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Showa Univ. 1. Med. Sci. 5(2), 137146, December 1993 Review Comparative Morphology of Skeletal Muscles in Man and Macaque Seuchiro INOKUCHI, Tadanao KIMURA, Masataka Suzuiu, Junji ITO and Hiroo KUMAKURA Abstract: Anatomical features (shape, weight and myofibrous composition) of skeletal muscles reflect the muscular functions. In this study, we histologically analyzed the myofibrous composition of corresponding muscles and compared their functions in man and the macaque. Among the intrinsic hand and foot muscles, the hand muscles for gripping and pinching movements were well developed in humans and those for hanging were well developed in the hands and feet of the macaque. Of the spinohumeral muscles, morphological differ- ences (thickness of muscle layer and muscle fiber size) were observed in different regions. In the muscles in the upper and lower extremities, the relative weight and myofibrous composition were correlated with the characteristics of their respective motions. There were differences in myofibrous composition among those muscle groups due to bipedal or quadrupedal walking by man and the macaque. The differences in the myofibrous composition of the muscles for vocalization (laryngeal muscles), and the innervation ratios of the muscle fibers reflected vocal ability. Key words: comparative anatomy, muscle composition, skeletal muscle, macaque Introduction In general, the number and the thickness of skeletal muscle fibers vary within each muscle, due to differences in functions. Muscle fibers are known to be thicker in muscles engaged in powerful movements and thinner in muscles engaged in fine movements1. On the other hand, muscle fibers are divided histochemically into 3 types, type I, type II and intermediate type2). Fig. 1 shows a muscle section stained with Sudan Black B. The thinner type I fibers, the so-called red fibers or slow twitch fibers, are stained black. The thicker type II fibers, the so-called white fibers or fast twitch fibers, are not stained. The occurrence of the three types of muscle fibers in a given muscle varies among species. In this paper, the dif- ferences of muscle fiber composition are compared in some muscles of the human and macaque. Materials and Methods For the muscle fiber composition of the muscle, the number, the size and the density of muscle fibers were estimated for transverse muscle sections. The muscle samples from Department of Anatomy, Showa University School of Medicine, 1-5-8 Hatanodai, Shinagawa-ku, Tokyo 142, Japan, 138 Seiichiro INOKUCHI, et al. Fig. 1. Muscle fibers stained by Sudan Black B (•~ 200). extremities were routinely fixed in formalin, embedded in celloidin, cross-sectioned into 20 ƒÊm thickness and stained with hematoxylin and eosin (H.E.) dye. The samples from laryngeal muscles were embedded in gelatin and stained with Sudan Black B, and the muscle fibers were differentiated into 3 types: Type I, II and intermediate. Results and Discussion 1. Lower extremity muscles3) Figure 2 shows the total number and the size of muscle fibers in the femoral muscle of man and the macaque. They were generally rather thin in macaques and comparatively thick in humans. According to the individual differences, muscles were divided into four groups: those in which the total number and the size of the muscle fiber were much larger in man (e.g. mm. vastus lateralis, adductor longus and adductor brevis); those in which the size in macaques was almost the same as in man (e.g. mm. semitendinosus, rectus femoris and gracilis); those in which the total number in macaques was almost the same as in man (such as mm. vastus intermedius); and those in which there were no remarkable differences between humans and macaques (this group ircluded the biceps femoris and sartorius muscle). From these comparisons, it can be generalized that the bi-articular muscles of the macaque are equivalent to those in man, whereas there is a tendency for the other macaque femoral muscles to be fewer and have thinner fibers than those in humans. In the lower leg, shown in Fig. 3, three muscle types can be observed. These are muscles corresponding to the vastus lateralis type (mm. gastrocnemius, soleus, and tibialis anterior); those of the vastus intermedius type (mm. extensor hallucis longus, extensor digitorum lon- gus, plantaris, flexor hallucis longus and fiburalis brevis); and those comparable to m. biceps femoris (mm. popliteus, fibularis longus, tibialis posterior and flexor digitorum longus). Muscles that differ widely in myofibrous composition between man and macaque are Skeletal Muscles in Man and Macaque 139 Fig. 2. Comparison between the total number and the size of muscle fibers of thigh muscles in the macaque (•~) and in humans (•œ) . Fig. 3. Comparison between the total number and the size of muscle fibers of the lower leg muscles in the macaque (•~) and in humans (•œ). those that participate in special functions of the lower extremities, e.g. bipedalism in man and quadripedalism in the macaque. 