200 THE DOG-FISH 



Further back the transverse processes become 

 more prominent and project ventrally as haemal 

 processes, causing the under surface of the column 

 to appear grooved. 



In the tail the ribs are absent, and the haemal 

 processes of each vertebra meet and unite mid- 

 ventrally, thereby completing the haemal arch. 



At the hinder end of the tail, opposite the caudal 

 fins, the haemal arches are produced downwards into 

 median haemal spines of some length and breadth. 



The haemal arches develop independently of the 

 vertebral centra, but become connected with them at 

 a very early date. 



8. The neural arches are made up of elements of three 

 kinds; the neural processes, neural plates, and 

 neural spines, 



a. The neural processes, forming the bases of the 



neural arches, are blunt processes projecting 

 upwards from the sides of each vertebra opposite 

 its middle. 



b. The neural plates are a series of thin hexagonal 



plates of cartilage which form the sides of the 

 neural canal (fig. 37). On each side there are 

 twice as many neural plates as there are vertebrae, 

 the neural plates being placed alternately opposite 

 the middles of the vertebrae, and opposite the in- 

 tervertebral intervals. The former or vertebral 

 neural plates are fused in the adult with the tops 

 of the neural processes, and have their posterior 

 borders notched for the passage of the ventral 

 roots of the spinal nerves. The dorsal roots of the 

 nerves pass out through notches in the posterior 

 borders of the intervertebral neural plates. 



c. The neural spines are a series of median nodules 



of cartilage, wedged in between the dorsal ends of 

 the neural plates so as to complete the neural 

 arches dorsally. 



