VERTEBRAL COLUMN. 



271 



incased. When ossified, these several rings arc termed vo'- 

 Uhrx; and the entire column which they compose is the 

 Spine. Fig. 177 shows the form of one of the verteb^ of 

 the back in the human skeleton. Fig. 178 is a side view of 

 four vertebrae joined together, and Fig. 179 is a vertical sec- 



tion of the same part of the spine, showing the canal formed 

 by the rings. From the constancy with which the spinal 

 column is found in all animals of this type, and from the 

 uniformity of the plan on which, amidst endless variations, 

 it is modelled, it has been chosen as the distinctive charac- 

 ter of this great assemblage of animals, which have, accord- 

 ingly, been denominated the Verlebrata, or Vcrtcbratcd 

 Jinimals, 



Nor is the spine of less importance when viewed in its 

 mechanical relations to the rest of the skeleton. It is the 

 great central beam of the fabric, establishing points of union 

 between all its parts, and combining them into one conti- 

 nuous frame-work: it is the o;eneral axis of all their motions, 

 the common fulcrum on which the principal bones of the 

 extremities are made to turn: it furnishes lixed points of at- 

 tachment to all the large muscles which act upon these bones 

 as levers, and, also, to those which move the trunk itself. 



If this column had been perfectly rigid, the whole frame- 

 work would Ti^'C been exposed to inconvenience and even 

 danger, amidst the shocks it must encounter during all tlic 

 quick and sudden movements of the body. Not only must 

 its mechanism be framed to sustain these shocks, but also to 



