THE FROG 125 



canal ; and the transverse processes, a pair of long lateral projec- 

 tions. 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 anterior surface of the neural 

 arch is a pair of articular projections called the prezygapophyses ; 

 on the posterior surface is a pair of corresponding postzygapophy- 

 ses. It is by these projections that the vertebrae are locked to- 

 gether. 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 processes 

 (although occasionally they have been found), no prezygapophy- 

 ses, 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. 



Between each two vertebrae on each side, and between the ninth 

 vertebra and the urostyle, is an interval called the intervertebral 

 foramen, through which a spinal nerve passes. In the urostyle, on 

 each side near the forward end, is the foramen of the posterior 

 spinal nerve. 



Exercise 32. Draw a view of the ventral aspect of the vertebral 



column. 

 Exercise 33. Remove the second vertebra, clean it thoroughly, and 



draw a view of its hinder end. 



The skull is composed of two regions, the cranium and the vis- 

 ceral skeleton. The former incloses and protects the brain and 

 the organs of special sense ; the latter surrounds the mouth and 

 pharynx, forming the framework of the jaws and tongue. 



Both these regions are composed exclusively of cartilage at 

 an early period of the life of the animal, and certain portions of 

 them remain cartilaginous. As the young animal develops, how- 

 ever, certain parts of the cartilage are replaced by bone. Besides 

 these cartilage bones, as they are called, other bones also make 



