[Illustration FIG. 2. CERVICAL VERTEBRAE OF AN OX (_Bos taurus_).
A, is the fifth; B, the fourth; C, the third. x 1/4 (Camb. Mus.)
1. neural spine. 2. transverse process. 3. hypapophysis. 4. convex anterior face of the centrum. 5. concave posterior face of the centrum. 6. prezygapophysis. 7. postzygapophysis. 8. vertebrarterial canal. 9. neural canal. 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=.
In fish and snakes the vertebral column is divisible into only two regions, an anterior trunk region, whose vertebrae bear ribs, and a posterior tail region, whose vertebrae are ribless.
Before giving a general account of the adult skull it will be well to briefly describe its development.
Shortly after its appearance, the central nervous system becomes surrounded by a membranous mesodermal investment which in the region of the spinal cord is called the =skeletogenous layer= or =perichordal sheath=, while in the region of the brain it is called the =membranous cranium=. Ventral to the central nervous system is the notochord, which extends far into the region of the future cranium, and like the nervous system, is enclosed by the skeletogenous layer. The primitive cartilaginous cranium is formed by histological differentiation within the substance of the membranous cranium and always consists of the following parts: