388 THE MECHANICAL FUNCTIONS. 



the vertebrae of the back in the human ske- 

 leton. Fig. 178 is a side view of four vertebrae 

 joined together, and Fig. 179 is a vertical section 



A 



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 

 character of this great assemblage of animals, 

 which have accordingly been denominated the 

 Vertehrata, or Vertehrated Animals. 



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 con- 

 thiuous frame-work : it is the general axis of 

 all their motions, the common fulcrum on which 

 the principal bones of the extremities are made 

 to turn : it furnishes fixed points of attachment 



