.346 



THE MECHANICAL FUNCTIONS. 



parallel to each other the whole length of the body : 

 but in process of time these two filaments unite, 

 and constitute a single spinal cord : and the pri- 

 mary type of the skeleton is determined by the 

 peculiar form of this, the central organ of the ner- 

 vous system. 



In laying the foundations of the skeleton, then, 

 the tirst object is to provide for the security of the 

 spinal cord ; and this is accomplished by enclosing 

 it within a series of cartilaginous rings, which are 

 destined to shield it during its growth, and, by 

 their subsequent ossification, to protect it most 

 etfectually from all injurious pressure. It is this 

 part of the skeleton, accordingly, of which the 

 rudiments appear the earliest in the embryo animal. 

 Tliese rings form a column, extending in a longi- 

 tudinal direction along the trunk ; retracing to us 

 the series of horny rings, in which the bodies of 

 worms, of insects, and indeed of all the Arliculala, 

 are encased. When ossified, these several rings 

 are termed vertebrcB ; and the entire column which 

 they compose is the Spine. Fig. 177 shows the 



form of one of the vertebrae of the back in the 

 human skeleton. Fig. 178 is a side view of four 



