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 {190}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_.