distal border of the pronator teres. The mostradial branch is the main trunk and the mostulnar one is the UABM branch (Saito 2020; Saito
et al., 2021). The last one, the anterior interosseous nerve, comes out from the radio-posterior
portion of the latter.
The UABM runs ulno-distally toward theulnar side of the wrist and comes close to, butdoes not unite with the ulnar nerve. It bifurcates
there, and its ulnar bundle goes distally along theradial border of the hypothenar muscles. Itsradial one goes around the hook of the hamate
deep into the palm and then radial-ward as thedeep branch of the ulnar nerve does in the humanhand. The ulnar bundle further bifurcates, one
being the ulnar digital nerve of the little ﬁnger
and the other the 3rd common digital nerve goingto the 4th interdigital space. The radial bundle
ﬁrst gives oﬀ a branch to the hypothenar muscles
and then branches to the lumbrical muscles of thelittle and ring ﬁngers, and then to both dorsal andpalmar interossei; 4th DI (dorsal interosseous),
3rd PI (palmar interosseous) and 3rd DI, 2nd PI,
in the 4th and 3rd intermetacarpal spaces (Fig.
2). It goes further radially to give branches toboth the transverse and the oblique heads of the
adductor pollicis (ADP). Its terminal branch goes
through the interval between those two heads of
ADP and comes out to the post-adductor space toinnervate both dorsal and palmar interossei (2ndDI, 1st PI) in the 2nd intermetacarpal space andﬁnally both heads of the 1st DI in the 1st intermetacarpal space. So, the radial bundle of the
UABM innervates all ulnar intrinsic muscles.
It is essential to discriminate between thosenervous branches and the vascular ones. The
route and branching pattern of metacarpal arteries from the deep palmar arterial arch composedof the deep branch of the radial artery and that of
the ulnar artery (Lampe, 1951; Hollinshead,Median nerve branch innervating ulnar intrinsics in a gorilla1969) resemble those of this radial bundle ofUABM. It was conﬁrmed that those branches
innervating intrinsic muscles were continuouswith the radial bundle of UABM.
b) Components
It was found that most nerve ﬁbers of the
UABM come from the medial cord and somefrom the lateral cord (Fig. 4A). Intraneural dissection of the medial cord revealed that one halfof fasciculi going to UABM come from themedial division of the middle trunk·C7 root
(Eisler) and the other half from the lower
trunk·C8 root (Fig. 4B). Several fasciculi of C8
root going to the UABM pass anterior or posterior to the ulnar nerve; two thick fasciculi anterior, and two thick and one thin fasciculi posterior (Fig. 5). Three fasciculi from the lateral cord
join the UABM (Fig. 5).
3) The ulnar nervea) Component
The ulnar nerve emerging from the medialcord is composed of fasciculi from C7 and C8
roots; ﬁve of 12 fasciculi of the medial divisionof the middle trunk·C7 root and three thick and a
few thin fasciculi from the lower trunk·C8 root
(Fig. 4A). There was not found such a nerve bundle coming from the lateral cord to form theulnar nerve as seen on the Plate 22 of Ravenʼsbook (Gregory, 1950). The fasciculi going to theulnar nerve through the medial division of the
middle trunk (Eisler) does contain motor ﬁbers
going to the forearm muscles (FCU, FDPV) but
does not those to intrinsic muscles of the hand.
b) Motor branches to FCU muscle
The ulnar nerve sends three motor branches tothe ﬂexor carpi ulnaris (FCU): two come from athick fasciculus at the level of the elbow; one tothe proximal portion of the muscle and the otherto its middle portion, and the third comes fromthe distal portion of the same fasciculus to the
distal portion of the muscle (Fig. 6).

