Bull. Natl. Mus. Nat. Sci., Ser. A, 47(3), pp. 135–155, August 20, 2021
DOI: 10.50826/bnmnszool.47.3_135

Anatomical Study of the Right Forearm and Hand of One Western
Gorilla (Gorilla gorilla) for Comparison with Humans
with Respect to Motions of the Thumb and Fingers
Hidehiko Saito1, 2*, Tadasu K. Yamada3 and Yuko Tajima3
1
Nishiyama Hospital,
500 Nishiyamacho, Nishi-ku, Hamamatsu, Shizuoka 432–8001, Japan
2
Retired Hand Surgeon, Seirei Hamamatsu Hospital,
2–12–12 Sumiyoshi, Naka-ku, Hamamatsu, Shizuoka 430–8558, Japan3
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 14 January 2021; accepted 23 June 2021)

Abstract The right forelimb of a western gorilla (Gorilla gorilla) was dissected with particularattention to the motor function of ﬁngers of this species. The anatomical structure of the forearmand hand of our specimen of gorilla is not much diﬀerent from that of the human. The palmar aponeurosis is absent. Ligamentous ﬁbers, instead, extend to the palm from the ﬂexor retinaculum toform the ligamentous septa along the ﬂexor tendons. The short thumb seems to be carefully calculated to fulﬁll both roles of locomotion and feeding. The underdeveloped ﬂexor pollicis longus
muscle bifurcates from the ﬂexor digitorum profundus muscle of the index ﬁnger. The extensor
indicis is also underdeveloped; its tendon does not reach the index ﬁnger but attaches to the wrist.
The extensor apparatus of the ﬁngers composed of both the extrinsic and intrinsic systems is as
well developed as in the human hand. During knuckle walking with the metacarpophalangeal
joints in extension and the proximal interphalangeal joints in deep ﬂexion, the former joint of eachﬁnger seems to be stabilized with integrated functions of the ﬁnger extensor, interosseous and lumbrical muscles, and sagittal bands as well as the groove at the metacarpal head. The ulnar antebrachial branch of the median nerve with a ﬁne motor communicating branch from the ulnar nerve
innervates the ulnar intrinsic muscles of the hand, which are innervated by the deep branch of theulnar nerve in the human.
Key words: anatomy, gorilla hand, ﬂexor pollicis longus, extensor indicis, median nerve, innervation of intrinsic muscles, all median hand.

Introduction
It is said that the human species had separated
from the common ancestor of the human and thegorilla about ten to seven million years ago(Hayama, 1999; Watanabe, 2008). It is interesting to know how much diﬀerence has developedin the hands between two species for this periodof many years. In this regard, we assume that the© 2021 National Museum of Nature and Science

human evolved the hand into the work device
and obtained the ability of precision grip while
the gorilla uses the forelimb for locomotion andfeeding and possesses the hand suitable forknuckle walking. Those diﬀerences between two
species might result from the anatomical architecture possessed by the common ancestor ofboth species in the process of phylogeny. However, only a few comparative studies of motor
functions of ﬁngers between the gorilla and thehuman hands to explain this phylogenetic prob-

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lem have been reported up to the present (Diago
et al., 2012), although Kaplan (1946) stated that
this comparison might be extremely helpful forboth the surgeon and the anthropologist.
In this connection, the author (H.S.). has seen
the patients in clinical practice who cannot ﬂex
the thumb and the index ﬁnger individually (seealso Fahrer, 1981), and also those who do notshow any paralysis of the intrinsic muscles of thehand in spite of the injury of the ulnar nerve
around the elbow suggesting variation in innervation of those muscles (see also Mannerfelt,
1966; Okubo et al., 1977). Those are considered
to be closely related to ﬁndings seen in this specimen of a gorilla: underdeveloped ﬂexor pollicis
longus muscle (uFPL) and innervation of intrinsic muscles of the hand by the ulnar antebrachialbranch of the median nerve (UABMN).
In this paper, therefore, there are two objectives; one is to describe and show in ﬁgures theanatomical architecture of the forearm and handof this specimen of a gorilla in detail, and theother is to correlate those with the hand functionsof a gorilla, based on comparison with the human
hand.
Material and Method
The right forelimb of a female western gorilla(Gorilla gorilla), estimated 40 years old that diedat Ueno Zoo in Tokyo was used for dissection.
The forelimb had been amputated at the level ofthe lateral margin of the cervical spines. Dissection was carried out under 3.5 times magniﬁcation of surgical loupes, and also 4–16 times magniﬁcation of an operating microscope especiallyfor intraneural funicular dissection.
The ﬁndings were ﬁrst compared with thosedescribed in Ravenʼs book “The Anatomy of theGorilla” (Gregory, 1950), to know whether theﬁndings seen in this speciﬁc material are common or not in other materials of gorilla. Then
those were compared with descriptions and ﬁgures in the established textbooks of the anatomyof the human (Lampe and Netter, 1951; Boileau
Grant and Basmajian, 1965; Kaplan, 1965; Hol-linshead, 1969; Lampe, 1974; Anderson, 1983).
The ﬁndings diﬀerent from the human handand forearm were analyzed from the viewpointof phylogeny of the primates based on the pertinent papers and information available throughthe Internet. Relevant literature (Jones, 1946;
Hayama, 1999; Yamagiwa, 2005; Shima, 2014)
was consulted to consider the anatomical andfunctional signiﬁcance of those.
For simplicity, the name of the thumb or ﬁngers of the human hand substituted that of thecorresponding digit of the gorilla.
Results
The ﬁndings described below are signiﬁcantly
diﬀerent from those seen in the human hand andforearm.
1) Appearance
The thumb is short and thin. The tip of thethumb reaches only the distal margin of the palm
while the human thumb can reach the distal portion of the proximal phalanx of the index ﬁnger(Fig. 1). The thumb is thinner than the index
through little ﬁngers. Both thenar and hypothenar
eminences are a little hypoplastic compared tothose in the human hand.

Fig. 1. The short and thin thumb, and two skincreases not seen in the human hand. It is diﬀerent from the human hand to see the hypothenar
and the transeminential creases.

