Page:Encyclopædia Britannica, Ninth Edition, v. 2.djvu/172

158 APE [ANATOMY- Kidges for muscular attachment, or other bony promi nences, more or less disguising the rotundity of the cranium, are very generally developed, except in the smallest species. Such ridges are met with at once at their very maximum of extent in Troglodytes gorilla. In that animal an Fia. 15. Side view of the skull of adult Orang (Simia salyrus). From Trans. Zool, Soc. vol. i. pi. 53. enormous sagittal ridge traverses the middle of the sinciput antero-posteriorly, joining at its hinder end a prodigious lambdoidal crest for the insertion of the muscles of the neck. A very largely developed supra-orbital ridge runs transversely above the orbits, which it obliquely overhangs. The orang has no supra-orbital ridge, but in the other large Simiadce all these ridges are more or less developed, and moderate sagittal and lambdoidal ridges are found in Mycetes, Cebus, Pithecia, and some other of the Cebidcc. The mastoid process never attains in apes the large rela tive size it has in man; but it is prominent in the baboons and larger Macaci, as well as in Troglodytes, its develop ment bearing relation to the size and weight of the head. As the mastoid diminishes we find that the under surface of the petrous bone assumes a swollen or " bullate" condition. The orbits are in T. gorilla much as in man, but in the orang they are more rounded. They become very large in Hylobates, but attain an enormous size in Nyctipithecus. The extent to which each orbit opens into the adjacent temporal fossa, i.e., the size and shape of the spheno- maxillary fissure, varies considerably. It is narrow and much elongated in the gorilla and in Cynocephalus; it is short in the Semnopithecinoe and in Ateles, but opens widely so as to expose the vidian foramen. It is most completely closed of all in Mycetes, where it sometimes all but disappears entirely. The olfactory chamber is narrow and deep in the lower /Simiadce. The cribriform plate of the ethmoid is generally separated from the presphenoid by the junction of the orbital plates of the frontal. Such, however, is not the case in the orang and chimpanzee, though it is so in the gorilla and in Hylobates. The plane of the foramen magnum, as compared with the basi-cranial axis, varies with the projection of the occiput. It generally forms a less open angle with that axis than in man, but in Chrysothrix the angle is yet more open than in the human skull. The zygomata are arched both outwards and upwards in the gorilla and some baboons, but decrease in relative as well as absolute size in the smaller forms, notably in Chrysothrix. No long slender styloid process is normally attached to the skull, though such may be the case in Cyno- ceplialus. The vaginal process never attains the size it does in man, though it is considerable in the gorilla, and some times noticeable mSimia. Abonymeatus auditorius externus exists in all the Simiadce, and is absent in the whole of the Cebidce. In Nyctipithecus the ali -sphenoid is almost shut out from the parietal by the close approximation of the squamosal to the malar. In Cebus there is often a large malar foramen. The premaxillary bones have throughout the apes a dis tinctness of development and a relative size not found in man, the sutures separating them from the maxillaries remaining visible, with the exception of the chimpanzee, after the adult dentition has been attained. The maxil laries themselves develop great swollen tuberosities in the Cynocepliali, and a similar structure is developed even in Macacus niger. The nasal bones are small, and generally flatter than in man. In the orang they are quite flat. They are convex in some of the Semnopitheci and in the Cynocephali, but the proboscis monkey has its nasal bones no more developed than have other species of its genus. The nasals seem to attain their maximum of relative size in Mycetes. The lower jaw, or mandible, is always in one piece in adults. It is most man-like in the siamang, since we there alone find a slight chin. On the other hand, in Hylobates the angle is produced downwards and backwards, and we find the same in Hapale. Its maximum of relative size is attained in Mycetes, where the very broad ascending raniua serves to protect and shelter the enormously developed body of the hyoid. Air cells may be developed, as in the gorilla, in the parts adjacent to the mastoid. Frontal sinuses are generally absent in the Simiadce, being replaced by a coarse diploe. We find them, however, in the Cehidce, and in Cebus FIG. 16. Side views of skull and hyoid bone of Howling Monkey (Afycetct seniculus}. From De Blamville. they are often lai ger than they are in man. The total length of the cerebral cavity is never so much as 2^ tunes the length of the basi-cranial axis. The sutures of the skull become entirely obliterated with age. In the gorilla the sutures assume a more dentated structure than is found in any other ape or in man. The internasal suture becomes early obliterated

in most of the Simiadve. The tentorium is sometimes