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A

B

Fig. 4. Components of UABM. A. The UABM is
composed of ①fasciculi from the medial division of the middle trunk·C7 root and ②fasciculi
from the lower trunk·C8 root, and some from
③the lateral cord. B. Intraneural dissection of
the medial cord revealed that one half of fasciculi going to UABM come from the medial division of the middle trunk·C7 root (Eisler) andthe other half from the lower trunk·C8 root.

c) Connecting branches with UABM
In the proximal forearm, a thin branch comesout the radial margin of the ulnar nerve andmeets a branch from the ulnar margin of UABM
to make a neural arch that gives oﬀ motor
branches to the ﬂexor digitorum profundus(FDP) of the ring and little ﬁngers (Figs. 6, 7A).
A slightly thicker branch comes oﬀ the ulnar
nerve, a little distal to the third branch to FCU
and bifurcates into two nerve branches; a verythin proximal one and a little thicker distal one(Fig. 7B). A thin proximal one unites with theradial bundle (motor fasciculi) of the UABM and202

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Fig. 5. The route of fasciculi composing the UABM. Several fasciculi of C8 root going to the UABM pass anterior or posterior to the ulnar nerve, two thick fasciculi anterior, and two thick and one thin fasciculi posterior.
Three fasciculi from the lateral cord join the UABM.

Fig. 6. Motor branches to FCU of the ulnar nerve. Three motor branches to the ﬂexor carpi ulnaris (FCU); two tothe proximal and middle portion of the muscle, and another to its distal portion. A thin branch is also seen tomeet with a branch from UABM to make a neural arch.

the other thicker distal one unites with its ulnar
bundle (sensory fasciculi).
Those ﬁndings reveal that the ulnar nerve con-tains motor fasciculi going to the FDP of the ringand little ﬁngers, FCU and very few motor fasciculi to the intrinsic muscles through a very thin

Median nerve branch innervating ulnar intrinsics in a gorillaconnection branch going to the radial bundle(motor fasciculi) of the UABM.
d) Fasciculi at the level of the wrist
The distal dissection of the remaining bundleof the ulnar nerve showed three fasciculi at thelevel of the wrist. The ﬁrst terminates in the fatty
tissue at the proximal border of the hypothenar
muscles, the second extends along the ulnar border of the hand and the third extends dorsallypassing under the tendon of the ﬂexor carpi ulnaris (FCU). All three are thought to be sensory
branches.
Discussion
1) Brachial plexus
The most common type of the human brachial
plexus is composed of fasciculi from C5 to Th1
roots. The brachial plexus in this specimen was
found to be composed of mainly C5 to C8 roots
and the thin nerve root of C4. There was notfound any nerve bundle from Th1 or Th2 root.
Ravenʼs book “The Anatomy of the Gorilla”(Gregory, 1950) showed a thin nerve bundle ofTh1 root to join the lower trunk on the plate 22.
So, the brachial plexus of this specimen is considered to be such a type as the preﬁxed type inhumans.
Proximal dissection of the UABM toward thelevel of the cervical roots revealed that mostnerve ﬁbers innervating the ulnar intrinsic muscles of the hand come through the middle
trunk·C7 root while most come through the ulnar
nerve emerging from the lower trunk composedof C8 and Th1 roots in the common type of thehuman.
In this specimen, the subclavian artery was
found to split the middle trunk into two bundles,
upper and lower (Fig. 3B). The upper bundle
consists of the anterior and posterior divisions.
The lower bundle is the medial division (Eisler)
going to the medial cord. This medial division of
the middle trunk is not seen in the common typeof the human brachial plexus shown in “Correlative Neuroanatomy and Functional Neurology”