Anatomy of the Forearm and Hand of One Gorilla

2) Palmar aspect of the hand and forearm
Skin creases: The thenar crease exists along
the thenar eminence and so do two distinct palmar creases, proximal and distal in the distal partof the palm. The single transverse line is notobserved in the distal palm. It is occasionallyseen in the human hand and called “simiancrease” (Shiono and Azumi, 1981) as it isreported to be a characteristic skin crease in apes
(van Mensvoort, 2002). The hypothenar crease
along the hypothenar eminence and a transverse
skin crease across the thenar and hypothenar
eminences (“transeminential crease” coined by
the author: H.S.) are skin creases not seen in thehuman hand (Fig. 1).
Palmaris longus and palmar aponeurosis:
The palmar aponeurosis in the human hand is the
subcutaneous aponeurotic structure spreadingover the center of the palm in a fan shape extending from the tendon of the palmaris longus (PL).
In this specimen of gorilla, there is not such astructure in the palm except for its most distal
part called the natatory ligament that is the subcutaneous ﬁbrous tissue in each interdigital web(McFarlane, 1985) (Fig. 2A). Neither the muscle
nor tendon of PL is found in the forearm of thisspecimen.
Ligamentous ﬁbers arising from the ﬂexor
retinaculum: The ligamentous bundles arefound to arise from the distal margin of the ﬂexor
retinaculum* (transverse carpal ligament†). Those
extend distally along ﬂexor tendons of each digital ray and then dorsally on each side of the tendons attaching to the fasciae of the adductor pollicis and interosseous muscles to form
ligamentous septa (Fig. 2B, C). This ligamentous
structure covers the ﬂexor tendons in the area
between the digital sheath and the distal marginof the ﬂexor retinaculum, where only the synovial sheath covers each ﬂexor tendon underneaththe palmar aponeurosis in the human hand.
Flexor pollicis longus: The ﬂexor tendon ofthis short and thin thumb is much thinner thanthat of the ﬂexor digitorum superﬁcialis (FDS) or
the ﬂexor digitorum profundus (FDP) (Fig. 3A).
Its proximal muscular portion bifurcates from the137

Fig. 2. The subcutaneous structures of the palm.
A. The palmar aponeurosis is absent except for
its most distal parts: natatory ligaments (whitearrows); B. The ligamentous ﬁbers (whitearrows) arising from the distal margin of the
ﬂexor retinaculum; C. Ligamentous septum (awhite arrow) along the ﬂexor tendon formed
with those ligamentous ﬁbers.

distal portion of the muscle of FDP of the index
ﬁnger (Fig. 3B). Pulling FDP index ﬁnger proximally at its myotendinous junction by using asurgical retractor revealed that the thumb and theindex ﬁnger were ﬂexed simultaneously to make

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Fig. 4. Finger ﬂexor sheath and pulley system. A.
The entire sheath of the ﬁnger ﬂexor of the ringﬁnger is shown with the names of individual
pulleys. The very thick and wide A2′ pulley (awhite thick arrow) is located between A2 andC1; B. The inner surface of the sheath showingthat A2′ pulley is twice as wide as A3.

Fig. 3. Underdeveloped thumb ﬂexor. A. The tendon of the thumb ﬂexor is much thinner thanthat of FDS or FDP; B. Its muscle belly (uFPL)
bifurcates from the distal portion of FDP muscle of the index ﬁnger; C. Pulling FDP index
ﬁnger at its myotendinous junction with aretractor revealed that the thumb and the index
ﬁnger were ﬂexed simultaneously.

a tip pinch (Fig. 3C). So, the muscle bifurcatingfrom the FDP muscle of the index ﬁnger isjudged to be the underdeveloped ﬂexor pollicis
longus (uFPL).
Origin of FDP index ﬁnger muscle: The
muscle of FDP of the index ﬁnger originates atthe palmar aspect of the radius where FPL muscle does in the human forearm.
Finger ﬂexor sheath and pulley system: The

thick parts of the tendon sheath of a ﬁnger ﬂexor
are called “pulley” which prevents the ﬂexor tendon from bowstringing out when the ﬂexor muscle contracts and the tendon gets tight on bending
the ﬁnger. The thick ring-like pulley is called“annular pulley (A)” and relatively thinner andcruciate-form pulley is called “cruciate pulley(C)” in accordance with the pulley system in thehuman hand. Unlike the pulley system in thehuman hand described by Doyle (1988), there isanother very thick and wide pulley between A2
at the proximal portion of the proximal phalanx
and C1 pulley at its distal portion. This additional
annular pulley found in this gorilla is tentatively
termed A2′ (Fig. 4A). It is twice as wide as theA3 pulley located at the level of proximal interphalangeal (PIP) joint (Fig. 4B).
Palmar aspect of the proximal phalanx:
Quite diﬀerent from the human hand, there wasan osseous crest along each edge of the palmar
aspect of the proximal phalanx making thegroove between those crests where the ﬂexor tendon sits in (Fig. 5). The annular pulley mentioned above attaches to each crest of the proxi-

Anatomy of the Forearm and Hand of One Gorilla

Fig. 5. Palmar aspect of the proximal phalanx. An
osseous crest is seen along each edge of the palmar aspect of the phalanx and the groove isformed between those crests. The head of theproximal phalanx slants palmarward.

mal phalanx.
The head of the proximal phalanx slants palmarward so that the base of middle phalanxslides down deeply to increase the ﬂexion angleof PIP joint while the extension angle is limited
even when it slides up the proximal phalanx headdorsalward maximally.
Thenar muscles: The abductor pollicis brevis(APB) is well developed and its muscle belly
extends as far as the midportion of the proximal
phalanx of the thumb. By lifting that muscle, theopponens pollicis muscle is found to attach at the
1st metacarpal. The ﬂexor pollicis brevis (FPB)
muscle lies over the ﬂexor pollicis longus tendon
and attaches to the radial sesamoid at the palmar
aspect of the metacarpophalangeal (MP) joint of
the thumb. The inner head of this muscle
described in Ravenʼs book (Gregory, 1950) is notfound in this specimen when it is deﬁned to pass
deep to the ﬂexor pollicis longus tendon as in thehuman hand as described in Hollinsheadʼs book
(Hollinshead, 1969).
Motor branch of the thenar muscles: The
common branch of the recurrent branch to innervate the thenar muscles and the radial palmar digital nerve of the thumb comes apart from the radialside of the median nerve in the distal portion of
the carpal tunnel and then soon bifurcates. The
recurrent branch penetrates through the very distal
portion of the ﬂexor retinaculum and goes deeply

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into the thenar muscles across the muscle ﬁbers of
APB. The branches coming from the ulnar nerve
to communicate with the recurrent branch of themedian nerve, called Riche-Cannieu anastomosis
occasionally seen in the human hand (Mannerfelt,
1966; Spinner, 1972), are not present.
Ulnar antebrachial branch of the median
nerve and innervation of intrinsic muscles:
The most signiﬁcant ﬁnding is that all intrinsic
muscles other than three thenar muscles andradial two lumbrical muscles innervated by
motor branches from the main trunk of themedian nerve are innervated by the “ulnar antebrachial branch” of the median nerve, while
those are innervated by the deep branch of theulnar nerve in the human hand (Saito, 2020) ¶.
In this specimen, the median nerve trifurcates atthe level of the distal border of the pronator teresin the proximal forearm. The most radial one is
the main trunk and the most ulnar one is thebranch concerned. The latter is not seen in thehuman forearm and tentatively called the ulnar
antebrachial branch of the median nerve(UABM) (Fig. 6 proximal). The branch between
those two, the anterior interosseous nerve, comes
oﬀ the radio-posterior portion of UABM. UABM
runs radio-proximally to ulno-distally in the forearm and comes close to but does not unite withthe ulnar nerve on the ulnar side of the wrist(Fig. 6 distal). It bifurcates there; its ulnar bundle
goes distally along the hypothenar muscles, andthe radial bundle goes radially as the deep branchof the ulnar nerve does in the human hand (Fig. 6
distal). The ulnar bundle further bifurcates, one
being the ulnar digital nerve of the little ﬁnger
and the other the 3rd common digital nerve goingtoward the 4th interdigital space. The radial bundle ﬁrst branches oﬀ to the hypothenar muscles
and then to the lumbrical muscles of the little andring ﬁngers, and then to both dorsal interosseous(DI) and palmar interosseous (PI) muscles (4thDI, 3rd PI and 3rd DI, 2nd PI) in the 4th and 3rd
intermetacarpal spaces (Fig. 7). It goes further
radially to give branches to both the transverse
and the oblique heads of the adductor pollicis(ADP). The terminal branch of this bundle goes

