STRUCTURE OF THE VERTEBRATES 149 



ventral portion of the centrum is another pair of transverse 

 processes serving as points of muscular attachment. In the caudal 

 region of the dogfish these processes grow toward the ventral 

 median line, fuse, and form a ventral haemal arch through 

 which passes the aorta. 



The vertebrae of the living Elasmobranchs are more spe- 

 cialized than those of many fossil fish and amphibia, and the 



Intercentrum 



Hntercentrum 



A. Primitive Stegocephalian 

 (Cricotiis) 



B. Stegocephalian 

 (Eryops) 



Neural 

 arch 



C. Tail Vertebrae of Cat 



D. Lumbar Vertebrae of Mammal 



Fig. 86. Comparative Anatomy of Bony Vertebrae. Primitive vertebrae 

 consist of two centra in each body segment (A) ; the intercentra are pushed 

 ventrally (B) in many amphibia and reptiles; in the tail region of some 

 mammals (C) the intercentra are left as chevron bones, and are lost 

 completely in mammals without a tail. 