203(Chusid and McDonald, 1964). Eisler (1890)
showed the medial division of the middle trunk
going to the medial cord in Figs. 8 and 9 of his
book “Das Gefäss- und Periphere Nervensystem
des Gorilla” and described it as the lower strong
anterior branch of N. cervicalis VII in the text.
Narakas (1977) also illustrated the medial
division of the middle trunk in the Figs. 9–19
presented in his chapter of the surgical management of brachial plexus injuries of the human inDaniel and Terzisʼ “Reconstructive Microsurgery”. He describes that the large central fascicleis almost constantly found in the root C7 and thatit contains the majority of axons that will form
the median nerve. In that ﬁgure, the medial cord
is drawn as the thick bundle continuing straightfrom the middle trunk. The thinner anterior division of the lower trunk composed of C8 and Th1
merges into this thick medial cord. Kusakabe etal. (1965) reported the similar structure of themedial cord of the brachial plexus in a chimpanzee they dissected. In their case of a chimpanzee,
the anterior branch of C7 root becomes the very
short medial cord by merging with the anterior
branch of the lower trunk C8·Th1 roots and a thinbundle from the lateral cord. This short medialcord soon sends the median nerve. So, themedian nerve is composed of mainly the mostnerve ﬁbers of the C7 root and some nerve ﬁbers
from the lower trunk C8 and Th1 roots and thelateral cord. It is quite diﬀerent from the common type of the human brachial plexus in which
the middle trunk·C7 root does not participate
directly in constituting the median nerve,
although its small bundle participate in throughthe lateral cord.
Nishimura et al. (1965) and Kusakabe (1976)
describe the brachial plexus of the gibbon; that is
diﬀerent from that of the chimpanzee or thehuman. The anterior branches of the three trunks:
upper, middle and lower constitute the fasciculus
anterior (the anterior cord) from which themedian nerve comes out.
In our case of a gorilla, the nerve ﬁbers emerging from the anterior horn cell of the C7 segment
has to get the route going to the intrinsic muscles

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A

B

Fig. 7. Connection between UABM and ulnerve nerve. A. In the proximal forearm, a thin branch from the ulnar
nerve meets a branch from the UABM to make a neural arch that gives oﬀ motor branches to the ﬂexor digitorum profundus (FDP) of the ring and little ﬁngers. B. A thin proximal branch (hollow arrow) from the ulnar
nerve unites with the radial bundle (motor fasciculi) of the UABM and the other thicker distal one (solid
arrow) unites with its ulnar bundle (sensory fasciculi).

of the hand. The medial division of the middle
trunk is considered to be its route going to themedial cord and ﬁnally intrinsic muscles. The
intraneural dissection of UABM toward the cer-vical roots revealed that nearly 6/10 of the nerve
ﬁbers of the medial root of UABM went to themedial cord and ﬁnally to the middle trunk·C7
root. So, the medial division of the middle trunk

Median nerve branch innervating ulnar intrinsics in a gorilla

was proved to be the route through which thosenerve ﬁbers go to the medial cord.
Hollinshead (1969) illustrated the variations inarterial relations to the human brachial plexus
but there was not any schema showing the same
variation as seen in our specimen in which thesubclavian artery split the middle trunk into twobundles.
Regarding the contribution of the middle
trunk·C7 root to the medial cord in the humanbrachial plexus, Hollinshead (1969) quoted
Kerrʼs observation in which the medial cordreceives a contribution from the C7 root trough athin branch in a few cases.
2) Ulnar antebrachial branch of the median
nerve (UABM)
In Ravenʼs book (Gregory 1950), the samenerve bundle as UABM seen in this specimen isdrawn on the plate 40 as “Ramus anastomoticus
n. medianus cum ulnaris”. This branch is also
named as “the ramus ulnaris of the nervus medianus” in the text of this book. This ramus isdescribed as follows: it courses distally andreceives two anastomosing branches from theulnar nerve and then it divides into superﬁcial
and deep branches eight centimeters above thepisiform. Its deep ramus passes deep to the pisohamate ligament; beneath this ligament it gives
oﬀ twigs to the m. palmaris brevis, m. abductor
digiti quinti, m. ﬂexor digiti quinti brevis and m.
opponens digiti quinti. The remainder of thenerve then swings laterally distal to the os hamatum, giving twigs to the third and fourth lumbricales, the mm. interossei and the m. adductor
pollicis; caput transversum and caput obliqum.
On the other hand, the ulnar nerve is describedin this book as follows. “As the ulnar nerve
passes behind the medial epicondyle it gives oﬀ
branches to the proximal part of FCU, and asmall branch to the elbow joint, next, a branch to
FDP that joins a branch from the median nerve
before it enters the muscle, and then a large
branch to FCU after it has sent two branches tothe ramus ulnaris of the median nerve.” Thisroute of the ulnar nerve is nearly identical to that205

