(C) The Anatomy of Apes (cont.)
The Muscles of the Ape
The muscles of apes are very similar in number, distribution, an form to those of man, except that in the long-tailed forms (e.g., Semnopithecus) the muscular bundles answering to the coccygeal muscles of man are so greatly developed as to form eight sets of caudal muscles. The latissimus dorsi commonly sends on a slip, called the dorso-epitroclear, as far as the olecranon. Often there is a rhomboideus capitis, and muscle, called levator claviculae, almost always descends from the cervical transverse processes to the outer part of the clavicle. The flexor longus pollicis and the flexor digitorum profundus are always more or less united. The extensor indicis commonly sends a tendon to the third digit as well as to the index, and at the same time the extensor minimi digiti sends a tendon to the fourth digit as well as to the fifth. An extensor primi internodii pollicis is never developed, but the extensor ossis metacarpi pollicis is often doubled, even in the chimpanzee. In the orang the flexor longus pollicis sends a tendon only to the index. In Hylobates the supinator longus is inserted into the middle of the radius, and there is an inductor tertii internodii indicis going from the metacarpal of the index to its ungual phalanx. Often the extensor brevis pollicis and the abductor longus become more or less united. In spite of the rudimentary condition of the thumb in Ateles, its muscles exist, though in a rudimentary condition, but in Hapale the opponens pollicis is wanting.
The lower limb in the orang generally wants the ligamentum teres, which seems to be sometimes absent in the gorilla also. The glutaei muscles of apes are feeble and small, and are inserted low down on the femur. Only in the gorilla are they large enough to cause a small buttock to protrude over the ischiatic tubercosities, but even in this ape the buttocks do not meet so as to conceal the anus.
Apes have an extra muscle, called the scansorius, which passes down from the edge of the ilium to the great trochanter of the femur, and must act as a powerful rotator of the thigh inwards.
The gracilis is much broader than in man, and is inserted lower down on the tibia than in him, as is also the case with the semi-membranosus and semi-tendinosus. The short head of the biceps femoris is generally wanting. The gastrocnemei and soleus are flatter than in man, and the latter has only a fibular origin. In the lower Simiadae the plantaris passes over the pulley-like end of the os calcis, and goes to the plantar fascia.
No ape seems to have any peroneus tertius, but we may find even in Semnopithecus a slender peroneus quinti digiti passing behind the outer malleolus, and going to the metatarsal of the fifth digit, while in the Cebidae we may also have a peroneus quarti digiti going similarly to the fourth digit.
The tibialis anticus may be divided, as even in the chimpanzee, like its homotype the extensor ossis metacarpi pollicis.
The muscles of the foot, with the exception of the interossei, resemble the muscles of the foot of man, and not those of his hand. As regards the interossei even, the difference is very slight. It consists in the insertion of the tendon of that dorsal interosseous mass which is interprosed between and third metatarsals, into the proximal phalanx of the third digit ( as in the human hand), instead of into that of the second digit (as in the human foot). The hallux in the orang is, in spite of its imperfect development, provided with an opponens muscle. The flexor brevis digitorum pedis does not, in apes, arise exclusively from the os calcis, and the flexor accessorius arises from the surface of the deep flexor tendons. The last named muscle may be wanting, as sometimes at least in Hylobates. A muscle, called the abductor ossis metacarpi quinti, exists even in the chimpanzee. In the lower Cebidae, and especially in the Hapalinae, the inerossei become true flexores breves, and altogether cease to be visible on the dorsum of the foot.
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