VERTEBRAL COLUMN. 347 



vertebra" joined together, and Fig. 179 is a vertical 

 section 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 ani- 

 mals 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 cha- 

 racter of this great assemblage of animals, which 

 have accordingly been denominated the Vertehrata, 

 or Verteb rated Animals. 



The spine is of great importance in its mecha- 

 nical 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 continuous framework : it is the general 

 axis of all their motions, or the common fulcrum 

 on which the principal bones of the extremities 

 are made to turn : it furnishes tixed points of attach- 

 ment 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 framework would have been exposed to 

 inconvenience, and even danger, amidst the shocks 

 it must encounter during all the quick and sudden 

 movements of the body. Not only must its mecha- 

 nism be framed to sustain these shocks, but also to 

 accommodate itself to various kinds of flexions 

 and twistings of the trunk. While these objects 

 are provided for, care must at the same time be 

 taken that the spinal marrow it encloses shall, 

 amidst all these motions, remain secure from pres- 

 sure ; for so delicate is its structure, that the least 

 degree of compression would at once interrupt its 