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Fig. 6. The ulnar antebrachial branch of the median nerve (UABM). proximal: The median nerve trifurcates atthe level of the distal border of the pronator teres. The most ulnar one is the branch UABM; distal: It runs
ulno-distally in the distal forearm and comes close to but does not unite with the ulnar nerve on the ulnar sideof the wrist. It bifurcates there to the radial and ulnar bundles.

through the interval between those two heads of
ADP and comes out to the postadductor 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.
3) The dorsal aspect of the hand and forearm
Knuckle of the proximal interphalangeal
joint (PIP joint): The skin at the dorsal aspect ofthe middle segment of each ﬁnger is thickened so
as to make a ʻknuckle padʼ around PIP joint
where chronic irritation has been inﬂicted byknuckle walking (Mackey and Cobb, 1994). The
subcutaneous loose areolar tissue is also thickened to make a bursa. PIP joint of every ﬁnger is
not fully extended passively. Maximum passive
extension of PIP joint is limited in each ﬁnger,
especially in the index ﬁnger where it is limited
to −30 degrees.
Superﬁcial extensor muscles: At the dorsoradial aspect of the proximal forearm, the extensor
digitorum* (extensor digitorum communis†) (EDC)
arises from the common muscle mass of theextensor-supinator group and then becomes aslightly thinner muscle bundle composed ofproper muscles of individual tendons going to
the index through the ring ﬁnger. EDC tendon
going to the little ﬁnger (EDCV) comes apartfrom the midportion of the one to the ring ﬁnger(EDCIV) at the dorsum of the hand as seen in thehuman hand (Fig. 8A). This EDCV tendon connects EDCIV tendon with the extensor digiti minimi (EDM) tendon. A thin, oblique and shortﬁbrous band is also seen between the distal portion of EDCV tendon and EDCIV tendon at MP
joint level (Fig. 8B). There are no such transverse or oblique bands between EDCII andEDCIII, EDCIII and EDCIV tendons known as theconnexus intertendineus* (junctura tendinum†) inthe human hand.
There is a very thin tendon slip along the ulnar
margin of EDC tendon to the index ﬁnger(EDCII), which is not the extensor indicis*(extensor indicis proprius†) (EIP) tendon but
rather the additional tendon slip of the former.
Along the ulnar border of EDC muscle in theforearm, there is an independent muscle belly ofthe extensor digiti minimi (EDM), proper extensor of the little ﬁnger. There is a proper tendon
sheath of this muscle at the dorsum of the hand alittle distal to the distal margin of the extensor

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141

Fig. 7. Innervation of intrinsic muscles by the radial bundle of UABM. The radial bundle ﬁrst gives oﬀ branches
to the hypothenar muscles and to the lumbrical muscles of the little and ring ﬁngers (marked with asterisks),
and then to interossei (4th DI, 3rd PI and 3rd DI, 2nd PI).

retinaculum* (dorsal carpal ligament†) as seen in
the human hand (Fig. 8A).
Deep extensor muscles: In the deep layer of
forearm extensor muscles, the origin of theabductor pollicis longus muscle (APL) is identiﬁed at the proximal radius and ulna. An accessory muscle bundle coming from the common
muscle mass of EDC was found to join this muscle at its myotendinous junction to give theappearance of the hemipennate muscle: tendinous at radial border and muscular at ulnar border. The tendon of this APL attaches to the radial
part of the 1st metacarpal base and a thin tendinous slip coming oﬀ the ulnar margin of the main
tendon extends more distally and attaches to theproximal phalanx base of the thumb (Fig. 8C).
Either the muscle or tendon of the extensor polli-

cis brevis (EPB), distinct and independent fromthe above-mentioned APL, is not found. The
extensor pollicis longus (EPL) muscle was foundulno-dstally to the muscle of APL. A small slender muscle is found at the dorsoradial aspect of
the ulna, ulno-distally to the origin of the extensor pollicis longus (EPL). This muscle wasjudged to be the underdeveloped extensor indicis* (extensor indicis proprius†) muscle (uEIP)
although its tendon did not reach the index ﬁnger, based on reasons described in the discussionsection (Fig. 9).
Interosseous muscles: Dorsal and palmar interosseous muscles of the hand are as well developedin the specimen of our gorilla as in the humanhand. The arrangement of each interosseous muscle corresponds to that in the human hand.

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Fig. 9. Underdeveloped EIP. A small slender muscle sitting at the dorsoradial aspect of the ulna is
judged to be underdeveloped EIP muscle
although its tendon attaches to the dorsal aspectof the wrist (a white block arrow).

Fig. 8. Extensor tendons of the ﬁngers and thumb.
A. EDCV tendon comes oﬀ EDCIV tendon. The
proper tendon sheath of EDM is seen. EIP tendon is not seen. Connexus intertendineus is notseen between EDCII and III, III and IV tendons; B.
A thin and short ﬁbrous band (a white arrow) isseen between the distal portion of EDCV and
EDCIV tendons at MP joint level; C. APL tendon attaches to the radial part of the 1st metacarpal base. A thin tendinous slip of its ulnar
margin extends more distally and attaches to theproximal phalanx (a thick white arrow).

In the specimen of our gorilla, the proportionof the insertion either to the proximal phalanx
base or to the interosseous hood (Tubiana andValentin, 1964) in each interosseous muscle isshown in Table 1A. The 1st DI muscle inserts tothe interosseous hood in the equal proportion to
the proximal phalanx, quite diﬀerent from thehuman hand (Fig. 10A, B). The proportion of theinsertion of the abductor digiti minimi muscle(ADM) to the interosseous hood is as little as in
the human hand (Table 1A, B).
Extensor apparatus of the ﬁnger: The anatomical structure of the extensor apparatus ofeach ﬁnger in the specimen of our gorilla nearlycorresponds to that of the human hand shown by
Tubiana and Valentin (1964) (Fig. 11A). In eachﬁnger, the muscle belly of DI or PI extends distally over the deep transverse metacarpal ligament (DTML) and converges with that of thelumbrical coming distally under the DTML tomake the common tendinous band. From thatband and the interosseous tendon itself, the membranous ﬁbers expand dorsally to make a triangular aponeurotic sheet (Fig. 11B). The hood-likestructure is formed together with the triangular
sheet from the opposite side, incorporating EDC
tendon at its central axis (Fig. 11B). Adherent to
the proximal margin of this interosseous hood,there is a sagittal band on each aspect of MP
joint. The sagittal band arises from DTML andextends dorsally and attaches to the margin of theextensor tendon.

