216 THE DOG-FISH. 



Each centrum is a ring, thin at its ends; but 

 thickened on its inner side in the middle of its 

 length, so that the cavity is hourglass-shaped 

 (fig. 49, p. 238). The inner surface of the ring is 

 lined by a layer of bone. 



The notochord persists throughout the whole 

 length of the column: it is greatly constricted 

 opposite the middle of each vertebra, but thickened 

 almost to the full width of the column in the inter- 

 vertebral spaces. 



2. The heemal arches. 



In the anterior part of the vertebral column the 

 centra are flattened ventrally, and produced laterally 

 into blunt horizontal transverse processes, which bear 

 at their ends movably articulated cartilaginous ribs 

 about half an inch in length. 



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 hiemal 

 processes of each vertebra meet and . unite mid- 

 ventrally, thereby completing the hfemal arch, which 

 encloses the caudal artery and vein. 



At the hinder end of the tail, opposite the caudal 

 fins, the haemal arches are produced downwards into 

 median hsemal spines. 



The haemal arches develop independently of the 

 vertebral centra, but become connected with them at 

 a very early date. 



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



kinds : neural processes, neural plates, and neural 

 spines. 



a. The neural processes, forming the bases of the 

 neural arches, are blunt processes projecting 



