The skeleton of the limbs or extremities is composed of two groups of parts—the skeleton of the extremities proper and the zone-skeleton, which connects these with the vertebral column. The zone-skeleton of the arms (or fore legs) is the shoulder-zone; the zone-skeleton of the legs (or hind legs) is the pelvic zone.
The vertebral column (Fig. 327) in man is composed of thirty-three to thirty-five ring-shaped bones in a continuous series (above each other, in man’s upright position). These vertebræ are separated from each other by elastic ligaments, and at the same time connected by joints, so that the whole column forms a firm and solid, but flexible and elastic, axial skeleton, moving freely in all directions. The vertebræ differ in shape and connection at the various parts of the trunk, and we distinguish the following groups in the series, beginning at the top: Seven cervical vertebræ, twelve dorsal vertebræ, five lumbar vertebræ, five sacral vertebræ, and four to six caudal vertebræ. The uppermost, or those next to the skull, are the cervical vertebræ (Fig. 327); they have a hole in each of the lateral processes. There are seven of these vertebræ in man and almost all the other mammals, even if the neck is as long as that of the camel or giraffe, or as short as that of the mole or hedgehog. This constant number, which has few exceptions (due to adaptation), is a strong proof of the common descent of the mammals; it can only be explained by faithful heredity from a common stem-form, a primitive mammal with seven cervical vertebræ. If each species had been created separately, it would have been better to have given the long-necked mammals more, and the short-necked animals less, cervical vertebræ. Next to these come the dorsal (or pectoral) vertebræ, which number twelve to thirteen (usually twelve) in man and most of the other mammals. Each dorsal vertebra (Fig. 165) has at the side, connected by joints, a couple of ribs, long bony arches that lie in and protect the wall of the chest. The twelve pairs of ribs, together with the connecting intercostal muscles and the sternum, which joins the ends of the right and left ribs in front, form the chest (thorax). In this strong and elastic frame are the lungs, and between them the heart. Next to the dorsal vertebræ comes a short but stronger section of the column, formed of five large vertebræ. These are the lumbar vertebræ (Fig. 166); they have no ribs and no holes in the transverse processes. To these succeeds the sacral bone, which is fitted between the two halves of the pelvic zone. The sacrum is formed of five vertebræ, completely blended together. Finally, we have at the end a small rudimentary caudal column, the coccyx. This consists of a varying number (usually four, more rarely three, or five or six) of small degenerated vertebræ, and is a useless rudimentary organ with no actual physiological significance. Morphologically, however, it is of great interest as an irrefragable proof of the descent of man and the anthropoids from long-tailed apes. On no other theory can we explain the existence of this rudimentary tail. In the earlier stages of development the tail of the human embryo protrudes considerably. It afterwards atrophies; but the relic of the atrophied caudal vertebræ and of the rudimentary muscles that once moved it remains permanently. Sometimes, in fact, the external tail is preserved. The older anatomists say that the tail is usually one vertebra longer in the human female than in the male (or four against five); Steinbach says it is the reverse.
Fig. 329—Three dorsal vertebræ, from a human embryo, eight weeks old, in lateral longitudinal section. v cartilaginous vertebral body, li inter-vertebral disks, ch chorda. (From Kölliker.)
Fig. 330—A dorsal vertebra of the same embryo, in lateral transverse section. cv cartilaginous vertebral body, ch chorda, pr transverse process, a vertebral arch (upper arch), c upper end of the rib (lower arch). (From Kölliker.)
In the human vertebral column there are usually thirty-three vertebræ. It is interesting to find, however, that the number often changes, one or two vertebræ dropping out or an additional one appearing. Often, also, a mobile rib is formed at the last cervical or the first lumbar vertebra, so that there are then thirteen dorsal vertebræ, besides six cervical and four lumbar. In this way the contiguous vertebræ of the various sections of the column may take each other’s places.
In order to understand the embryology of the human vertebral column we must first carefully consider the shape and connection of the vertebræ. Each vertebra has, in general, the shape of a seal-ring (Figs. 164–166). The thicker portion, which is turned towards the ventral side, is called the body of the vertebra, and forms a short osseous disk; the thinner part forms a semi-circular arch, the vertebral arch, and is turned towards the back. The arches of the successive vertebræ are connected by thin intercrural ligaments in such a way that the cavity they collectively enclose represents a long canal. In this vertebral canal we find the trunk part of the central nervous system, the spinal cord. Its head part, the brain, is enclosed by the skull, and the skull itself is merely the uppermost part of the vertebral column, distinctively modified. The base or ventral side of the vesicular cranial capsule corresponds originally to a number of developed vertebral bodies; its vault or dorsal side to their combined upper vertebral arches.