3. The lumbar vertebrae which are large and ribless.
4. The sacral vertebrae which are fused with one another and united with the pelvis.
5. The caudal or tail vertebrae which are small.
Except in the sacral region the vertebrae are movably articulated to one another, while their centra are separated from one another by cartilaginous intervertebral discs.
General characters of a vertebra.
Take as a type the fourth lumbar vertebra. It may be compared to a short tube whose inner surface is smooth and regular, and whose outer surface is thickened and drawn out in a variety of ways. The basal part of the vertebra is the centrum or body which forms the thickened floor of the neural canal. Its two ends are slightly convex and are formed by the epiphyses, two thin plates of bone which are at first altogether distinct from the main part of the centrum, but fuse with it as the animal grows older; its sides are drawn out into a pair of strong transverse processes, which project forwards, outwards, and slightly downwards. The neural arch forms the sides and roof of the neural canal, and at each end just above the centrum bears a pair of intervertebral notches for the passage of the spinal nerves, the posterior notches being considerably deeper than the anterior. The neural arch is drawn out into a series of processes. Arising from the centre of the dorsal surface is a prominent median neural spine or spinous process, which projects upwards and slightly forwards; its anterior edge is vertical, while its posterior edge slopes gradually. At the two ends of the neural arch arise the two pairs of zygapophyses or articulating surfaces, which interlock with those of the adjacent vertebrae. The anterior or prezygapophyses look inwards, and are large and concave; they are borne upon a pair of large blunt outgrowths of the neural arch, the metapophyses. The posterior or postzygapophyses are slightly convex and look outwards and downwards; they are borne upon backwardly projecting outgrowths of the neural arch. Lastly there are a pair of minute projections arising from the posterior end of the neural arch, below the postzygapophyses. These are the anapophyses. In young individuals the development of all the processes of the various vertebrae is less marked, and the epiphyses are obviously distinct.
Fig. 69. A, atlas and B, axis vertebra of a Dog (Canis familiaris) (after von Zittel).
| 1. transverse process of atlas. | 6. anterior articulating surface |
| 2. vertebrarterial canal. | of centrum. |
| 3. foramen for exit of spinal | 7. centrum. |
| nerve. | 8. transverse process of axis. |
| 4. neural spine. | 9. postzygapophysis. |
| 5. odontoid process. |