GENERAL CHARACTERS OF THE VERTEBRATA. 329 



and 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 "haemal." The correspondence of the typical bony 

 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. 118, c). 2. A number of vertebrae 



Fig. 1 1 8. Skeleton of the Beaver (Castor fiber], showing the different- regions of 

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

 Sacrum ; t Caudal region. 



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 



