THE SKIN AND SKELETON. 



141 



the corresponding surfaces of the preceding and succeed- 

 ing vertebrae. The bases of the arch are notched in front 

 and behind, so that when two vertebrae are put together a 

 round opening {intervertebral foramen) appears between 

 the pair, giving passage to the nerves issuing from the 

 spinal cord. From the sides of the arch, blunt transverse 

 processes project outward and backward, called diapophy- 

 ses. Such are the main elements in a representative ver- 

 tebra. The haemal arch is not formed by any part of the 

 vertebra, but by the ribs and breast-bone. Theoretically, 

 however, the ribs are considered as elongated processes 

 from the centrum (pleurapophyses), and in a few cases a 

 hcemal 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 towards 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 extremities 

 (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. But in the 

 higher animals there are six kinds of vertebrae : cranial, 

 cervical, dorsal, lumbar, sacral, and caudal. 



The cranial vertebra? form the skull. 77 They are greatly 

 modified, as the neural arches are expanded to enclose the 

 brain. The number of distinct bones composing the skull 

 is greatest in Fishes, and least in Birds : this arises partly 



