26 



CLASSIFICATION 



former consists of backbone, skull, ribs, and breast-bone. It will be 

 perhaps best to begin with the first of these. 



The Axial Skeleton. 



I. The Backbone. — A glance at the backbone, or, as it is 

 often called, the vertebral column (fig. 4), shows that it is 

 thrown into a series of curves, which converts 

 it into a kind of spring for breaking the shocks 

 to which the body is constantly liable, and which 

 if transmitted to the brain would often result in 

 stunning, or it might be even more serious injury. 

 A further obvious fact is that we have not here 

 to deal with a single bone, but a large number of 

 them, piled on top of one another. Each of these 

 vertebrae (fig. 5) is irregular in shape, consisting 

 of a bung-shaped body (centrum) and a curved arck 

 from which several projections or processes grow 

 out. The successive bodies are placed one above 

 the other on the ventral side of the backbone, and 

 are separated from one another by pads of gristle 

 which act as buffers, allowing at the same time as 

 much flexibility as is consistent with a proper 

 degree of firmness. Successive arches are further 

 linked together by the overlapping of their pro- 

 cesses. It will further be noted that in this way a 

 kind of tunnel is formed in which 

 the spinal marrow is lodged and 

 efficiently protected. This is 

 essential, for it is an exceedingly 

 delicate organ, and any severe 

 injury inflicted upon it would 

 result either in death or else in 

 paralysis of the parts posterior 

 to the damaged region. The 

 vertebrae differ among them- 

 selves in size and shape, and are 

 divisible into groups belonging to different regions of the body. 

 The first seven, beginning at the head end, are the neck-vertebrce, 

 and of these the first and second have a special interest (fig. 6). 

 The first, or atlas, supports the rounded skull, just as, to compare 



Fig. 4. — Backbone 

 7, 19, 24, 29, 33, Last 

 vertebras of neck, chest, 

 loin, sacral, and tail re- 

 gions; ah^ ventral cur- 

 vatures; ggg^ pads of 

 gristle between bodies 

 of vertebrae. 



Fig. S-— A Dorsal 

 Vertebra 



a, body; 3 c, arch; dg, pro- 

 cesses; A, spinal canal. 



