(C) The Anatomy of Apes (cont.)
The Brain of the Ape
The absolute size of the brain never in any ape approaches 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 1/2 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 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 Simiadae the posterior cornu is relatively shorter than in man, it again becomes elongated in the Cebidae, 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 Hylobates syndactylus. In the latter the cerebellum is slightly uncovered, but it is so considerably in the former. In Chrysothrix the posterior 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 Simia and Troglodytes, on the contrary, it is veryrichly convoluted. A hippocampus minor is present in all apes, and in some of the Cebidae it is much larger relatively than it is in man, and is absolutely 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 proportion 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 reappears 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, it is very short in Pithecia. In the orang and chimpanee 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 Cebidae than in the Simiadae. In all apes below the Simiinae, each lateral lobe of the cerebellum gives off a small lobule, which is received into a special fossa of the petrous bone. Certain prominences of the medulla oblongata, termed corpora trapezoidea, which are found in lower mammals, begin to make their appearance in the Cebidae.
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