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143

Table 1. Proportion of the insertion either to the proximal phalanx or to the interosseous hood in each interosseous muscle of our gorilla (A) and human (Eyler & Markee) (B).
B. Human (Eyler & Markee)*A. Our gorilla
MuscleProx. phalanx

Inteross. hood

MuscleProx. phalanx

Inteross. hood

1st DI
1st PI
2nd DI
3rd DI
2nd PI
4th DI
3rd PI
ADM

5/10
3/10
6/10
4/10
0/10
5/10
2/10
9/10

5/10
7/10
4/10
6/10
10/10
5/10
8/10
1/10

1st DI
1st PI
2nd DI
3rd DI
2nd PI
4th DI
3rd PI
ADM

100%
0
60
6
0
40
10
90

0%
100
40
94
100
60
90
10

*Quoted from Hollinshead (1969)

nates from the lateral aspect of the proximal phalanx and extends distally and dorsally along thelateral band and then the terminal extensor tendon (Landsmeer, 1949; Milford, 1968).
Metacarpophalangeal joint: The extensor tendon sitting on the top of the MP joint of the middle ﬁnger is divided longitudinally and so is thedorsal capsule of the joint beneath it to expose themetacarpal head. There is a shallow groove along
and a little proximal to the dorsal margin of thearticular cartilage of the metacarpal head (Fig.
12A). It is observed that the dorsal edge of theproximal phalanx base sits in the groove byextending the MP joint passively (Fig. 12B).
Discussion
The anatomical structure of the hand of agorilla was not much diﬀerent from the humanhand. Speciﬁc ﬁndings diﬀerent from the human
hand are discussed below.
Fig. 10. The insertions of the ﬁrst dorsal interosseous muscle. A. The distal portion of the 1st DI
muscle is voluminously muscular and passes
into the tendinous portion of the interosseous
hood at the level of the proximal portion of theproximal phalanx; B. The tendinous insertion to
the proximal phalanx base is exposed by reﬂecting the extensor apparatus. The tendon has been
divided to explore the deeper structure and thenre-sutured.

The oblique retinacular ligament (ORL) isdeveloped enough to be identiﬁed in each ﬁnger,
especially at the radial aspect (Fig. 11C). It origi-

Absence of palmaris longus muscle andpalmar aponeurosis
According to Ravenʼs book (Gregory, 1950),
PL muscle is slender, measuring 12 cm, and ispartly fused with the ﬂexor carpi radialis and thepronator teres. This description, however, does
not mean the existence of the palmar aponeurosis: the distal extension of PL. It is further statedthat PL tendon fans out and mingles with the
deep fascia of the forearm and attaches to theradius, and that very few of the tendinous ﬁbers
reach the lateral part of the transverse carpal liga-

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Fig. 11. Extensor apparatus and retaining ligaments. A. Extensor apparatus consists of two
components: extrinsic compornent (dark green)
and intrinsic component (light green). Ligamentous structures are shown in orange color. Theinterosseous hood is the proximal hood-like
portion of the aponeurotic expansion of theinterosseous tendon (a thick arrow). (modiﬁed
from Tubiana and Valentin, 1964); B. On theradial side of the middle ﬁnger, the interosseous
hood is shown. Adherent to the proximal margin of this interosseous hood, a sagittal band
expands between DTML and EDC tendon; C.
The oblique retinacular ligament (ORL) is welldeveloped at the radial aspect.

ment† (ﬂexor retinaculum*). This description inRavenʼs book suggests that there are no suchaponeurotic ﬁbers spreading over the palm as thepalmar aponeurosis in the human hand. Chapman(1878) reported that PL was absent in a young
male gorilla. Hepburn (1892) did dissections onfour species of anthropoid, one each, and foundthat PL was present in the orangutan, the chimpanzee and the gibbon but not in the gorilla.
Zihlman et al. (2011) reported that PL was absent
in all ﬁve gorillas on which he did dissection.
Those descriptions suggest that both PL muscle and the palmar aponeurosis are usually absent
in the gorilla (Diogo et al., 2017). The palmar
aponeurosis in the human is the structure that
protects the ﬁnger ﬂexor tendons and neurovascular bundles lying beneath it. It is thought thatthe ligamentous bundles arising from the ﬂexor
retinaculum and their extension: ligamentous
septa covering the ﬂexor tendons in this specimen substitute for the role of the palmar aponeurosis in the human hand (Fig. 2B, C).
In the human, the incidence of absence of PL
muscle is reported to be 12.9% (Reimann et al.,1944) or about 11% (Tountas and Bergman,1993). Although the absence of PL muscle isconsidered to mean the absence of its distal
extension: the palmar aponeurosis as well,
Ghoshal et al. (2019) reported a case where thepalmar aponeurosis arose from the distal borderof the ﬂexor retinaculum in spite of the absenceof the muscle and tendon of PL.
The short and thin thumb, underdeveloped
FPL and absence of EPB muscle
The short and thin thumb is a salient characteristic of the hand of a gorilla (Fig. 1). It is shortenough to avoid the tip of the thumb hitting
against the ground during PIP knuckle walking(Fig. 13A, B). The hands of a gorilla are notcompletely freed from the role of locomotion.
The thumb length of the gorilla seems to be carefully calculated to fulﬁll both roles of locomotion
and feeding.
Shima (2014) referred to the thickness index of
the thumb proposed by Susman (1994). It is the
proportion (percent) of the width of the head tothe length of the thumb metacarpal. He measured
the indices in early hominid fossils (Austropithecus afarensis, Paranthropus robustus, Homo erec-Anatomy of the Forearm and Hand of One Gorilla

145

Fig. 12. Dorsal aspect of the metacarpophalangeal joint. A. A shallow groove (white arrows) is seen along thedorsal margin of the articular cartilage of the metacarpal head; B. The dorsal edge of the proximal phalanx
base sits in the groove by extending the MP joint passively; C. Grooves are deep enough at the 3rd and 4th
metacarpals (white arrows), but not at the 2nd and 5th in the skeletal specimen of another gorilla stored in
National Museum of Nature and Science.

