SKELETON 



49 



vertebral canal. Each arch is composed of a pair o f neural plates 

 (neixrapophyses) which meet above the cord and are continued dor- 

 sally as the neural spine. A similar haemal arch extends from the 

 ventral side of the centrum around the blood-vessels of the tail. 

 This consists of similar paired plates (hsemapophyses) and a haemal 

 spine. In the trunk region centrum and neural arch are similar, but 

 the haemal arch is either incomplete (surrounding the various viscera 

 in fishes) or is entirely absent. 



In the higher vertebrates other parts are frequently added to 

 those just mentioned (fig. 42). Most common of these are articular 

 processes (zygapophyses) on the anterior and posterior sides of the 



Fig. 41. Fig. 42. 



Fig. 41. — Diagrams of {A and B) fish vertebras and (C) vertebra from higher groups. 

 b, basal stumps; c, capitular head of rib; et, centrum; d, diapophysis; fr, fish rib; ha, 

 hsmal arch; no, neural arch; p, parapophysis; r, rib; t, tubercular head. 



Fig. 42. — Two caudal vertebrae of alligator, c, centrum; ha, haemapophysis; hs, 

 hsmal spine; na, neurapophysis; ns, neural spine; poz, prz, post- and prezygapophyses; 

 /, transverse process. The arrow passes through the neural arch. 



neurapophyses, which aid in articulating more firmly the several 

 vertebrae. In most vertebrates higher than fishes there are also 

 transverse processes which extend laterally in the planes of the 

 partitions (myosepta) between the muscles. There may be two of 

 these transverse processes on either side of a vertebra, a more dorsal 

 diapophysis (fig. 41, C) arising from the neural arch, and a more 

 ventral parapophysis extending from the side of the centrum. The 

 true ribs, to be described later, articulate with the ends of these 

 processes. 



The ends of the centra by which the successive vertebrae are ar- 

 ticulated show, in different vertebrates, four distinct conditions (fig. 

 43). As in most fishes, they may be hollow at both ends (amphi- 



