SPINO-CEREBRAL AXIS. 387 



animals : for amidst great differences of structure 

 and of functions, an analogy is still retained 

 among the several forms of the nervous system, 

 characterising these two great divisions of the 

 animal kingdom. In the embryo state of the ver- 

 tebrata the central parts of that system consist 

 of two separate filaments running parallel to each 

 other the whole length of the body : but in pro- 

 cess of time these two filaments unite, and con- 

 stitute a single spinal cord : and the primary 

 type of the skeleton is determined by the pecu- 

 liar form of this, the central organ of the nervous 

 system. 



In laying the foundations of the skeleton, 

 then, the first object is to provide for the secu- 

 rity 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 effectually from all injurious pres- 

 sure. It is this part of the skeleton, accordingly, 

 of which the rudiments appear the earliest in the 

 embryo animal. These rings form a column 

 extending in a longitudinal 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 Articulata, are encased. When 

 ossified, these several rings are termed vertebra ; 

 and the entire column which they compose is 

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



