While the vertebræ differ greatly in form and appearance in the various regions of the neck, body and tail, in adaptation to the various degrees of mobility and strength which are required of them, yet they are all constituted upon the same easily recognizable plan. The principal mass of bone in each vertebra is the body, or _centrum_, which is typically a cylinder, or modification of that form, and the two ends of the cylinder are the _faces_, by which the successive vertebræ are in contact with one another. In the living animal, however, the successive centra are not in actual contact, but are separated by disks of _cartilage_ (gristle) which greatly add to the elasticity of the column. From the upper surface of the centrum arises an arch of bone, the _neural arch_, enclosing with the centrum the _neural canal_, through which runs the spinal cord. As already mentioned, the protection of the spinal cord is essential to the life of the animal, yet this protection must be combined with a certain flexibility, both lateral and vertical. Mere contact of the centra, even though these be held in place by ligaments, would not give the column strength to endure, without dislocation, the great muscular stresses which are put upon it. Additional means of articulation between the successive vertebræ are therefore provided, and these vary in size, form and position in different regions of the backbone, in nice adjustment to the amount of motion and degree of strength needed at any particular part of the column. Of these additional means of articulation, which are called the _zygapophyses_, each vertebra has two pairs, an anterior and a posterior pair, placed upon the neural arch. From the summit of the arch arises the _neural spine_, a more or less nearly straight rod or plate of bone, which may be enormously long or extremely short, massive or slender, in accordance with the muscular attachments which must be provided for. Finally, should be mentioned the _transverse processes_, rod-like or plate-like projections of bone, which arise, one on each side of the vertebra, usually from the centrum, less commonly from the neural arch; these also differ greatly in form and size in the various regions of the column. Anatomists distinguish several other processes of the vertebra, but for our purpose it is not necessary to take these into consideration. [Illustration: FIG. 10.—First dorsal vertebra of Wolf from the front. _cn._, centrum. _r._, facet for the head of the rib. _r′._, facet for the tubercle of the rib. _tr._, transverse process. _pr.z._, anterior zygapophyses. _n.sp._, neural spine.]