

VEKTEBRAL COLUMN 199 



boiling point for a few moments, in order to soften the connec- 

 tive tissue, and then scraping or brushing away the soft parts 

 until the skeleton is clean. Specimens so prepared must not 

 be allowed to dry, but should be kept in weak spirit. 



A. The Vertebral Column, or ' Backbone,' 



A typical vertebra consists of a centrum, traversed by the 

 notochord ; a dorsal or neural arch, surrounding the spinal 

 cord ; and a ventral or haemal arch surrounding the body- 

 cavity. The caudal or tail vertebrae of the dog-fish are 

 perfectly typical, while the anterior or trunk vertebrae have 

 the haemal arch modified or absent. 



The structure of the vertebral column is best made out by 

 cutting longitudinal, transverse, and horizontal sections through 

 various parts of its length, and comparing these with one 

 another. 



1. The centra are short cylinders of cartilage, about as long 



as they are wide, connected together by intervertebral 

 ligaments of strong connective tissue, which allow 

 slight movement between the successive vertebrae 

 and free flexion of the column as a whole. 



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. 37). The inner surface of the ring is very 

 densely calcified. 



The notochord persists throughout the whole 

 length of the column : it is moniliform in shape, 

 being greatly constricted opposite the middle of each 

 vertebra, but dilated almost to the full width of the 

 column in the intervertebral spaces. 



2. The haemal 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. 



