332 MANUAL OF ZOOLOGY SECT. 



When the skeletons of these three animals are examined 

 and compared, it will be found that they are constructed on 

 the same general plan with differences in details. In the 

 Dog-fish the skeleton is composed mainly of cartilage : in 

 the others mainly of bone. In all there is a rod-like axis, 

 the spinal or vertebral column supporting the trunk and tail, 

 but not continued into the head, where its place is taken 

 by the skull. The spinal column consists of a row of 

 similar segments, the vertebra, which articulate with one 

 another. Each vertebra consists of a ventral solid portion, 

 the centrum or body ; an arch of bone or cartilage, the 

 neural arch, situated on the dorsal side of the centrum, and 

 certain processes. The series of centra form together a 

 strong axial support for the entire body and tail ; the 

 series of neural arches enclose a canal, the neural canal, 

 on the dorsal side of the centra. By the interlocking of 

 certain articulating processes of the neural arches the 

 vertebrae in the Lizard and Rabbit are yet more firmly 

 united together. 



In the Dog-fish the centra (Fig. 195, c.) have deeply 

 concave anterior and posterior faces, so that when the 

 vertebrae are in position there are hollows of considerable 

 extent between the centra formed by the apposition of these 

 concave faces. This form of centrum is termed ampbicoclous. 

 The entire spinal column is distinguishable into two regions 

 the region of the trunk in front and the region of the tail 

 (caudal region) behind. In the region of the trunk the 

 vertebrae bear very small ribs in the form of short rods of 

 cartilage ; in the caudal region ribs are absent, but each 

 vertebra bears, in addition to the neural arch, a ventrally 

 situated arch of similar shape the hccmal arch (h. a.). 



In both the Lizard and the Rabbit the vertebrae are 

 composed almost entirely of bone. In the former (Fig. 



