1. The Axial Skeleton.
A. The Vertebral Column.
This consists of a series of some fifty-two vertebrae, all completely ossified.
It is divisible into two regions only, viz. the trunk region, the vertebrae of which bear movable ribs, and the caudal or tail region, the vertebrae of which do not bear movable ribs.
Trunk vertebrae.
These are seventeen in number; the ninth may be described as typical of them all. It consists of a short deeply biconcave centrum whose two cavities communicate by a narrow central canal. From the dorsal surface of the anterior half of the centrum arise two strong plates, the dorsal or neural processes, which are directed obliquely backwards and meet forming the dorsal or neural arch. This is produced into a long backwardly-directed dorsal or neural spine.
From the lower part of the anterior edge of each neural arch arise a pair of blunt triangular projections which overhang the posterior half of the preceding centrum, and bear a pair of flattened surfaces which correspond to the anterior or prezygapophyses of most vertebrae, they differ however from ordinary prezygapophyses in the fact that they look downwards and outwards. From the posterior end of the centrum arise a pair of short blunt processes each of which bears an upwardly- and inwardly-directed articulating surface corresponding to a postzygapophysis.
The two halves of the ventral arch form a pair of large ventri-lateral processes which arise from the anterior half of the centrum and pass outwards and slightly backwards and downwards.
Behind these there arises on each vertebra a second outgrowth which is small and flattened, and like the ventri-lateral process serves to protect the air-bladder. The surface of the centrum is marked by more or less wedge-shaped depressions, one in the mid-dorsal line, and two on the ventral surface immediately mesiad to the bases of the ventri-lateral process. There are also a number of smaller depressions.