Fig. 13. Knuckle walk. A. The trunk tilts backward because of longer forelimbs; B. The knuckle walk of gorillas
is walking with MP joints in extension and PIP joints in deep ﬂexion bearing the weight on the dorsal aspectsof the middle phalanges of the hands. Sketches are drawn imitating the walking style shown in movies of
NHK BS TV program (Yamagiwa, 2012).

tus and Homo sapiens neanderthalensis), andcompared their indices to those of two extant species: chimpanzees and modern humans. He con-ﬁrmed that the three hominids (P. robustus, H.
erectus, and H. s. neanderthalensis) found withstone and bone tools showed their thumb thick-146

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Fig. 14. Grasping pattern of the hand. A. Grasping a bough of the tree; B. Holding a piece of fruit. Sketches are
drawn imitating the hand motion shown in movies of NHK BS TV program (Yamagiwa, 2012).

ness indices close to 30, similar to that of Homosapiens. So, he insisted that this index could be
one of criteria to detect the capability for reﬁned,
humanlike precision grasping in fossil hominid
species of the Plio-Pleistocene. Contrary to his
proposal, Ohman et al. (1995), who added the dataof gorillas, claimed that the thumb thickness index
indicated the strength of grasping rather thanreﬁned precision grasping essential for tool making, because the size of joint corresponds to howmuch stress is loaded on it.
Apart from this discussion, the thickness index
of the thumb is the head width/length ratio of theﬁrst metacarpal sitting in the palm. It is not theindex of the part projected out of the palm (MPjoint to the thumb tip). So, this index does notcorrespond to the appearance of “short and thinthumb” of a gorilla.
The well-developed APB in the thenar eminence suggests that placing the thumb into palmar abduction or opposition is feasible as in thehuman hand. The pentadactyly with the thumbplaceable into opposition was one feature in theevolution of the human hand (Hayama, 1999).

Movies and photos taken during the ﬁeldwork by
Yamagiwa (2012) teach us how a gorilla uses thethumb when it grasps the bough of a tree or holds
a fruit to eat. The pattern of grasping a bough is
not the hook grip but the power grip in which the
thumb is placed in the position of palmar abduction with a bough placed in the ﬁrst interdigitalspace (Fig. 14A). The pattern of holding a fruit is
the whole hand grasp with the thumb placed in
opposition (Fig. 14B).
The relative segment length of the palm of agorilla seems to be longer than that of the humanhand. Feix et al. (2015) conﬁrmed that the relative segment length of the 2nd metacarpal, and of
the proximal and middle phalanges of the index
ﬁnger was longer than that of the human hand.
This makes the entire length of the index ﬁnger
ray (2nd metacarpal plus three phalanges) longenough for the tip of the index ﬁnger to meet thatof the short thumb (Fig. 3C). They estimated thethumb-index ﬁnger precision grip and manipulation potential in extant and fossil primates usinga kinematic model based on the bony hand morphology. They described that gorillas have theAnatomy of the Forearm and Hand of One Gorillaclosest workspace to humans. This study, however, assumes that the interphalangeal (IP) jointof the thumb moves actively in the same range as
in humans. When FPL muscle is absent, active
ﬂexion of IP joint is not feasible. Thus, the workspace would be smaller than that calculated by
the kinematic model mentioned above.
Chapman (1878) reported that FPL wasentirely absent in a young male gorilla. Straus(1942) described, based on his own dissections
and data in the literature available to him, thatFPL tendon was absent in ﬁve, rudimentary andinutile in six and a half of 16 gorillas with therate of absent plus non-functioning tendon being
71.9% in all. Diogo et al. (2012) reported theﬁndings of their own dissections of ﬁve maingroups of hominoids; FPL was absent in orangutans, gorillas and chimpanzees but present inhylobatids and humans. Diago et al. (2017)
described that the tendon to digit I was present
but this tendon mainly originated from the fascia
lying over the thenar muscles in a great part ofgorillas dissected by other authors as well as infour hands of their own gorillas.
In the specimen of our gorilla, the muscle of
FDP of the index ﬁnger originates at the palmar
aspect of the radius where FPL muscle does in
the human forearm. It is considered that FDP
muscle of the index ﬁnger occupied the empty
place of the origin of FPL muscle.
Active ﬂexion of the thumb IP joint is not feasible in those cases without FPL muscle. The
motion of the thumb is controlled by thenar muscles and three forearm muscles of APL, EPL and
EPB if present. Hepburn (1892) described a very
interesting ﬁnding regarding FPL in a gorilla.
There was a tendinous band occupying the position of the long ﬂexor of the thumb, one end ofwhich attached to the carpus and the other to thebase of the distal phalanx. Bunnell (1956)
described similar ﬁndings seen in the chimpanzee
and the orangutan; there is only a narrow tendon
from the distal phalanx of the thumb to the transverse carpal ligament†. In both cases of Hepburnand Bunnell, such a tendon acts as a tenodesis,
ﬂexing the thumb automatically on extension of147

the wrist or radial abduction of the thumb.
In the specimen of our gorilla, the muscle ofuFPL comes oﬀ the distal portion of the muscle
belly of FDP of the index ﬁnger (Fig. 3B). The
own muscle belly of uFPL is much shorter than
the fully developed FPL muscle. Its stretch-contraction amplitude must be short. However, thisanatomical structure of uFPL muscle is a kind ofdigastric muscle. So, the insuﬃcient amplitudeof the distal muscle belly could be augmented bythat of the proximal one common with FDP muscle of the index ﬁnger. This structure also lets thethumb and the index ﬁnger ﬂex simultaneously
and facilitates a pinch motion between those twodigits (Fig. 3C). Phylogenetically, uFPL muscle
bifurcating from the FDP index muscle is considered to be the incomplete form of individualization from the FDP muscle (Kodera, 2010). Thishypothesis might be supported by the report of
Diogo et al. (2012) that FDP muscle sends tendons to the ﬁrst through the ﬁfth digits in species
of non-hominoid primates. Contrarily, it might beon the halfway to the complete absence of FPL
as in most of gorillas reported above.
There are some persons who cannot ﬂex thethumb and the index ﬁnger individually. Simultaneous ﬂexion of both the thumb and the index ﬁnger by a common ﬂexor muscle might be the character that the common ancestor of both the humanand the gorilla had in the process of phylogeny.
Chapman (1878) described that there existed a
distinct extensor primi internodii pollicis (Extensor pollicis brevis*: EPB) in a gorilla of his specimen. Straus (1941) casted some doubt on thisdescription “distinct” because other authors suchas Duckworth, Eisler, Hepburn, Pira and Sommer
described that EPB merely was a derivative of
the metacarpal division of the abductor. Gibbs etal. (2000) reported, based on the statistical analysis of soft tissue characters in higher primates byan extensive literature search, that EPB muscle
with the tendon inserting on the base of the proximal phalanx of the thumb as in the humanexisted only in the gorilla out of four anthropoids: the gibbon, the orangutan, the gorilla andthe chimpanzee. Straus (1941) described that

