xi VERTEBRAL COLUMN 497 



Vertebrates, and a pair of longer, backwardly-projecting pos- 

 terior horns or thyro-hyals, attached to the larynx and repre- 

 senting the lower ends of the first branchial arch. 



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 con- 

 nected by synovial articulations, as in the frog, but inter- 

 posed between them are elastic intervertebral 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 characteristic of the 

 vertebrae of all or nearly all mammals : they unite com- 

 paratively late with the centrum proper, and so in dis- 

 articulated 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 respectively 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 centrtini (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 vertebrae are in their 



PRACT. ZOOL. K K 



