FIGURE 2.338. Skeleton of the breast-fin of a young Selachius. The radii of the median fin-border have wholly disappeared. The shaded part on the right is the section that persists in the five-fingered hand of the higher Vertebrates. (b the three basal pieces of the fin: mt metapterygium, rudiment of the humerus, ms mesopterygium, p propterygium.) (From Gegenbaur.)) The bony skull (cranium), the head-part of the secondary axial skeleton, develops in just the same way as the vertebral column. The skull forms a bony envelope for the brain, just as the vertebral canal does for the spinal cord; and as the brain is only a peculiarly differentiated part of the head, while the spinal cord represents the longer trunk-section of the originally homogeneous medullary tube, we shall expect to find that the osseous coat of the one is a special modification of the osseous envelope of the other. When we examine the adult human skull in itself (Figure 2.332), it is difficult to conceive how it can be merely the modified fore part of the vertebral column. It is an elaborate and extensive bony structure, composed of no less than twenty bones of different shapes and sizes. Seven of them form the spacious shell that surrounds the brain, in which we distinguish the solid ventral base below and the curved dorsal vault above. The other thirteen bones form the facial skull, which is especially the bony envelope of the higher sense-organs, and at the same time encloses the entrance of the alimentary canal. The lower jaw is articulated at the base of the skull (usually regarded as the XXI cranial bone). Behind the lower jaw we find the hyoid bone at the root of the tongue, also formed from the gill-arches, and a part of the lower arches that have developed as "head-ribs" from the ventral side of the base of the cranium. Although the fully-developed skull of the higher Vertebrates, with its peculiar shape, its enormous size, and its complex composition, seems to have nothing in common with the ordinary vertebrae, nevertheless even the older comparative anatomists came to recognise at the end of the eighteenth century that it is really nothing else originally than a series of modified vertebrae. When Goethe in 1790 "picked up the skull of a slain victim from the sand of the Jewish cemetery at Venice, he noticed at once that the bones of the face also could be traced to vertebrae (like the three hind-most cranial vertebrae)." And when Oken (without knowing anything of Goethe's discovery) found at Ilenstein, "a fine bleached skull of a hind, the thought flashed across him like lightning: 'It is a vertebral column.'" (FIGURE 2.339. Skeleton of the fore leg of an amphibian. h upper-arm (humerus), ru lower arm (r radius, u ulna), rcicu apostrophe, wrist-bones of first series (r radiale, i intermedium, c centrale, u apostrophe ulnare). 1, 2, 3, 4, 5 wrist-bones of the second series. (From Gegenbaur.) FIGURE 2.340. Skeleton of gorilla's hand. (From Huxley.)