DEVELOPMENT OF THE SKELETON. 



28l 



the intervertebral discs (ligamenta intervertebralia), and 

 at the same time, are connected by joints, so that the 

 entire vertebral column forms a firm and solid axis, which 

 is, however, flexible and elastic, capable of moving freely 

 in all directions. In the various regions of the trunk, 

 the vertebras differ in form and connection, so that the 

 following are distinguished in the human vertebral column, 

 beginning from above : seven neck- vertebras, twelve breast- 

 vertebras, five lumbar-vertebras, ^.ve cross-vertebras, and 

 four to five tail-vertebras. The uppermost, those directly in 



Fig. 254. — Third neck-vertebra of man. 

 Fig. 255. — Sixth breast- vertebra of man. 

 Fig. 256. — Second lumbar-vertebra of man. 



contact with the skull, are the neck-vertebras (Fig. 254), 

 and are distinguished by a hole found in each of the 

 two lateral processes. There are seven neck-vertebras in 

 Man, as in nearly all other Mammals, whether the neck 

 is long, as in the Camel and the Giraffe, or short, as in the 

 Mole and the Hedgehog. The fact that the number of these 

 neck-vertebras is always seven, — and there are but few 

 exceptions (explicable by adaptation), — is a strong argu- 

 ment for the common descent of all Mammals ; it can only 

 be accounted for as a strict transmission from a common 



