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Home , Hypothenar eminence, Muscles of the hand, Thenar eminence

Anthropol.Sci. 102(Suppl.), 85-95,1994

Intrinsic Muscles of the Japanese Monkey, Macaca fuscata

TOSHIHIKO HOMMA AND TATSUO SAKAI

Department of Anatomy, School of Medicine, Juntendo University, Hongo 2-1-1, Bunkyo-ku, Tokyo 113, Japan

Received December 24, 1993

•ôGH•ô Abstract•ôGS•ô Anatomy of the intrinsic hand muscles in the Japanese monkeys was

studied with an improved method to dissect the muscles and in water after removal of the skeletal framework. The contained four muscles, namely m. abductor pollicis brevis, m. opponens pollicis, m, flexor pollicis brevis, and m. adductor pollicis. The contained four muscles, namely m. palmaris brevis, m. abductor digiti minimi, m. flexor digiti minimi brevis, and m. opponens digiti minimi. Majority of the thenar muscles are fused more or less with each other, so that clear separation of these muscles was difficult. The lumbrical muscles arose from the palmar parts of four main of the deep flexor muscle to the second, third, fourth, and fifth digits. The mm, contrahentes arose mainly from a medial tendinous septum attached on the palmar surface to the third metacarpus, and included three muscles destined to the second, fourth and fifth digits. The interossei were found on the radial side of the second, third, fourth and fifth as well as on the ulnar side of the second, third and fourth finger. The intrinsic hand muscles in the Japanese monkey received innervation either from the median or the ulnar . Branching pattern of these nerves to the individual muscles was fundamentally similar to those in man except for the fact that the median and the in the Japanese monkey do not communi cateto make a loop in the thenar muscles. A separate branch from the to the deep head of m. flexor pollicis brevis (accessory recurrent nerve) was described for the first time in the macaque monkey.

•ôGH•ô Key Words•ôGS•ô: hand muscle, innervation, median nerve, ulnar nerve, Japanese

monkey

INTRODUCTION

The primate can grasp and manipulate objects by which move

more freely than those in other mammals. The movement of fingers is executed by

extrinsic and intrinsic hand muscles. The extrinsic hand muscles are located in the

, send tendons to the palmar and dorsal aspects of the hand, and move the

joint and interphalangeal joints. The intrinsic hand muscles are located in the

palm, inserted onto the base of each finger, and move the metacarpophalangeal

(MP) joints. Anatomy of the hand muscles has been studied in several species of primates 86 HOMMAAND SAKAI including gorilla (Raven, 1950), chimpanzee (Champneys, 1872; Sonntag, 1923), orangutan (Sullivan and Osgood, 1927; Homma and Sakai, 1991a), rhesus monkey (Howell and Straus, 1933; Jacobi, 1966), and crab-eating monkey (Kimura and Takai, 1970). Forster (1916) studied anatomy of the intrinsic hand muscles in the primates comparatively. A detailed description of anatomy of the hand muscles in the anthropoid apes was published by Hepburn (1892). Outline of the intrinsic hand muscles in primates has been elucidated by these authors. However, findings in these studies frequently contradict each other in detail, probably due to difficulty in dissecting the intrinsic hand muscles in narrow spaces of the hand. In a series of studies on the human and nonhuman primate hands (Homma and Sakai, 1991a, b, 1992), we employed an improved method to dissect the intrinsic hand muscles and their distributing nerves in water after removal of the skeletal framework. With this method allowing better identification and visualization of the muscles and nerves, we reported several new findings on branching pattern of the median and ulnar nerve in the hand. In the present study, we dissected the intrinsic hand muscles and their distribut ingnerves in the Japanese monkey with this improved dissection method. The findings are compared with those in other monkeys in the Macaca genus in litera ture,namely the rhesus monkey (Howell and Straus, 1933; Jacobi, 1966) and crab eatingmonkey (Kimura and Takai, 1970).

MATERIALS AND METHODS Six hands of three adult Japanese monkeys supplied from The Primate Research Institute of Kyoto University, Aichi, Japan were examined. After removal of the superficial part of the hand, including the and the long flexor tendons, the individual thenar and hypothenar muscles were identified, and then all the hand muscles with their distributing nerves were removed from the skeletal framework as a whole. Each specimen included all the intrinsic hand muscles and their distributing nerves. The specimens were fixed with insect pins on a gum board and immersed in water for further dissection. Distributing pattern of the nerves was analyzed after removal of the epineurium.

