VERTEBRATA. 



129 



haemal arches. Hence it follows that the ribs in a fish and 

 those in a higher vertebrate a bird or man, for example 

 are not identical; i.e., are not homologous. The centra of 

 the vertebrae may be hollow at either end (amphicoelous) as 

 in fishes, or they may be hollow behind and rounded in 

 front (opisthoccelous) as in the salamanders; or again they 

 may be rounded in front and concave behind (proccelous) 

 as in many reptiles; or lastly, they may have flat surfaces, 

 as in most mammals. 



The vertebral column is capable of division into regions. 



/ 



FIG. 55. Diagram of the skeleton of a mammal, showing regions of verte- 

 bral column, etc. d, cervical; e, thoracic; /, lumbar; gr, sacral; /j, 

 caudal vertebrae ; i, scapula ; /c, humerus ; Z, radius ; m, carpus ; n, ulna ; 

 o, metacarpus; p, pelvis; r, femur; *s. fibula : f, tibia; tt, tarsus ; v, meta- 

 tarsus; it?, phalanges; j/, sternum. 



In the fishes there are two of these, trunk and caudal, the 

 former being distinguished by bearing ribs. In the Batra- 

 chia a cervical region is distinguished from the trunk by the 

 absence of transverse processes from its single vertebra, 

 while the caudal is separated from the trunk by a sacral 

 region, the vertebra of which is connected with the bones 

 (girdle) supporting the hind limbs. In the higher verte- 



