In the higher vertebrates pads of fibrocartilage—the intervertebral discs—are commonly interposed between successive centra, these or parts of them often ossify, especially in the trunk and tail, and are then known as inter centra.
[a]Fig. 2. Cervical vertebrae of an Ox] (Bos taurus).
A, is the fifth; B, the fourth; C, the third. X ¼ (Camb. Mus.)
| 1. neural spine. | 6. prezygapophysis. |
| 2. transverse process. | 7. postzygapophysis. |
| 3. hypapophysis. | 8. vertebrarterial canal. |
| 4. convex anterior face of the centrum. | 9. neural canal. |
| 5. concave posterior face of the centrum. | 10. inferior lamella of transverse process. |
The vertebrae of the higher forms can generally be arranged in the following five groups, each marked by certain special characteristics:
1. The cervical or neck vertebrae. These connect the skull with the thorax, and are characterised by relatively great freedom of movement. They often bear small ribs, but are distinguished from the succeeding thoracic vertebrae by the fact that their ribs do not reach the sternum. The first cervical vertebra which articulates with the skull is called the atlas, but a study of the nerve exits shows that the first vertebra is not serially homologous throughout the Ichthyopsida, so that it is best to reserve the term atlas for the first vertebra in Sauropsida and Mammalia.
2. The thoracic vertebrae (often called dorsal) bear movably articulated ribs which unite ventrally with the sternum.
3. The lumbar vertebrae are generally large, and are often more movable on one another than are the thoracic vertebrae. They bear no ribs.
4. The sacral vertebrae are characterised by the fact that they are firmly fused together, and are united with the pelvic girdle by means of their transverse processes and rudimentary ribs.
5. The caudal or tail vertebrae succeed the sacral. The anterior ones are often fused with one another and with the sacrals, but they differ from true sacral vertebrae in that there are no rudimentary ribs between their transverse processes and the pelvic girdle. They often bear V-shaped chevron bones.