RESULTS The intrinsic hand muscles in the Japanese monkeys were divided into five groups, namely thenar muscles, hypothenar muscles, lumbrical muscles, mm. contrahentes and interossei. The thenar and hypothenar muscles made thenar and hypothenar eminences, respectively. The lumbrical muscles were associated with the deep flexor . The interossei occupied spaces between neighboring meta carpalbones. The mm. contrahentes were found in the layer between the deep HandMuscles of theJapanese Monkey 87 flexor tendon and the interossei.

(1) Thenar muscles (Figs. 1,2) The thenar eminence included the thenar pad as well as four of the muscles that moved the , namely m. abductor pollicis brevis (AbPB), m. opponens pollicis (OP), m, flexor pollicis brevis (FPB), and m. adductor pollicis (AdP). The bellies of these muscles were fused more or less with each other, so that clear separation of these muscles was difficult. The AbPB was located in a superficial radial part of the thenar eminence and divided into a superficial and a deep layer. The superficial layer originated from within the thenar pad, and the deep layer began on the flexor retinaculum. These layers fused with each other to terminate onto the radial sesamoid bone of the thumb. The OP were located beneath the AbPB and on the radial side of the FPB. It originated from the flexor retinaculum and the trapezium, and terminated onto the first metacarpal bone. The FPB possessed two heads. The superficial head was located on the ulnar side of the AbPB, and originated from the trapezium. The deep head was found between the superficial head and the AdP, and originated from the trapezium and trapezoid. The deep head was mingled with the AdP near the origin, but distin guishedfrom the latter by the fact that the deep head traversed beneath the long flexor tendon to the pollex and fused with the superficial head and the AbPB to terminate onto the radial sesamoid bone of the thumb. The AdP was found on the ulnar side of the thenar eminence, and originated mainly from a medial tendinous septum which separated the origin of the AdP and that of the m. to the fourth digit and which was anchored on the palmar surface of the shaft of the third metacarpal bone. A small part of the AdP originated from the articular capsule of the second and third MP joints and from the deep transverse metacarpal ligament between them. The major part of the AdP termi natedonto the ulnar sesamoid bone of the thumb, and the minor part extended over the sesamoid bone to terminate onto the proximal part of the distal phalanx of the thumb.

(2) Hypothenar muscles (Figs. 1, 2) The hypothenar eminence contained four muscles, namely m. palmaris brevis (PB), m. abductor digiti minimi (AbDM), m. flexor digiti minimi brevis (FDMB), and m. opponens digiti minimi (ODM). The PB was embedded within the hypothenar pad and covered the other three hypothenar muscles. It originated from the flexor retinaculum and terminated in the skin on the ulnar margin of the hypothenar eminence. 88 HOMMA AND SAKAI

Figs. 1 and 2. Schematic drawings showing the intrinsic hand muscles and their distributing nerves in the Japanese monkey in situ. In Fig. 1, the superficial flexor tendons are cut off at the level of the proximal interphalangeal joint, and the superficial layer above the deep flexor tendon (tD) is depicted. In Fig. 2, the deep flexor tendons are cut off and the intermediate layer above the deep branch of the ulnar nerve (nU) is exposed. In the thenar eminence, m. abductor pollicis brevis (Ab), superficial head of m. flexor pollicis brevis (Fs) and m. adductor pollicis (Ad) as well as the thenar fat pad (asterisk) are indicated. In the hypothenar eminence, m. abductor digiti minim (Am), m. flexor digiti minim brevis (Fm) and m. opponens digiti minimi (Om) are indi cated.In the superficial layer (Fig. 1), the lumbrical muscles (L2, L3, L4 and L5) are obvious, whereas in the intermediate layer (Fig. 2), mm. contrahentes (C4 and C5) are exposed. The median nerve (nM) sends off the recurrent nerve (single arrow) and the accessory recurrent nerve (double arrow) to the thenar muscles as well as branches to the lumbrical muscles of the second and third finger (arrowheads). Branches of the ulnar nerve (nU) are depicted in detail in Figs. 3 and 4. *5 and *6 indicate branches of the deep branch of the ulnar nerve to the lumbrical muscles of the fourth and fifth finger, respectively. tS2, stump of the superficial flexor tendon to the second finger.