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such a part of the abductor muscle complex as
inserts upon the phalangeal portion of the thumbseemed to have been found only in the gorilla(56%, 9/16) and the gibbon (7%, 1/15). He also
referred to Keith who wrote that the tendon of
APL sent a slip to the proximal phalanx of thethumb in one chimpanzee out of 20 reported,
although he has not encountered such statement
in any other publications. The ﬁnding seen at thedorsoradial aspect of the wrist in our gorilla isjust the same as Keithʼs description of the tendonof the long abductor in that chimpanzee (Fig.
8C). Another unique ﬁnding in our specimen isthat an accessory muscle bundle coming from thecommon muscle of EDC joins the muscle bellyof APL at its myotendinous junction. We are notsure that this muscle bundle is the EPB muscle,
because it comes oﬀ the superﬁcial extensor
mass rather than the deep one. Straus (1941)
claims that EPB indubitably is a diﬀerentiatedportion of APL, the tendon of which has
migrated distally on to the proximal phalanx ofthe thumb. According to Tountas and Bergman(1993), EPB was reported absent only in 6.3% ofcases in the human. So, the existence of EPB
seems to be closer to the human hand.
Underdevelopment of both FPL and EPB muscles of the thumb seen in the specimen of ourgorilla seems to be a transitional form between
the hand of anthropoids and the human hand.
This form, however, must be speciﬁcally evolved
and adapted for gorillas to live in the wild.
Extensor apparatus, interosseous hood, sagittal band and oblique retinacular ligament
Tubiana and Valentin (1964) showed the anatomical structure of the extensor apparatus in thehuman hand schematically with precise names ofeach detailed component (Tubiana, 1981) (Fig.
11A). The extensor apparatus is composed of two
muscular systems: the extrinsic system (EDC,
EIP, EDM muscles arising from the forearm) andthe intrinsic system (interosseous and lumbricalmuscles originating from the hand proper). The
aponeurotic expansion of the interosseous tendon
forms a triangular sheet. The proximal transverse

ﬁbers of this triangular sheet form the interosseous hood together with those ﬁbers from the
opposite side (Tubiana and Valentin, 1964) (Fig.
11A, B). According to their description, the proximal portion of the entire dorsal expansion(extensor expansion) is called “interosseoushood”. This hood-like structure covers the dorsal
portions of the proximal phalanx and MP joint.
The anatomical structure of the extensor apparatus of each ﬁnger in the specimen of our gorillanearly corresponds to that of the human hand
shown by Tubiana and Valentin (1964) (Fig.
11A). The intrinsic components of the extensor
apparatus in each ﬁnger arise mainly from both
DI and PI muscles, and the lumbrical muscle on
the radial side. The main role of each interosseous muscle depends on its location (Lampe andNetter, 1951; Lampe, 1974). The 1st DI muscle
is located on the radial side of the 2nd MP joint
in the 1st intermetacarpal space. In the human
hand, it inserts entirely to the radiopalmar cornerof the proximal phalanx base and works as
abductor and ﬂexor of the 2nd MP joint (Table
1B). In the specimen of our gorilla, however, one
half of the component of 1st DI inserts to theproximal phalanx base and the other does into
the interosseous hood. (Fig. 10A, B, Table 1A).
In the human hand, the 3rd DI muscle located on
the ulnar side of the 3rd MP joint inserts mostly(94%) to the interosseous hood while that of ourgorilla sends only a little more than half of theinsertion (6/10) to it (Table 1 A, B).
EDC tendon extends distally and trifurcates
into the extensor middle band and, radial andulnar extensor lateral bands at the level of theproximal phalanx (Fig. 11A). The extensor middle band extends distally and merges with theinterosseous middle bands coming from bothsides to make the central extensor tendon that
attaches to the middle phalanx base. The extensor lateral band on each side merges into theinterosseous lateral band described below.
The interosseous tendons on the both sides
send the membranous ﬁbers dorsally and thecord-like ﬁbers distally (Fig. 11A, B). The cordlike portion bifurcates to the interosseous middle

Anatomy of the Forearm and Hand of One Gorilla

149

Fig. 15. The mechanism to block hyperextension of the MP joint with the function of the sagittal band. A. Whenthe ﬁnger is ﬂexed, the sagittal band passes over the joint axis of the metacarpal head in coordination with the
distalward movement of the extensor tendon; B. When the MP joint is extended and surpasses the range ofextension, the traction force of the ﬁnger extensor is blocked with the tightened sagittal band that transmits theforce palmarward so that the proximal phalanx base does not move further to hyperextension.

band and the interosseous lateral band. The lumbrical muscle coming under DTML on the radialside of each ﬁnger joins the interosseous lateralband. The interosseous lateral band joins theextensor lateral band to make the lateral band. It
extends further distally to unite with the lateralband coming from the other side to form the terminal extensor tendon that attaches to the distal
phalanx.
In the human hand, the sagittal band arises
from the DTML and extends dorsally andattaches to the margin of the extensor tendon.
This band seen on each side of the MP joint connects the extensor tendon with DTML to stabilize
the former on the top of the MP joint as
described by Tubiana and Valentin (1964), andMilford (1968). The same aponeurotic band isalso named the transverse lamina by Landsmeer(1955). In addition to stabilizing the extensor
tendon on the top of MP joint, the sagittal bands
have the other role to restrict the hyperextensionof MP joint. The sagittal bands transmit the traction force of the ﬁnger extensor palmarward to
DTML when the MP joint surpasses the range ofextension (straight position) so that the proximal
phalanx base does not move further to hyperex-

tension (Fig. 15A, B). This anatomical structure
also exists in the specimen of our gorilla. We
consider that the same mechanism as in thehuman hand works in gorillas, especially on the
MP joints with shallow metacarpal grooves.
Another noteworthy ﬁnding was that theoblique retinacular ligament (ORL), one of theretaining ligaments of the ﬁnger, was well developed in each ﬁnger especially at the radial aspect(Fig. 11C). It arises from the radial or ulnar aspectof the proximal phalanx along the attachment of
the ﬂexor sheath and extends distally and dorsally
along the margin of the lateral band, and thenmerges into the terminal tendon at the dorsal
aspect of DIP joint. This anatomical structure corresponds to ORL in the human hand (Landsmeer,
1949; Milford, 1968). As ORL passes palmar to
the rotation axis of PIP joint and then dorsal to therotation axis of DIP joint, it loses tension when
PIP joint is ﬂexed so that DIP joint becomes ﬂexible. When PIP joint is extended, it tenses so that
DIP becomes extended. So, ORL facilitates simultaneous extension or ﬂexion of both PIP and DIP
joints (Landsmeer, 1963). Haines (1951) called
this ligament “link ligament”.
Those anatomical evidences of the extensor

