VERTEBRAL COLUMN. 223 



the body becomes divided transversely into segments, the 

 centra or bodies of the vertebrae, the flexibility of this region 

 being thereby increased. 



Above the centra neural arches are formed, which sur- 

 round and protect the brain and spinal cord ; and below them 

 haemal arches, which tend to encircle the body-cavity. 



Besides this axial skeleton a visceral skeleton is developed 

 around the anterior part of the alimentary canal, and skeletal 

 elements are also formed in relation with the fins. 



The greater part of the skeleton consists throughout life 

 of cartilage. It is hardened in many places by deposition of 

 calcareous salts. 



The skeleton of the dog-fish is best prepared by taking 

 a specimen which has been some days in spirit, dipping it in 

 hot water a few degrees below boiling-point for a few minutes, 

 in order to soften the connective tissue, and then scraping or 

 brushing away the soft parts until the skeleton is clean. Speci- 

 mens so prepared must not be allowed to dry, but should be 

 kept in weak spirit. 



A. The Vertebral Column. 



A trunk vertebra consists of a centrum, traversed by the 

 notochord ; a dorsal or neural arch, surrounding the spinal 

 cord ; and transverse or haemal processes. In the caudal or 

 tail vertebrae of the dog-fish the haemal processes unite 

 ventrally, forming a haemal canal which encloses the caudal 

 artery and vein. 



The structure of the vertebral column is best made out by 

 cutting sagittal, transverse, and horizontal sections through 

 various parts of its length with a strong knife, and comparing 

 these with each other. 



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

 as they are wide, connected together by interver- 

 tebral ligaments of strong connective tissue, which 

 allow slight movement between the successive 

 vertebrae and free flexion of the column as a whole. 



