The opening for the passage of the spinal cord is situated towards the posterior portion of the base of the cranium, and is thus further from the centre than in Man. Except among the Gibbons, the vertebral column shows in the sacral region indications of that curve--or concavity in the back between the two convexities of the neck and loins--which is one of the distinctive characters of the human skeleton. The processes for the interlocking of the vertebræ, which are large in the lower Anthropoids, are much reduced in the Man-like Apes, and become inconspicuous in Man. The breast-bone is flat, and resembles that of Man, and, in all, except the Orang, is composed of two bones. The {146}arm-bone is often shorter than the fore-arm. The _radius_ and _ulna_ can be completely rotated. The articulating surface of the _trapezium_, the wrist-bone (_carpus_), to which the thumb is attached, has a rounded face like that of the _ento-cuneiform_ bone in the ankle (_tarsus_), a form which, as already pointed out (Vol. I., p. 11), was in the Lemuroids correlated with an opposable great-toe, so here it is correlated with a true opposable thumb. In the Monkeys and Lemuroids this bone is not generally rounded, and they have not the thumb opposable in the strict sense that it is among the higher Apes. The thigh-bone (_femur_) is shorter than the arm-bone (_humerus_); and the foot is very long; yet the absolute length of the _tarsus_ is never so great as in Man; it is the rest of the foot which is so much longer relatively in Apes. The _ento-cuneiform_, or articulating bone of the ankle for the great-toe, has a sub-cylindrical surface, which gives a great range of motion to that digit, towards and from the plane of the foot. The brain of the Apes closely resembles in general form and structure that of Man; but the cerebral hemispheres differ in being much elongated and depressed, and the cranial capacity of the skull, which is never less than 55 cubic inches in any normal human subject, is in the Chimpanzee 27½ cubic inches; in the Gorilla 35 inches; in the Orang 26 inches; and in the Gibbons very much less. The cerebrum has its surface richly convoluted; and its posterior lobes always entirely over-arching the cerebellum, except in the Siamang (_Hylobates syndactylus_).