XII 



PHYLUM CHORDATA 



335 



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 h&mal arch. 



In both the lizard and the rabbit the vertebrae are com- 

 posed entirely of bone. In the former the centra have 

 concave anterior and convex posterior surfaces and the 

 vertebrae are accordingly said to be precocious. In the lat- 

 ter the surfaces are flat, and the discs of fibro-cartilage, the 

 inter-vertebral discs, are intercalated between the vertebrae. 



FIG. 210. Vertebrae of Lizard. A, anterior, B, posterior, view of a thoracic ver- 

 tebra; C, lateral, D, anterior, view of atlas vertebra; E, lateral view of axis. 

 cent, centrum; hyp, hypapophysis of axis; lat, lateral piece of atlas; lig. liga- 

 mentous band dividing the ring of the atlas into two; neur, neural arch of atlas; 

 od, odontoid process; pr. zy, pre-zygapophysis; pt. zy, post-zygapophysis; rb, 

 rib; sp, spine; -vent, ventral piece of atlas. 



In both the spinal column is divisible into five regions, the 

 cervical, the thoracic, the lumbar, the sacral, and the caudal. 

 The cervical region is the most anterior. In the rabbit the 

 vertebrae of the cervical region are devoid of ribs; in. the 

 lizard they have short ribs with the exception of the first 

 three. The first and second vertebrae in both the rabbit 

 and the lizard are specially modified in connection with the 

 movements of the head on the trunk. The vertebrae of the 



