CHAPTER XXI 



THE VERTEBRAL COLUMN, RIBS, AND STERNUM 



The primitive skeletal stiffening of the body is the notochord. 

 In Amphioxus this extends to the extreme anterior end of the 

 body ; in Petromyzon it does not reach further forwards than 

 the region of the infundibulum, but in this position it persists 

 throughout life. In the remaining vertebrates, the notochord 

 usually disappears in the skull region. 



Surrounding the notochord are two sheaths, the so-called 

 elastica interna and the elastica externa. These are of import- 

 ance in some forms in connexion with the formation of the 

 vertebral column. 



Amphioxus has no structures comparable to vertebrae, 

 but they appear first in Petromyzon in the form of little paired 

 pegs or struts on each side of the nerve-cord, rising up from the 

 notochord. There are two pairs of these pegs to each segment 

 as a rule. The notochord in Petromyzon is continuous and 

 unconstricted, a primitive feature. 



A properly formed vertebral column appears first in the 

 Selachii. Each vertebra is composed of a neural arch formed 

 from a pair of basidorsals, and a pair of basiventrals which in 

 the region of the tail form a haemal arch. Between them, the 

 basidorsals and basiventrals form the body of the vertebra or 

 centrum, which constricts the notochord, and usually obliterates 

 it altogether except between one centrum and the next. 

 Alternating with the basidorsals are the interdorsals, and in 

 some, interventrals are present. The basidorsals and basi- 

 ventrals perforate the elastica externa and cartilage-cells invade 

 the notochordal sheath. Such centra are called chordal, and 

 they occur in the Selachii, in the sturgeons and in the 



302 