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apparatus suggest that gorillas could use theirﬁngers for manipulating, grasping or pinching
objects in a similar manner to humans. It also
supports that they could exert strong power on
the ﬁnger pulps enough to hold the surface of the
thick trunk of the tree while climbing it as shownby the ﬁeldwork of Yamagiwa (2005, 2012).
Knuckle walk and stabilization of the MP
joint
The skin at the dorsal aspect of the middle
segment of each ﬁnger is so thick as to be calleda ʻknuckle padʼ, where chronic irritation had been
inﬂicted (Mackey and Cobb, 1994). This is a
proof that this gorilla had walked with knuckles.
The knuckle walk of gorillas is walking with MP
joints in extension and PIP joints in deep ﬂexion
bearing the weight on the dorsal aspects of the
middle phalanges of the hands (Fig. 13A, B).
Stability of both PIP and MP joints are secured
mainly with coordination of the forces of theextensors and ﬂexors during the stance phase of
knuckle walking. The mechanism to stabilize the
MP joints seems to be essential during knuckle
walk in which the MP joints are stabilized inextension (straight position), and the PIP joints inover 90° ﬂexion.
Tuttle (1969) listed major features associatedwith knuckle walking in the African apes as follows: (1) conﬁguration of the carpal bones and the
distal radius, (2) increased development of thewrist ﬂexors, (3) extension of the articular cartilage onto the dorsal aspects of the 2nd through 5th
metacarpal heads, (4) prominent ridges on the dorsal aspects of the metacarpal heads, (5) shorteningof the long digital ﬂexor muscles, (6) prominent
development of the lumbrical and interosseousmuscles and (7) friction skin over the dorsal
aspects of the middle phalanges. The ﬁndings seenin the right hand of our gorilla correspond to thefeatures: (3)(4)(6) and (7) described above. Items(1)(2)(5) are beyond the scope of this paper since
we did not do any osteoarticular study of the wrist
joint or any quantitative muscle study of wrist or
ﬁnger ﬂexors.
The groove and ridge located along the dorsal

margin of the articular cartilage of the metacarpal
head seen in our specimen correspond to the feature (4) listed above. The groove is calledʻknuckle walking grooveʼ that is the evidence ofwalking on knuckles (Fig. 12A). The role of thegroove and the ridge behind it might be to prevent MP joints from going into hyperextension
by bony blockage. It is observed in the middle
ﬁnger of this specimen that the dorsal edge of theproximal phalanx base sits in the groove when
MP joint is extended passively (Fig. 12B). This
groove, however, seems to be too shallow toblock the proximal phalanx base moving over the
ridge of the groove into hyperextension. Observation of the skeletal specimen of another gorillastored in National Museum of Nature and Science revealed that grooves are deep enough at
the 3rd and 4th metacarpals but not at the 2nd
and 5th (Fig. 12C).
The sagittal bands play a role to restrict hyperextension of MP joint in the human hand (Green,1982). Those transmit the traction force of the
ﬁnger extensor palmarward just when MP joint
surpasses the range of extension so that the proximal phalanx base does not move to further
hyperextension (Fig. 15A, B). This anatomical
structure also exists in the specimen of ourgorilla as described above. We consider that thesame mechanism as in the human hand works ingorillas, especially on MP joints with shallow
metacarpal grooves.
Retaining structures of the ﬁnger ﬂexor
An osseous crest along each edge of the palmar aspect of the proximal phalanx and thegroove between those crests make the place
where the ﬂexor tendon sits ﬁrmly (Fig. 5). Eachpulley of the ﬂexor tendon sheath attaches to thecrests of the palmar aspect of the proximal phalanx. The thick and wide annular pulley A2′ ofthe ﬂexor sheath, which is not seen in the human
hand, is considered to reinforce the retaining
structure of the ﬂexor sheath to prevent the ﬂexor
tendon from bowstringing and slipping out of theosseous groove (Fig. 4A, B), when the ﬂexor tendon gets extremely tight on powerful graspingAnatomy of the Forearm and Hand of One Gorillasuch as climbing the tree or grasping a boughwhile living in the wild (Fig. 14A) (Jones, 1946;
Yamagiwa, 2005, 2012).
Underdeveloped EIP and pointing
A small slender muscle sitting at the dorsoradialaspect of the ulna in the deep layer of the forearm
extensors is judged to be the underdeveloped EIP(uEIP) muscle, although its thin tendon attaches tothe dorsal aspect of the wrist joint (Fig. 9). The
following descriptions support this judgment.
There is a description of M. extensor indicis
proprius† (EIP) in the gorilla in Ravenʼs book
(Gregory, 1950), stating that it is a slender, spindle-shaped muscle existing between EPL muscle
and the ulna, and that its tendon divides and mostof it joins EDCII tendon while the remainder consists of two slender tendons; one of these inserts
on the dorsum of the hamatum and the other on
the capitatum. It occupies the same location of
the origin of EIP as in the human forearm
described by Kaplan (1965). Ueba (1985)
described that there is a variation of EIP in
human specimens in whom the tendon does notreach the index ﬁnger but inserts to the dorsal
aspect of the second metacarpal.
Chapman (1878) reported that the extensor
indicis* (EIP) existed in a young male gorilla.
Hepburn (1892) described, from the ﬁndings of
his own dissection of four species of anthropoids,
one of each species, that EIP muscle in thegorilla was as slender as in the human, while itshowed a considerably greater development withplural tendons inserting to other digits as well as
the second digit in other species of anthropoids:
the chimpanzee, the orangutan and the gibbon.
After extensively reviewing literature on soft tissue anatomy of the ﬁve extant hominoid genera,
Gibbs et al. (2000) reported that the most common pattern of the insertion of EIP is to the second digit in the gorilla. Aversi-Ferreira et al.
(2010) reported that EIP existed in the gorilla
based on an extensive review of the literature andon the dissections by themselves. Diogo et al.
(2017) showed in their atlas of gorilla that thetendon of this muscle attached to only the 3rd151

and 4th metacarpals on one side while to theindex ﬁnger as well as to the 3rd and 4th metacarpals on the other side.
Tan and Smith (1999) stated, by reviewing literature of anomalous extensors muscles of thehuman hands, that the deep portion of the forearm extensors to which EIP belongs appears tobe highly unstable and undergoes considerable
evolutionary change in the great variation in different species of primates. They showed in thesketches the anomalous ﬁnger extensors in thehuman: the extensor digitorum brevis manus, theextensor medii proprius and the extensor indicis
et medii communis. Yoshida (1995) also reported
the anomalous extensors in the human such as
the extensor pollicis et indicis accessories andextensor indicis radialis, both of them arisingfrom the deep layer of the forearm extensors.
Considering those pieces of information, EIP
muscle with the insertion to the index ﬁnger
exists in most of gorillas but that of our gorilla isspeciﬁcally underdeveloped; its tendon does notreach the index ﬁnger.
The fully developed EIP is essential to extend
the index ﬁnger while other ﬁngers being ﬂexed
in such a motion of pointing. When EIP is functioning, MP joint of the index ﬁnger is extended.
When EIP muscle is underdeveloped and its tendon does not reach the proximal phalanx of theindex ﬁnger as in our gorilla, theoretically, intrinsic muscles alone can extend both PIP and DIP
joints but not MP joint. The question is whether agorilla has the ability to point to the particular
object. According to Leavens and Hopkins(1999), Patterson (1978) reported that a gorilla(born in captivity, initially mother reared and language trained) pointed to itself and the object orfood placed at a distance. It is not clear which
was used, the index ﬁnger alone or the wholehand with outstretched ﬁngers. Pointing with all
outstretched ﬁngers is feasible mainly with thefunction of EDC even when EIP muscle or tendon is absent.

