A Hand-book to the Primates, Volume 2 (of 2)

by Henry O. Forbes

This genus contains those Apes which stand highest, next to Man, in the animal kingdom. This proximity, however, refers only to his external conformation and his anatomical structure.

The Chimpanzees approach very closely to the Gorilla in structure. Indeed the Gorilla was at first placed in the same genus as the Chimpanzee, which was much earlier known to science than its larger cousin, although an excellent description of the Gorilla, under the name of Pongo, was brought to this country by Andrew Battell, an English prisoner of the Portuguese in Angola, early in the seventeenth century, and published in "Purchas his Pilgrimage," in 1613, a story which for the first time referred definitely to the Chimpanzee.

The body is heavily built, but shorter and less robust than that of the Gorilla. The crown is depressed, and the supra-orbital ridges, from which rise stiff strong eye-brows, are prominent, but not remarkably so. The eye-lids are wrinkled, and their margins set with eye-lashes. The nose, of which the ridge is shorter than in the Gorilla, is depressed in the middle, flatter at the extremity, and, as in the last-named species, is furrowed longitudinally, its nostrils looking more downward and forwards. The lips are extremely mobile and protrusile, the upper one broad and the lower one retreating from the mouth, and not forming a true human-like chin, though it is more prominent than in the Orang. The cheeks are more wrinkled than in that Ape. The ears are large and projecting from the side of the head, and often carry a lobule. They are strangely like those of Man, and, as Mr. Darwin has remarked, the Chimpanzee never moves or erects its ears, so that they are equally rudimentary, as far as that function is concerned, as in Man. The shoulders and chest are broad, and indicate great strength. Their lower limbs are longer in proportion than in the Orang. The foot, which is anatomically in no respect a hand, is sometimes shorter than the latter, the great-toe is thick, opposable, and thumb-like, the other four toes are united together by a web, the heel is somewhat developed, and the whole of the sole of the foot is applied to the ground when walking. The arms, of which the humeral segment is about equal in length to the fore-arm, are long, but reach only a little below the knee—their span being about a half more than the height of the body. The hands, which are wonderfully human in form, are broad, comparatively short, and less hook-like than in the Orang. The hair on the arm and fore-arm converges towards the elbow, as in the Gorilla and Orang. The thumb is short in comparison with the same digit in Man, and, as in the human hand, the middle finger is the longest; the outer four fingers being united by a web reaching up to the first joint. The palm of the hand can be applied flat to the ground; but though the Chimpanzees can stand or run erect on the flat sole of the foot, they prefer to advance leaning forward, supporting themselves on the knuckles of the hand. They have no callosities on the ischiatic bones, on which they sit.

The female Chimpanzees are slightly smaller than the males, but the disparity between them is much less than between the two sexes of the Gorilla. The nose and teeth are less prominent, and the belly is more tun-shaped. The young males also exhibit fewer differences from the adult than among the Gorillas, though differing in many points of their soft anatomy and osteology. The nose lengthens, and its extremity widens, while the face becomes more prognathous with increasing years. In the young the frontal bone is low and flat. The skull in the Chimpanzee is elongated, and small in proportion to the body; the forehead is smaller, the crown more rounded than in the Gorilla, and the back of the head convex.[^[2]] The central (sagittal) crest, so strongly developed in the Gorilla and the Orang, is here wanting; the supra-orbital ridges which extend across the face, and the occipital prominences for the back-muscles, though large, are also less marked. The orbits have a circular rim, and are less prominent than in the Gibbons. The nasal bones are but slightly arched, and the openings for the nostrils round and small. The jaws, which are smaller, proportionately to the cranium, in this genus, than in any other of the Simiidæ, protrude far forward, but the symphysis of the lower jaw is smaller than in the Gorilla, and its two halves low and wide. The bones of the skull are much hollowed out into cavities (sinuses) in the forehead, nose, and jaws, all of which communicate with each other. The plane of the foramen magnum (for the passage of the spinal cord) is oblique to the plane of the base of the skull.

The volume of the cranium is from twenty-six to twenty-seven cubic inches, or about one-half of the lowest capacity of a normal human cranium. A styloid process is more or less distinctly visible in the Chimpanzees.

The canine teeth are long and conical, but less than in the Gorilla; and the diastema, or gap, between them and their neighbouring teeth is smaller than in the other Apes. The molar teeth are four-cusped, and have the oblique ridge already described extending from the front inner to the hind outer cusp; and the middle lower molar has five cusps, both these dental characters being similar to those in Man. The anterior lower pre-molar, however, is pointed, and has a long sharp anterior edge, as in the Cercopithecidæ.

The vertebral column begins to show the S-shaped flexure, characteristic of Man's back-bone; it presents also a human character in the form of its second neck vertebræ, and there are thirteen pairs of ribs, as in Man. The hindmost vertebræ "give the impression of a rudimentary tail." (Hartmann.)

The humerus is nearly equal in length to the fore-arm; the wrist (carpus) has only eight bones (the central bone being absent), agreeing, therefore, with the number in Man.

All the ridges and grooves seen in the human brain are present in that of the Chimpanzee, but "they are simpler and more symmetrical, and larger in proportion to the brain." (Huxley.) The cerebellum, and the nerves also, are larger in proportion to the cerebrum than in Man; and certain structures (the corpora trapezoidea) which exist in the brains in the lower Mammalia are absent. These prominences, which are situated in that portion of the brain known as the medulla oblongata, at the summit of the spinal cord, disappear, as we have seen, in all the genera of higher rank than the Cebidæ, one of the lowest families of the Anthropoidea. The brain in its convolutions and in many other respects conforms to that of the Orang. This is especially the case in A. calvus.

The uvula, which is absent in the throat of the Orang, is pendulous in the Chimpanzees, as in Man. Large air-sacs are also present, and the hyoid bone is excavated posteriorly, suggesting the conformation of the same bone in Alouatta (the South American Howlers). The stomach is very similar to that of Man, and so are the digestive and reproductive organs. The round ligament, attaching the head of the thigh-bone into its pelvic socket, is present, and restricts the flexibility of the hind-limb of the Chimpanzees, compared with that of the Orang. Its presence, however, while acting somewhat less favourably in regard to the climbing capacities of these animals, whose habits are less essentially arboreal than the Orangs', beneficially assists them in walking, affording them a firmer support on the ground. In the Chimpanzee there is always a semi-lunar fold (plica semilunaris) in the inner corner of the eye, corresponding to the nictitating membrane (or third eyelid) of birds. In some of the Lemuroids it is well developed (suprà, vol. i., p. [90]), and is large in some races of men.