84 THE VERTEBRATE SKELETON. 



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 de- 

 scribed 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 pro- 

 duced 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 over- 

 hang the posterior half of the preceding centrum, and bear a 

 pair of flattened surfaces which correspond to the anterior 

 or pre-zygapophyses of most vertebrae, they differ however 

 from ordinary pre-zygapophyses 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 post-zygapophysis. 



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 out- 

 growth 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 de- 

 pressions, 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. 



