xi VERTEBRAL COLUMN 497 



of lower Vertebrates, and a pair of longer, backwardly- 

 projecting posterior horns or thyro-hyals, attached to the 

 larynx and representing the lower ends of the first 

 branchial arch (compare p. 44). 



The vertebral column includes about forty-five bony 

 vertebrae, each consisting of a centrum, a neural arch, 

 and various processes (compare pp. 36-38), but becoming 

 simplified towards the end of the tail. The centra have 

 flat anterior and posterior surfaces, and are not connected 

 by synovial articulations, as in the frog, but interposed 

 between them are elastic iniervertebral discs of fibro- 

 cartilage. In addition to the ossification which gives 

 ' rise to the main part of the centrum, a separate flat disc 

 of bone (Fig. '134, ep) is formed on the anterior and 

 posterior surface of each. These epiphyses are character- 

 istic of the vertebrae of all or nearly all Mammals : they 

 unite comparatively late with the centrum proper, and 

 so in disarticulated skeletons of young animals they 

 often come away from the main mass of the centrum 

 and remain attached to the intervertebral discs. 



In correspondence with the differentiation of the parts 

 of the body, the vertebral column is divisible into five 

 regions (Fig. 130) : the cervical in the neck, including 

 seven vertebrae, the first two of which called respec- 

 tively the atlas and axis are peculiarly modified in 

 order to allow the skull free movement ; the thoracic 

 in the thorax, twelve or thirteen in number, and bearing 

 ribs ; six or seven lumbar in the abdominal region ; three 

 or four sacral in the sacral region ; and about fifteen or 

 sixteen caudal in the tail. 



Examining one of the anterior thoracic vertebrae first 

 (Fig. 134), we see that the centrum (c) is continuous above 

 with the neural arch (n. a), the lower part of which, on either 

 side, presents an anterior and a posterior notch (i. v. n), so 

 that when the vertebra are in their natural position, an 