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No connecting bands in the radial part ofﬁnger extensor tendon complex
Connexus intertendineus* (junctura tendinum†) in the human hand are transverse or
oblique tendinous bands connecting individual
EDC tendons between EDC index ﬁnger (EDCII)
and EDC middle ﬁnger (EDCIII), EDCIII and
EDC ring ﬁnger (EDCIV). There are no such connecting bands seen between each extensor tendonof the index through ring ﬁngers in the specimenof our gorilla (Fig. 8A). EDC little ﬁnger (EDCV)
tendon coming apart from the midportion of
EDCIV tendon runs obliquely to the EDM tendon
at the MP joint of the little ﬁnger. A thin, obliqueand short ﬁbrous band is also seen between the
distal portion of EDCV tendon and EDCIV tendon
at MP joint level (Fig. 8B). Those are considered
to work as functionally as a part of connexus
intertendineus.
Connexus intertendineus is not seen, although
the intervening tissues between the individual
tendons might have been excised during preparation of the specimen, in photos of ﬁnger extensors in the atlas of Diogo et al. (2017). This
structure is also absent on the hand of a gorilla
shown in the book of Kaplan (1965). He explainsthat absence of this structure between the individual tendons of EDCII and EDCIII is ʻadvancedʼ
isolation of those tendons, referring to Strausʼs(1941) observation in the series of primates.
Straus described that the terminal tendons of
EDC are intimately interconnected in the hand in
most specimens of Tupaia, lemurs, Tarsius andboth groups of monkeys, and sometimes also in
gibbons, and that these connections are reduced
to the so-called juncturae tendinum† in anthropoid apes and man. He also described that eventhese junctures were said to be entirely lacking insome examples of the anthropoids.
Connexus intertendineus of the human hand isconsidered to play a role in keeping the extensor
tendons on the top of the knuckle of MP joints
especially on making a tight ﬁst, as explained
below. The transverse arch of the human hand
lengthens at the level of MP joints by making atight ﬁst because the 4th and 5th carpometacarpal

joints (CM joint) ﬂex and the knuckles of thering and little ﬁngers sink palmarward (Boileau
Grant and Basmajian, 1965; Zancolli, 1979).
This phenomenon also occurs on opposing the
thumb to the little ﬁnger (Kapandji, 1970). Eachtendon of EDCIII or EDCIV consists of two slips
in the human hand. So, connexus intertendineus
connects EDCII tendon with the radial slip of
EDCIII tendon and, the ulnar slip of EDCIII tendon with the radial slip of EDCIV tendon, leaving
each cleavage between two slips of the individual tendons going to the middle or ring ﬁngers
not bound. The tension on the extensor tendonsexerted by lengthening of the transverse arch of
the hand is absorbed by widening each cleavage.
Ulnarward pull of the extensor tendons transmitted through the connexus intertendineus is also
restrained by the checkrein eﬀects of the more
ﬁxed radial components of the extensor mechanism: the 2nd and 3rd CM and MP joints, andinterosseous muscles, interosseous hoods, sagittal bands, collateral ligaments on the radial sideof MP joints.
This functional role is the reason why connexus intertendineus is retained in the humanhand. In the hand of our gorilla, there was not
such a cleavage in each individual tendon goingto the index through ring ﬁngers, which consistsof one slip.
The absence of connexus intertendineus in thegorilla suggests immobility of the 4th and 5th
CM joints so that the transverse arch of the handdoes not lengthen at the level of the MP joints on
making a tight ﬁst. Immobility of those joints
might aﬀord support to the ring and little ﬁngers
on pushing oﬀ the hand (Jones, 1946). Further
study is needed to determine whether the 4th and5th CM joints in the hand of the gorilla aremobile or not.
The ulnar antebrachial branch of the median
nerve innervating the intrinsic muscles of thehand
The ulnar antebrachial branch of the median
nerve (UABM) seen in our gorilla is the mostsigniﬁcantly diﬀerent anatomical structure from

Anatomy of the Forearm and Hand of One Gorilla

the human forearm (Fig. 6 proximal, distal). Thisbranch conveys the motor ﬁbers to all intrinsic
muscles of the hand other than thenar muscles
and radial two lumbricals innervated by motor
branches from the main trunk of the median
nerve (Fig. 7). In the human forearm, thosemotor ﬁbers are conveyed through the ulnar
nerve. We do not know whether the innervationof those intrinsic muscles through the median
nerve seen in this specimen is the common pattern in the gorilla or not. “The ramus ulnaris of
the nervus medianus” is described in the text andclearly shown in the plate 40 as “ramus anastomoticus n. medianus cum ulnaris” in Ravenʼsbook (Gregory, 1950).
Martin-Gruber anastomosis connecting themedian nerve with the ulnar nerve in the forearmis not so uncommon in the human. Gruber
reported that the incidence of this anatomicalvariation was 17.9% in human cadavers (Leibovic and Hasting, 1992). Leibovic and Hasting(1992) analyzed the world literature of MartinGruber connections in the human forearm andclassiﬁed them into four types. Type II is theconnection sending motor branches from themedian to the ulnar nerve that conveys motor
ﬁbers to “ulnar intrinsic muscles (interossei,
ADP, FPB deep head, 3rd & 4th lumbricals andhypothenar muscles). UABM that conveys motor
ﬁbers to innervate ulnar intrinsic muscles, however, comes close to the ulnar nerve at the distal
forearm but does not unite with it. In our gorilla,
there are two thin connecting branches between
the ulnar nerve and UABM in the proximal forearm. A very ﬁne proximal branch connects theulnar nerve with the radial bundle (motor fasciculi) of UABM and the other thicker distal one
with its ulnar bundle (sensory fasciculi). Therefore, all ulnar intrinsic muscles are innervated by
UABM although the very ﬁne proximal branchcoming from the ulnar nerve might take a part in
innervating the intrinsic muscles. The motor
branches coming from the main trunk of themedian nerve innervate thenar muscles and two
radial lumbricals. So, all intrinsic muscles of thehand of this gorilla are mainly innervated by the153

median nerve.
This innervation of all intrinsic muscles by the
median nerve corresponds to the term “allmedian hand”, which Marinacci and von Hagen(1965) proposed. According to their report “themotor supply to the hypothenar-interosseousmuscle group was travelling in the median nerveat the elbow and crossed to ulnar side to supply
the muscles more usually innervated by the ulnar
nerve” in a case of neuroﬁbroma in the ulnar
nerve above the elbow. In the human upper limb,
there are other reports in which the intrinsic muscles are not paralyzed when the ulnar nerve is cutor anesthetized at the elbow or the proximal forearm (Mannerfelt, 1966; Okubo et al., 1977). The
explanation for this paradoxical phenomenon isthat the motor ﬁbers innervating those intrinsic
muscles come through the median nerve in thearm and then the Martin-Gruber anastomosis in
the forearm.
N.B.
Names marked with * correspond to P.N.A.
(Paris Nomina Anatomica)
Names marked with † correspond to B.N.A.
(Basel Nomina Anatomica)¶: This part was submitted in a short message
to American Society for Surgery of theHand Perspectives, January 2020 edition.
Acknowledgement
The author (H.S.) 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.
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