xii PHYLUM CHORDATA 489 



lumbar, the sacral, and the caudal. In the cervical region 

 there are seven vertebrae; in the thoracic twelve or some- 

 times thirteen, in the lumbar seven or sometimes six, in the 

 sacral four, and in the caudal about fifteen. 



The centra of the vertebrae in a young Rabbit consist of 

 three parts a middle part which is the thickest, and two thin 

 disks of bone the epiphyses anterior and posterior, applied 

 respectively to the anterior and posterior faces of the middle 

 part or centrum proper. Between successive centra in an 

 unmacerated skeleton are thin disc-like plates of fibre-cartil- 

 age the inter-vertebral discs. 



The first vertebra or atlas resembles the corresponding 

 vertebra of the Pigeon in being of the shape of a ring 

 without any solid centrum like that of the rest. On the 

 anterior face of its lateral portions are two concave articular 

 surfaces for the two condyles of the skull. The second 

 vertebra or axis bears on the anterior face of its centrum 

 a peg-like process the odontoid process which fits into the 

 ventral part of the ring of the atlas. 



The thoracic vertebrae all have elongated spines. The 

 transverse processes are short and stout ; each bears near its 

 extremity a small smooth articular surface or tubercular facet 

 for the tubercle of a rib. On the anterior and posterior 

 borders of each vertebra is a little semilunar facet, the 

 capitular facet, situated at the junction of the centrum and 

 the neural arch. The two contiguous semilunar facets 

 of successive vertebrae form between them a little cup- 

 like concavity into which the head or capitulum of a rib is 

 received. 



In the lumbar region the spines are comparatively short, 

 and the transverse processes and bodies are devoid of facets. 



The sacral vertebrae are firmly ankylosed together to form 

 a single composite bone, the sacrum. The first and second 