The AbDM occupied the ulnar part of the hypothenar eminence. It originated from the pisiform bone and terminated onto the ulnar sesamoid bone of the fifth digit. The FDMB was found on the radial side of the AbDM, originated from the flexor retinaculum, and terminated mainly onto the base of the proximal phalanx of Hand Muscles of the Japanese Monkey 89 the fifth digit. A part of the muscle fascicle of the FDMB joined the AbDM to terminate onto the ulnar sesamoid bone. The ODM were located deep beneath the FDMB. It originated from the flexor retinaculum and the hamatum and terminated onto the fifth metacarpal bone. The muscle fascicles originating from the flexor retinaculum and those from the hamatum tended to separate from each other.

(3) Lumbrical muscles (Fig. 1) The deep flexor muscle sent four main tendons to the second, third, fourth, and fifth digits as well as a branch tendon to the thumb. The lumbrical muscles arose from the palmar parts of the four main tendons and we named them L2, L3, L4 and L5, respectively. They passed on the radial side of the base of individual digits and terminated on the dorsal aponeurosis.

(4) Mm. contrahentes (Fig. 3) The mm. contrahentes and the AdP made a group of muscles which were located in a layer deep to the deep flexor tendons and superficial to the deep branch of the ulnar nerve. They arose mainly from a medial tendinous septum attached on the palmar surface to the third metacarpus. In the Japanese monkey, three mm. contrahentes were destined to the second (Ch2), fourth (Ch4) and fifth digits (Ch5). The Ch2 arose from distal part of the medial tendinous septum and capsule of the third MP joints, and terminated on the ulnar side of the base of the second proximal phalanx. It was much smaller than the Ch4 and Ch5, and was absent in one hand out of the six examined. The Ch4 arose from the medial tendinous septum, and passed through on the radial side of the fourth proximal phalanx. It fused together with the palmar interossei on the radial side of the fourth phalanx (Io4r, see later) to terminate mainly on the dorsal aponeurosis and in part on the radial side of the base of the fourth proximal phalanx. The Ch5 was found on the ulnar side of the Ch4, and arose from proximal part of the medial tendinous septum and from proximal part of the of the Ch4. It terminated mainly on the radial side of the base of the fifth proximal phalanx and in part on the dorsal aponeurosis.

(5) Interossei (Fig. 4) The second, third and fourth digits possessed interossei both on the radial and on the ulnar side, and the fifth digit possessed an interosseus on the radial side. Since the individual mass of the interossei either on the radial or on the ulnar side of each digit was not separated into clear subgroups, we found as a whole seven interossei in the Japanese monkey, namely Io2r, Io2u, Io3r, Io3u, Io4r, Io4u and Io5r. 90 HOMMA AND SAKAI

Figs. 3 and 4. Schematic drawings showing the intrinsic hand muscles and their distributing nerves in the Japanese monkey after removal of the skeletal framework. In Fig. 3, the intermediate layer above the deep branch of ulnar nerve (nU) is depicted, whereas in Fig. 4, the deep layer below the deep branch is exposed. Three thenar muscles out of the four, m. abductor pollicis brevis (Ab), superficial (Fs) and deep head (Fd) of m. flexor pollicis brevis and m. adductor pollicis (Ad), as well as the three hypothenar muscles, m. abductor digiti minim (Am), m. flexor digiti minim brevis (Fm) and m. opponens digiti minimi (Om), are indicated. In the intermediate layer (Fig. 3), mm. contrahentes to the fourth and fifth finger (C4 and C5) are obvious, whereas in the deep layer (Fig. 4), mm. interossei on the radial (I2r, I3r, I4r and I5r) and ulnar side (I2u, I3u and I4u) of each finger are depicted. The median nerve (nM) sends off the recurrent nerve (single arrow) and the accessory recurrent nerve (double arrow) to the thenar muscles. The ulnar nerve (nU) sends branches to the hypothenar muscles and interossei as well as to m. adductor pollicis (*1), to mm. contrahentes of the second (*2), fourth (*3) and fifth finger (*4), and to lumbrical muscles of the fourth (*5) and fifth finger (*6). aD1, aD2, aD3, aD4 and aD5 indicate the dorsal aponeurosis of the individual fingers.

