Page:Encyclopædia Britannica, Ninth Edition, v. 2.djvu/178

164 APE [ANATOMY. ligamcntum teres, which seems to be sometimes absent in the gorilla also. The gluicei 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 tuberosities, 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 rota tor 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 gastrocnemd and soleus are flatter than in man, and the latter has only a fibular origin. In the lower Siiniadce 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 Cebidce 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 inter ossei even, the difference is very slight. It consists in the insertion of the tendon of that dorsal interosseous mass which is interposed between the second and third meta- tarsals, 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 chim panzee. In the lower Cebidce, and especially in the ffapalince, the interossei become true flexores breves, and altogether cease to be visible on the dorsum of the foot. THE BRAIN. The absolute size of the brain never in any ape ap proaches that of man. Thus the cranial capacity is never less than 55 cubic inches in any normal human subject, while in the orang and chimpanzee it is but 26 and 27|- cubic inches respectively. The relative size of the brain varies inversely with the size of the whole body, but this is the case in warm-blooded vertebrates generally. The extreme length of the cerebrum never exceeds, as it does in man, two and a quarter times the length of the basi-cranial axis. The proportion borne by the brain to its nerves is less in the I apes than in man, as also is that borne by the cerebrum to the cerebellum. In general structure and form the brain of apes greatly resembles that of man. Each half of the cerebrum contains a triradiate lateral ventricle, and though in some Simiadce the posterior cornu is relatively shorter than in man, it again becomes elongated in the Cebidce, and in many of the latter it is actually longer relatively than it is in man. The posterior lobes of the cerebrum are almost always so much developed as to cover over the cerebellum, the only exceptions are the strangely different forms, Mycetes and Ilylobates syndactylus. In the latter the cerebellum is slightly uncovered, but it is so considerably in the former. In Chrysothrix the pos terior lobes are much more largely developed relatively than they are in man. The cerebrum has almost always a convoluted external surface. In this group, however, as in mammals generally, a much-convoluted cerebrum is correlated with a considerable absolute bulk of body. Thus in Hapale (and there only) we find the cerebrum quite smooth, the only groove being that which represents the Sylvian fissure. In Simla and Troglodytes, on the contrary, it is very richly convoluted. A hippocampus minor is present in all apes, and in some of the Cebidce it is much larger relatively than it is in man, and is abso lutely larger than the hippocampus major. Of all apes, the orang has the brain which is most like that of man; indeed, it may be said to be like man s in all respects, save that it is much inferior in size and weight, and that the cerebrum is more symmetrically convoluted and less complicated with secondary and tertiary convolutions. If the brain of Simia be compared with that of Troglodytes, we find the height of the cerebrum in front greater in pro portion in the former than in the latter; also the " bridging convolutions," though small, are still distinguishable, while they are absent in the chimpanzee. Nevertheless, this character cannot be of much importance, since it re appears in Ateles, while two kinds of the genus Cebus (so closely allied as to have been sometimes treated as one species) differ strangely from each other in this respect. The corpus callosum, in apes generally, does not extend so far back as in man, and it is very short in Pithecia. In the orang and chimpanzee there are, as in man, two corpora albicantia, while in the lower monkeys there is but one. The vermis of the cerebellum is larger in the Cebidce than in the Simiadce. In all apes below the Simiince, each lateral lobe of the cerebellum gives off a small lobule, which is received into a special fossa of the petrous bone. Cer tain prominences of the medulla oblongata, termed corpora trapezoidea, which are found in lower mammals, begin to make their appearance in the Cebidce. THE TEETH. The teeth of apes consist, as in man, of incisors, canines, premolars, and molars; but the series of teeth no where forms so perfect an arch as in man, the opposite series of grinding teeth tending to become more parallel. No ape has the teeth placed in one uninterrupted series in each jaw, as is the case in the human species, but there is always a small interval (diastema) between each upper canine and the adjacent incisor, and between each lower canine and the adjacent premolar. This condition is due to the excessive size of the canines, the interspaces giving passage to the apices of these teeth. . This prolongation of the canines into tusk-like weapons of offence and defence (especially developed in the males), makes a great difference between the aspect of the dentition in apes and man. The number of the teeth is the same as in man in the Simiadce. The Cebidce have an additional premolar on each side of each jaw, and the ffapalince, besides this, have a true molar the less. The incisors are always nearly vertical, save in the Pithecince, when their apices project strongly forward. The canines are always considerabi} longer than the incisors, except in the genus Hapale, where the lower incisors equal the canines in length. The pre molars differ structurally from the molars much as in man, save that the first lower premolar may be modified in shape to give passage to the upper canine, as is specially to be seen in Cynocephalus. The grinding surface of the molars consists generally of two transverse ridges, each end of each ridge projecting more than the intermediate part, and so giving rise to four tubercles. In Simia and Troglodytes, however, we find in the upper molars an additional structure, which also exists in man. This is a ridge which runs obliquely from the front inner tubercle

(or cusp), outwards and backwards to the hind outer