194 PHYLUM CHORD ATA 



The Axial Skeleton. The Vertebral Column. Strip the muscles 

 from the back. Disarticulate the head from the trunk. 



The vertebral column is composed of nine vertebrae and a long, 

 unsegmented bone called the urostyle, which forms its posterior 

 portion. Four regions may be distinguished in it : a cervical 

 region, consisting of the first vertebra ; a thoracolumbar region, 

 consisting of the succeeding seven vertebras ; a sacral region, con- 

 sisting of the last vertebra ; and the urostyle, which represents a 

 caudal region. 



A vertebra is made up of the following parts : the centrum, or 

 body, which is the cylindrical ventral portion ; the neural arch, 

 on the dorsal side of the centrum, which with it forms the neural 

 canal in which lies the spinal cord ; and the transverse processes, 

 a pair of long lateral projections. The neural arch is made up of 

 a pair of neural processes, which form its sides, and the median 

 neural spine, or spinous process, which forms its roof. On the an- 

 terior surface of the neural arch is a pair of articular projections 

 called the prezygapophyses ; on the posterior surface is a pair of 

 corresponding postzygapophyses. It is by these projections that 

 the vertebrae are locked together. Note the difference in size in 

 the transverse processes of the various vertebrae. 



The first, or cervical, vertebra is called the atlas. It differs from 

 the other vertebrae principally in that it has no transverse proc- 

 esses (although occasionally they have been found) , no prezyga- 

 pophyses, and a thinner centrum. On its anterior surface is a pair 

 of depressions into which fit the articular processes of the skull, 

 the condyles. The last vertebra, or sacrum, has large, transverse 

 processes with which the pelvic girdle articulates. It lacks post- 

 zygapophyses, and on the hinder surface of the centrum is a pair 

 of prominences which articulate with the urostyle. 



Exercise 26. Draw a view of the ventral aspect of the vertebral 

 column. 



Exercise 27. Remove the second vertebra, clean it thoroughly, and 

 draw a view of its hinder end. 



The Skull. The skull is composed of two regions, the cranium 

 and the visceral skeleton. The former incloses and protects the 