observed in our specimen (Figs. 6, 7A, 7B).
On the plate 40, the ramus anastomoticus n.
medianus cum ulnaris (UABMN), after receiving
the connecting branch from the ulnar nerve in the
mid-forearm, is indicated as “Ramus profundus
n. ulnaris” with a leading line shown at the levelof the wrist. Although this note indicated with aleading line causes a misunderstanding that
UABM unites with the main trunk of the ulnar
nerve, it does not. The main trunk of the ulnar
nerve is recognized on the plate 40. According to
the description in the text, it passes dorsally after
sending two communicating branches to UABM
and perforates the deep fascia between FCU andthe ulna 4 cm above the pisiform. It terminates as
the cutaneous nerve of the medial side of the dorsum of the hand.
Eisler (1890) described the course of the median
nerve in detail in “Gefäss- und periphere Nervensystem des Gorilla” but did not refer to such a
branch as UABM. Preuschoft (1964) did not referto such a branch of the median nerve either in “Die
Nerven der Vorderextremität des Gorilla”. Hepburn(1892) described the innervations of muscles in theforearm and cutaneous nerve supply of the hand in
the gorilla but did not mention such a branch as
UABM at all. So, UABM seems to be either thevariation or anomaly in the nervous system of theforelimb of the gorilla.
Such a branch of the median nerve as UABM
seen in our specimen of gorilla is not describedin any established textbooks of the human anatomy mentioned above, even in Tountas andBergmanʼs book (1993) “Anatomic Variations of
the Upper Extremity”.
Okubo et al. (1977) showed seven types(1st~3rd main types with two subtypes each and4th type) in photographs. Each connection
branch emerging from the anterior interosseous
nerve or the main trunk of the median nerve ispretty thick and not very short. Those branches,
however, join to the main trunk or muscularbranches of the ulnar nerve.
The UABM seen in our specimen takes its
own route to the hand to innervate ulnar intrinsic
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hand although it receives two connection
branches coming from the main trunk of theulnar nerve.
The connection between the median and theulnar nerves seen in the human forearm is called
Martin-Gruber
anastomosis.
Martinﬁrst
described this connection in 1763 and then Gruber reported in 1870 that it was found in 38(17.9%) of 212 limbs (Leibivic and Hastings,
1992). Okubo et al. (1977) quoted the incidence
reported being 10 to 44%.
It has been known that there are occasionallyseen cases in which the ulnar intrinsic muscles
are not paralyzed when the ulnar nerve is cut or
anesthetized at the elbow or the proximal forearm (Kaplan, 1965; Mannerfelt, 1966; Okubo etal., 1977). The explanation for this paradoxical
phenomenon is that the motor ﬁbers innervating
those intrinsic muscles come through the median
nerve in the arm and then the Martin-Gruber
anastomosis in the forearm.
In our gorilla, there are two connecting
branches between UABM and the ulnar nerve inthe proximal forearm. A very thin proximal
branch unites with the radial bundle (motor fasciculi) of UABM and the other thicker distal one
unites with its ulnar bundle (sensory fasciculi).
The very thin proximal branch coming from theulnar nerve might take a part in innervating ulnar
intrinsic muscles.
It is a critical anatomical variation put into themind on making a clinical diagnosis and treating
nerve injuries in the upper arm and around theelbow. Leibovic and Hasting (1992) classiﬁed
Martin-Gruber anastomosis into 6 types according to not only the pattern of communication
seen by the naked eye but also which nerve
ﬁbers, the median nerve or the ulnar nerve, innervate the intrinsic muscles by using an EMG technique.
The UABM seen in this specimen ﬁrst goes
toward the ulnar side of the wrist and it bifurcates into two bundles in the distal forearm. Itsradial bundle innervates all ulnar intrinsic muscles and so it resembles the type II of Leibovicʼsclassiﬁcation. It is not identical to that typebecause it goes through the own branch of themedian nerve, not through that of the ulnar nerve.
When such a branch coming out the median
nerve and innervating ulnar intrinsic muscles as
Leibovicʼs type Ib or II exists in the human, allintrinsic muscles (both radial and ulnar intrinsic
muscles) of the hand shall be innervated by nerve