2. Foot muscles4,5) Table 1 is a comparison of the total numbers of muscle fibers in the muscle groups of the foot. In the thenar muscle and the dorsal interossei, the number of muscle fibers was slightly higher in racaques, but those of the flexor and hypothenar muscles were much 140 Seiichiro INOKUCHI, et al. Table 1. Total number of muscle fibers in foot muscles in humans and macaque. m.f.: muscle fiber. Fig. 4. Muscle fiber size of foot muscles in the humans and macaque. lower than in humans. These differences obviously are due to prehensility in macaques and to bipedal walking in man. The average size of muscle fibers in the foot muscles is shown in Fig. 4. In macaques, the highest values are around 2000 ƒÊm2 and were found in the m, interosseus plantaris I, and in the oblique and transverse heads of the m. adductor hallucis muscle. These muscles act to close the toe grip (gather the toe). In man, the fibers of the m. adductor hallucis and flexor hallucis brevis reach the size of 3000 ƒÊm2. The fiber size in the m. flexor brevis digiti Skeletal Muscles in Man and Macaque 141 Table 2. Total number of muscle fibers in hand muscles of humans and macaque. m.f.: muscle fiber. Fig. 5. Mean size of muscle fibers of hand muscles in the human and macaque. I, and II are around 2500 ƒÊm2, and they are much thicker than the fibers of the cone- sponding muscles in the macaque foot. This strong development of the muscles may indicate the kick exerted by digiti I and II to produce propelling force in bipedal walking. 3. Hand muscles6 Table 2 is a comparison of the total numbers of muscle fibers in the hand muscles in man and macaque. In the monkey, the thenar muscle possesses relatively fewer, and the hypo- thenar muscles relatively more fibers than in humans. Figure 5 shows the mean size of the muscle fibers in the hand muscles. In the macaque hand, the thickest fibers (about 2000 ƒÊm2) were found in the m, interosseus plantaris I, II 142 Seiichiro INOKucHI, et a!. Table 3. Comparison of the thickness of muscle layer of truncobranchial muscles in humans and macaque. Table 4. Comparison of the muscle fiber size of truncobranchial muscles humans and macaque. Skeletal Muscles in Man and Macaque 143 Table 5. Comparison of the relative weight of the upper extremity muscle group on macaque and humans. and m. contrahens digitorum III. In the human hand, the largest values (1700 ƒÊm2) occur in the m. interosseus dorsalis I. Consequently, each of these muscle fibers in the macaque were thicker than those in humans. 4. Truncobrachial muscles7,8) Regarding the truncobrachial muscles, the thickness of the muscle layer, as can be seen from Table 3, is largest in the cranial portion of the anterior serratus muscle, the inter- mediate portion of the m. trapezius and the cranial portion of the m. rhomboideus major in humans. In the macaque, however, it was largest in the caudal portions of the anterior serratus muscle, the trapezius and the rhomboideus. The thickness of muscle fibers in the human truncobrachial muscles are shown in Table 4. It is the greatest in the cranial portion of the m. serratus anterior, in the intermediate portion of the trapezius muscle and the cranial portion of the m. rhomboideus. The white muscle fibers in the same muscles of the macaque are largest in the caudal portions of all muscles. The results suggest that the parts of the muscles that are enhanced in humans are those that are used for lifting objects and in macaques those that are used to suspend the trunk on the forelimb. 5. Upper extremity muscles9,10) Table 5 sets out the proportions of muscle groups on the free upper extremities. The data include information about the chimpanzee. The extensors of the elbow joint occupy the highest proportion in humans, followed by the flexors of the elbow and flexors of the wrist and finger which have similar values. In macaques, the extensors of the elbow and the flexors of the wrist and hand have similar values; no difference was found between the flexors of the elbow and the extensors of the wrist and fingers. The relative weights of in- dividual muscles in the upper extremity muscles are summarized in Fig. 6. The branchial triceps muscle is the largest, occupying 30.6%. This is followed by the mm. brachialis, biceps brachii, the deep digital flexors and the superficial digital flexors. It is tempting to relate the marked development of the extensors of the upper arm in humans to the throwing movement and the finger flexors, next in size, probably participate in the powerful movement of the hand by flexing the fingers or wrists. 6.

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