GENERAL CHARACTERS OF THE VERTEBRATA. 351 



aiid is completed in front by the breast-bone or sternum (b), 

 which in some cases— but not in man — develops a spine (the 

 haemal spine) which corresponds to the neural spine on the 

 opposite aspect of the vertebra, 



It follows from the above, that the typical vertebra consists 

 of a central piece or body from which two arches are given off, 

 one of which protects the great masses of the nervous system, 

 and is therefore said to be " neural ; " whilst the other pro- 

 tects the .main organs of the circulation, and is therefore said 

 to be "hsemal." The correspondence of the typical bony 



Fig. 126. — Skeleton of ttie Beaver {Castor ■Ji^er), showing tlie different regions of the 

 vertebral. column, c Cervical region ; d Dorsal region; d Lumbar region ; 5 Sacrum; 

 t Caudal region. 



segment or vertebra with the doubly tubular structure of the 

 body in all Vertebrates is thus too obvious to require to be 

 specially pointed out. 



As a general rule, the vertebral column is divisible into a 

 number of distinct regions, of which the following are recog- 

 nisable in man and in the higher Vertebrata: — i. A series of 

 vertebrae which compose the neck, and constitute the " cervical 

 region" of the spine (fig. 126, c). 2. A number of vertebrae 

 which usually carry well-developed ribs, and form the " dorsal 

 region " (d). 3. A series of vertebrae which form the region 

 of the loins,, or "lumbar region" (p). 4. A greater or les."! 