ﬁbers coming through the median nerve. Such atype of innervation is called as “all median hand”
proposed by Marinacci and von Hagen (1965).
So, laceration of the ulnar nerve at the medialaspect of the elbow or the proximal forearm,entrapment of the ulnar nerve with osteoarthritisof the elbow joint will not elicit functional disturbance due to intrinsic paralysis of the hand.
Hepburn (1892) stated, in his thesis on his
own anatomical dissection of four species ofanthropoids; Gorilla, Chimpanzee, Orangutan
and Gibbon, one head each, “In the Gorilla,
Chimpanzee and Gibbon, distinct branches ofcommunication existed between the median andulnar nerves in the forearm.” This statement suggests that the connection between the median andthe ulnar nerves is the invariable phenomenon inanthropoids.
3) Causative factors of UABM
The variation in the cervicobrachial region isimportant as far as the development of UABM isconcerned. Which cervical segment those nerve
ﬁbers innervating the ulnar intrinsic come out,
and which trunks or cords those come through;those two factors seem to be crucial. Which compartment of the upper arm those nerve ﬁbers
come down depends on those. The median nerve
comes down in the anterior compartment of theupper arm while the ulnar nerve in the posterior
one. Those two compartments are separated by
the thick fascia; the medial brachial intermuscular septum. Fasciculi composing UABM haveextended anteriorly along the median nerve overthe proximal margin of this septum at the earlystage of embryonic development. In this specimen, the subclavian artery splits the division
branch of the middle trunk into two and also
there was such a variation as high division of theMedian nerve branch innervating ulnar intrinsics in a gorillabrachial artery which bifurcates into the radial
and the ulnar arteries at the level a little proximal
to the midlevel of the biceps brachii. These vascular variations might have some relation toembryonic development of this variation of themedian nerve.
Another hypothetical cause is an accidental
change of the route during the embryonic development. If the neurons, the ﬁrst and the second,are destined to innervate the targeting muscles
from the beginning of the development, it isunderstandable that the route of the fascicularbundle took another route by accident whilenerve ﬁbers were being bundled. However, there
are two papers regarding the muscle and innervating nerve during the embryonic developmentthat are against this hypothesis. Akita (1997)
quoted the Stranznickyʼs (1963) experimental
result of the chicken and referred to the conclusive idea of Hollyday and Hamburger (1977) thatthe muscle and its innervating nerve is not destined.
Kusakabe (1976) describes in his paper entitled “The structure of brachial plexus in anthropoids” that there is the myotome in the limb
which is the primitive form of the muscle andthat the nervous tissue develops in each muscle
segment at the early stage of embryonic development. It repeats union and division to grow distally to be the nerve plexus unique to each species of the animal while a myotome graduallydevelops into the muscular tissue.
In this specimen, the mechanism of development of UABM is surmised as follows; the nervous tissue from the anterior horn cells of the C7,
one segment cranial than the common type in thehuman anatomy, reaches the myotome of intrinsic muscles of the hand earlier than that of the
C8, Th1 one or two segments below, and grows
distally with the extending nerve ﬁbers. Those
nerve ﬁbers were bundled with the median nerve
rather than the ulnar nerve during the embryonicdevelopment.
4) SigniﬁcanceThe signiﬁcance of this anatomical course of207

those motor ﬁbers through the median nerve is
not clear. The route of this nerve bundle(UABM) is advantageous to avoid the dysfunction of the hand caused by ulnar nerve palsy after
injuries at the medial aspect of the elbow, or withosteoarthritis of the elbow joint, to both of which
gorillas are very susceptible during their wild
lives. On the other hand, the median nerve injury
in the upper arm or at the elbow could cause
paralysis of all (both ulnar and radial) intrinsic
muscles of the hand.
Acknowledgements
The author is very grateful to Shinichiro
Kawada who supported him for providing andpreserving the specimen at the Department of
Zoology, National Museum of Nature and Science in Tsukuba, Japan.
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