72 THE VERTEBRATE SKELETON. 



1. THE AXIAL SKELETON. 

 A. THE VERTEBRAL COLUMN AND RIBS. 



The vertebral column consists of a series of some hundred 

 and thirty vertebrae, each of which is united with its pre- 

 decessor and successor in such a way as to allow a large 

 amount of flexibility. 



These vertebrae are developed round an unsegmented 

 rod, the notochord, which forms the axial support of the 

 embryo. The notochord remains continuous throughout the 

 whole vertebral column, but is greatly constricted opposite the 

 middle of each vertebra, and thus rendered moniliform. The 

 vertebrae are divided into two groups, an anterior group of 

 trunk vertebrae, and a posterior group of caudal or tail 

 vertebrae. 



A typical vertebra consists of a middle portion, the cen- 

 trum, a dorsal portion, the dorsal or neural arch, which 

 surrounds the spinal cord, and a ventral portion, the ventral 

 or haemal arch, which similarly encloses a space. 



The tail vertebrae of the dogfish have this typical arrange- 

 ment, the trunk vertebrae have the haemal arches modified. 



Each centrum is a short cylinder of cartilage surrounding 

 an hourglass-shaped cavity occupied by the notochord. The 

 neural arches are composed of three separate elements, the 

 vertebral neural plates (basidorsalia), intervertebral 

 neural plates (interdorsalia), and neural spines (supra- 

 dorsalia). 



The vertebral neural plates are in the adult fused with 

 their respective centra, and are notched behind for the exit 

 of the ventral (motor) roots of the spinal nerves. The inter- 

 vertebral neural plates are polygonal pieces alternating 

 with the vertebral neural plates; they are notched behind, 

 but at a more dorsal level than are the vertebral neural plates, 



