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. [Illustration FIG. 69. A, ATLAS AND B, AXIS VERTEBRA OF A DOG (_Canis familiaris_) (after VON ZITTEL). 1. transverse process of atlas. 2. vertebrarterial canal. 3. foramen for exit of spinal nerve. 4. neural spine. 5. odontoid process. 6. anterior articulating surface of centrum. 7. centrum. 8. transverse process of axis. 9. postzygapophysis.] THE CERVICAL VERTEBRAE. These are seven in number, as in almost all mammals. They are characterised by the fact that they have small ribs fused with them, forming transverse processes perforated by canals through which the vertebral arteries run.