greater in the orang-utan and chimpanzee than in some of the
langurs (Semnopithecus, fig. 1); the maximum development of this
feature occurs in the dog-faced baboons (Papio, fig. 2). In American
monkeys, with the exception of the howlers (Alouata, fig. 3), the
facial part is relatively smaller than in Old World monkeys and
apes; while in the squirrel-monkeys (Chrysothrix) it is even smaller
than in man himself. In none of the Old World group does the
forehead present that rounded and elevated contour characteristic
of man, although the height of this region is great in the orang-utan
(fig. 4). Curiously enough, American monkeys, especially those
included in Pithecia, are the most man-like in this respect. The skull
of the male gorilla is characterized by the great development
of the crests for muscular attachment, one of these (superciliary)
overhanging the orbits, a second (sagittal) traversing the middle
line of the upper surface, while a third (lambdoid) forms an inverted
V on the occiput, and affords attachment for the muscles of the neck.
In the gorilla the orbits are much as in man, but in the orang-utan
they are more rounded. They become very large in Hylobates,
but attain an enormous size in the American Nyctipithecus. The
extent to which each orbit opens into the adjacent temporal fossa,
i.e. the size and shape of the sphenomaxillary fissure, varies
considerably; this is narrow and much elongated in the gorilla and the
baboons, but short in the langurs and spider-monkeys. It is most
closed in the howlers, where it sometimes all but disappears entirely.
The mastoid process never attains the large relative size it has
in man; but it is prominent in the baboons and larger macaques,
as well as in the chimpanzee and gorilla, its development bearing
relation to the size and weight of the head. As the mastoid
diminishes, the under surface of the petrosal assumes a swollen or
bladder-like condition.
Fig. 3.—Skull and Hyoid-bone of a Howler-Monkey (Alouata). In nature the hyoid-bone, which is bladder-like, is placed between the two branches of the lower jaw.
The plane of the foramen magnum, as compared with the basicranial 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 cheek, or zygomatic, arches bend 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 the baboons. An external bony auditory meatus (or tube) is present in Old World but absent in New World monkeys. In all apes and monkeys the premaxillae have a distinctness of development and a relative size not found in man; the sutures separating them from the maxillae remaining visible, except in the chimpanzee, after the adult dentition has been attained. The maxillae develop great swollen tuberosities in the baboons and the black ape of Celebes. Thee nasal bones are small, and generally flatter than in man; being in the orang-utan quite flat. They are convex in some langurs and all baboons; but the proboscis-monkey has its nasals no more developed than those of other species. The nasals seem to attain their maximum of relative size in the howlers. The lower jaw, or mandible, is always in one piece in adults; and is most man-like in the siamang, which alone has a slight chin. On the other hand, in other gibbons the angle is produced downwards and backwards, as also in marmosets. Its maximum of relative size is attained in the howlers (fig. 3), where the broad ascending part 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 Old World group, being replaced by coarse cellular bone. In old age the sutures of the skull become obliterated, the one between the two nasals disappearing at an early age in Old World monkeys. In the spider-monkeys and howlers the tentorium, or membrane dividing the hemispheres of the brain from the cerebellum, becomes bony.
Fig. 4.—Skull of adult male Orang-utan (Simia satyrus).
The spinal column of apes and monkeys always lacks the S-like curvature of that of man, the nearest approach to this occurring in the baboons (fig. 2). The number of dorsal vertebrae varies from eleven in some species of Cercopithecus and Macacus to fourteen in certain gibbons or fifteen in the American night-apes (Nyctipithecus). In the American Cebidae the number seldom falls below thirteen; in the orang-utan it is twelve, as in man, but thirteen in the chimpanzee and gorilla. In most cases the dorsal and lumbar regions are about equal in length, but the lumbar region is the shorter in the man-like group, and less than half the length of the dorsal in the gorilla. The lumbar spinous processes are vertical, or project backwards in the man-like apes, gibbons and spider-monkeys; in the others they project forwards, especially in Cebidae. The lumbar transverse processes project outwards, more or less at right angles to the axis of the spine, or else forwards. The sacrum attains its greatest absolute length in the gorilla, but is relatively longer than in man in all the man-like group. Hylobates has the relatively longest sacrum. The number of vertebrae included in the sacrum varies more or less with age; with the exception of the Simiidae and Hylobatidae, there are generally only two or three; but in Ateles, Hylobates, and Uacaria there may be four; while in the Simiidae there are always five, and sometimes six. In most apes the sacrum and lumbar vertebrae lie in one slightly curved line, the gorilla and chimpanzee presenting in this respect a great contrast to the human structure. In the orang-utan the sacro-vertebral angle is rather more marked; but in some baboons it is so much so as almost to rival that of man.
Fig. 5.—Skeleton of South American Spider-Monkey (Ateles), to illustrate the length of the limbs and tail, and the slenderness of the former.
