352 COMPARATIVE ZOOLOGY 



opening (intervertebral foramen} appears between the 

 pair, giving passage to the nerves issuing from the 

 spinal cord. From the sides of the arch, blunt trans- 

 verse processes project outward and backward, called 

 diapophyses. Such are the main elements in a repre- 

 sentative vertebra. The hemal arch is not formed by 

 any part of the vertebra, but by the ribs and breastbone. 

 Theoretically, however, the ribs are considered as elon- 

 gated processes from the centrum (pleurapophyses), and 

 in a few cases a hemal spine is developed corresponding 

 to the neural spine. 



The vertebrae are united together by ligaments, but 

 chiefly by a very tough, dense, and elastic substance be- 

 tween the centra. The neural arches form a continuous 

 canal which contains and protects the spinal cord ; hence 

 the vertebral column is called the neuroskeleton. The 

 column is always more or less curved ; but the beautiful 

 sigmoid curvature is peculiar to man. The vertebrae 

 gradually increase in size from the head toward the 

 end of the trunk, and then diminish to the end of the 

 tail. The neural arch and centrum are seldom wanting ; 

 the first vertebra in the neck has no centrum, and the 

 last in the tail is all centrum. The vertebrae of the ex- 

 tremities (head and tail) depart most widely from the 

 typical form. 



The vertebral column in fishes and snakes is divisible 

 into three regions head, trunk, and tail. In the higher 

 animals there are six divisions of the vertebral column, 

 the skull, and cervical, dorsal, lumbar, sacral, and caudal 

 vertebra. 



The skull ul is formed of bones whose shape varies 

 greatly from that of typical vertebrae. The number of 

 distinct bones composing the skull is greatest in fishes, 

 and least in birds ; this arises partly from the fact that 

 the bones remain separate in the former case, while 