The interossei were found in a layer under the deep branch of the ulnar nerve. They arose from the metacarpus of the concerned and the neighboring digit as well as from carpal bones. They terminated on the base of the proximal phalanges and on the dorsal aponeurosis. HandMuscles of theJapanese Monkey 91

The Io2r, Io3r, Io3u and Io4u were found in the dorsal aspects of the interosseous spaces, and represented the dorsal interossei in the human anatomy. On the other hand, the Io2u, Io4r and Io5r were found in the palmar aspects of the interosseous spaces, representing the palmar interossei. Distal parts of the Io4r were fused with the Ch4.

(6) Innervation of intrinsic hand muscles (Figs. 3, 4) The intrinsic hand muscles in the Japanese monkey received Innervation either from the median or the ulnar nerve. The distribution pattern of the individual branches of these nerves was analyzed after removal of the epineurium. The median nerve branched into four motor and seven sensory branches after passing through the carpal tunnel. The first motor branch, the recurrent nerve, terminated on major parts of thenar muscles including the AbPB, the FPB and the OP, and the second motor branch, the accessory recurrent nerve, innervated a part of the deep head of the FPB, whereas the other part of the thenar muscles, namely the AdP, was innervated by the ulnar nerve. The third and fourth motor branches innervated the L2 and L3, respectively. The sensory branches distributed the radial and ulnar side of the first, second and third digits and the radial side of the fourth digit. The recurrent nerve was independent from the other branches. The accessory recurrent nerve and the ulnar sensory branch of the first digit made usually a common trunk. The lumbrical nerves (L2 and L3) tended to fuse with the radial sensory branches of the corresponding digits, respectively. The ulnar sensory nerve of a given digit and the radial sensory nerve of the adjacent digit on the ulnar side were usually covered together by epineurium, and called a common palmar digital nerve. The ulnar nerve divided into a superficial and a deep branch after passing through the carpal tunnel. The superficial branch was branched into a motor branch to the PB and three sensory branches which distributed the ulnar side of the fourth digit and the radial and ulnar side of the fifth digit. The deep branch sent off three branches to the hypothenar muscles (AbDM, FDMB and ODM), seven branches to the interossei (Io2r, Io2u, Io3r, Io3u, Io4r, Io4u and Io5r), two branches to the third and fourth lumbrical muscles (L4 and L5), three branches to the mm. contrahentes (Ch2, Ch4 and Ch5), and four sensory branches to the MP joints to become a terminal branch which innervated the AdP. In two cases out of the six examined, the deep branch ran on the surface of the FDMB, whereas in the other four cases it penetrated the FDMB. The sensory branches to the MP joint as well as the lumbrical branches (L4, L5) made usually a common trunk with the radial interosseous branches of the corre spondingdigits (Io2r, Io3r, Io4r and Io5r), respectively. The contrahentes branch to 92 HOMMAAND SAKAI the Ch5 arose in most cases directly from the deep branch, that to the Ch4 made a common trunk with the radial interosseous branch of the third digit (Io3r), and that to the Ch2 made a common trunk with the sensory branch of the third MP joint.

DISCUSSION

Anatomy of the hand muscles in the macaque monkeys has been studied so far in the rhesus monkey, Macaca mulatta (Howell and Straus, 1933; Jacobi, 1966), and in the crab-eating monkey, Macaca fascicularis (Kimura and Takai, 1970). The present study provides some new findings on the hand muscles of the Japanese monkey which are different from those on the other macaque monkeys.

Thenar muscles The AbPB in the Japanese monkey contains two layers, namely a superficial layer arising from within the thenar fat pad and a deep layer from the flexor retinaculum. The thenar fat pad is located on the flexor retinaculum so that these two layers can be distinguished. On the other hand, the AbPB in the other macaque monkeys as well as in human beings contains a single part mainly arising from the flexor retinaculum. The deep head of the FPB in the Japanese monkey terminates onto the radial sesamoid bone of the thumb, whereas that in the rhesus monkey terminated both on the radial and the ulnar sesamoid bone (Howell and Straus, 1933; Jacobi, 1966). We identified insertion of the deep head after removal of the skeletal framework, a maneuver which allows better visualization of the insertion. Difference of the findings may depend on the species difference. Considering confusion and diffi cultyin identification of the deep head as reviewed by Day and Napier (1961, 1963), we could not deny the possibility that the difference is resulted from the dissection method employed.

