GENERAL CHARACTERS OF THE VERTEBRATA. 375 



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

 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 " haemal." The correspondence of the typical bony 

 segment or vertebra with the doubly tubular structure of the 



Fig- iSS- Skeleton of the Beaver (Cas tor fiber), showing the different regions of the 

 vertebral column, c Cervical region ; d Dorsal region ; b Lumbar region ; j Sacrum ; 

 t Caudal region. 



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. 155, 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 " (b). 4. A greater or less 



