VERTEBRAL COLUMN. 



Ill 



duced by alterations in the notochordal sheath as well as in the 

 surrounding skeletogenous sheath (fig. 570, a). The latter gives 

 rise to cartilaginous or bony rings, which 

 represent the first rudiments of the vertebral 

 bodies. These rings constrict the notochord 

 till they assume the form of biconcave car- 

 tilaginous or bony discs, and become connected 

 with cartilaginous or bony arches which are 

 developed round the spinal cord and the 

 perivisceral cavity (fig. 570 a, b). Each 

 vertebra therefore consists of a principal 

 median portion, the body of the vertebra or 

 centrum, which frequently retains the re- 

 mains of the notochord in its axis ; of a 

 dorsal or neural arch, and a ventral or haemal 

 arch. The two limbs of the dorsal arch are 

 called neurapophyses, those of the ventral 

 arch haemapophyses, and the unpaired median 

 prolongation of each arch is known as the 

 spinous process (fig. 570, D, D'). The trans- 

 verse processes (pleurapophyses) which arise 

 from different parts of the vertebrae, either 

 from the neural arches or from the centra, 

 are not independent structures but merely 

 processes. The ribs, on the other hand, are 

 independent lateral bony or cartilaginous rods 

 which are attached either to the hsemapophyses 

 (fishes) or to the pleurapophyses, and embrace 

 the part of the body cavity containing the 

 viscera. 



Regions of the vertebral column. In the 

 higher Vertebrates the primitive homonomous 

 segmentation of the skeleton gives place to 

 a heteronomous segmentation which leads to 

 the origin of a number of regions. In this 

 point as in others there is a parallel between 

 the segmented Invertebrates and Vertebrates. 

 In the first place an anterior region or head 

 can always be distinguished from the posterior uniformly segmented 

 region or trunk (fig. 571) ; and this division corresponds with the 

 enlargement of the anterior part of the central nervous system to 



570. a, Diagram of 

 the vertebral column of 

 a Teleostean with inter- 

 vertebral growth of the 

 notochord. Ch, Noto- 

 chord ; Wk, bony verte- 

 bral bodies ; J, mem- 

 branous Intel-vertebral 

 portion, b, Vertebrae of 

 fish. K, Body of vertebra. 

 Ob, dorsal arch (neura- 

 pophysis) ; Ub, ventral 

 arch (hcemapophysis) ; D, 

 dorsal spinous process ; 

 D', ventral spiuous pro- 

 cess ; K, rib. 