Hypothenar muscles Howell and Straus (1933) and Kimura and Takai (1970) identified in the rhesus monkey and in the crab-eating monkey a smaller head of the AbDM which origi natedtogether with the FDMB from the flexor retinaculum. we found a similar muscle bundle in the Japanese monkey. However, we consider the "smaller head" as a part of the FDMB, because its head as well as belly are fused with the FDMB and it is innervated by a branch to the FDMB.

Mm. contrahentes In the Japanese monkey, we find that the Ch5 has a single head which originates mainly from the medial tendinous septum. On the other hand, Howell and Straus Hand Muscles of the Japanese Monkey 93

(1933) and Kimura and Takai (1970) found two heads of the Ch5 in the rhesus monkey and in the crab-eating monkey. However, their description as well as figures are so ambiguous that one cannot understand what the second head indi cates.

Interossei We identified four dorsal and three palmar Interossei in the hand of the Japanese monkey (present observations), of the orangutan (unpublished observations) and of the human hand, as generally accepted in the human anatomy. The same number of the Interossei has been also described in several species including the gorilla (Raven, 1950), the orangutan (Sullivan and Osgood, 1927), the snob-nosed monkey, Rhinopithecus roxellana (Patterson, 1942) and the Sulawesi black ape, Cynopithcus niger (Macaca nigra) (Patterson, 1942). The findings of seven interossei in the primate hand contradict Lewis (1965, 1989), who argued that the mammalian hand in general contained three layers of intrinsic hand muscles including a superficial layer of the contrahentes and two deep layers of the interossei and who counted four dorsal and seven palmar interossei. His arguments are mainly based on his own findings on anatomy of the hand of primates and those on the marsupial hands by Cunningham (1878, 1882). According to the three-layer theory, fusion of four dorsal and four palmar interossei makes apparent four dorsal interossei in the human hand. Anatomical reports on the mam malianhand in the 20th century are influenced by Cunningham (1878, 1882) and identified frequently 11 interossei. Dissection and identification of the interossei are extremely difficult because of their situation in narrow interosseous spaces. We improved the dissection proce dureby removing the skeletal framework and dissecting the remained specimens in water. With these technical improvements, we identified with certainty the intrinsic hand muscles and counted seven interossei in every species so far observed includ ingman, the orangutan and the Japanese monkey. Seven primordia of the interossei are also reported in human embryological studies by Ruge (1878) and Grafenberg (1906). The three-layer theory of the intrinsic hand muscles by Cunningham (1878, 1882) is not supported by our present observations.

Innervation Innervation of the intrinsic hand muscles in the macaque monkey has been reported in the rhesus monkey (Howell and Straus, 1933; Jacobi, 1966), in the crab-eating monkey (Kimura and Takai, 1970), and in the Formosan monkey (Akiyama, 1960). Findings in these studies are fundamentally compatible with those in the present study in that the median nerve innervates the AbPB, the FPB, the OP and the two radial lumbrical muscles, whereas the ulnar nerve innervates 94 HOMMAAND SAKAI the hypothenar muscles, the interossei, the AdP, the contrahentes and the ulnar two lumbricales. However, the branching pattern of these individual branches in the macaque monkey was reported for the first time in the present study. The branching pattern of the median and the ulnar nerve within the hand in the Japanese monkey is fundamentally similar to those in man except for the fact that the median and the ulnar nerve in the Japanese monkey do not communicate to make a loop in the thenar muscles. The present study described for the first time in the macaque monkey a separate branch to the deep head of the FPB (accessory recurrent nerve, Figs. 1, 2) which was described in detail in the human hand (Homma and Sakai, 1992). Similar distributing pattern of the median and the ulnar nerve including the accessory recurrent nerve between man and the Japanese monkey speaks for ex tremeconservatism of the peripheral nervous system within the hand. The deep branch of the ulnar nerve penetrated the FDMB on the course in man, the rhesus monkey and the crab-eating monkey. In the Japanese monkey, the deep branch passed on the surface of the FDMB in two cases and penetrated the FDMB in the other four cases examined. Brooks (1865/6) discussed that the part of the FDMB surface to the deep branch represented the contrahentes, whereas Lewis (1989) argued that the part represents the migrant FDMB and not the contrahentes. Variation of the course of the deep branch observed in the present study speaks for the argument by Lewis (1989).

ACKNOWLEDGEMENTS The authors are grateful to Dr. Tasuku Kimura, Primate Research Institute, Kyoto University, for supply of the animals used in the present study.

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