279 THE MECHANICAL FUNCTIONS. 



viding a solid basis for the attachments of the muscles which 

 are to effect their movements; and, thirdly, the giving pro- 

 tection to the vital organs, but more particularly to the cen- 

 tral parts of the nervous system. Of these, the last is the 

 circumstance that has the greatest influence in determining 

 the principles on which the osseous frame-work has been 

 constructed. In the nervous system of all the animals 

 coming under the denomination of vertebrata, the spinal 

 marrow, together with the brain, which may, indeed, be 

 considered as the anterior extremity of the spinal marrow, 

 only much enlarged by an additional mass of nervous sub- 

 stance, are the most important parts of that sj^stem, and the 

 organs which stand most in need of protection from every 

 kind of injury. These two portions of the nervous system, 

 when viewed as composing a single organ, have been deno- 

 minated the spi)io -cerebral axis, in contradistinction to the 

 analogous parts of the nervous system of articulated animals: 

 for, amidst great differences of structure and of functions, an 

 analogy is still retained among the several forms of the ner- 

 vous system, characterizing these two great divisions of the 

 animal kingdom. In the embryo state of the vertebrata, the 

 central parts of that system consist of two separate filaments, 

 running parallel to each other the whole length of the body: 

 but, in process of time, these two filaments unite, and con- 

 stitute a single spinal cord: and the primary type of the ske- 

 leton is determined by the peculiar 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 security of the spinal cord: and 

 this is accomplished by enclosing it within a series of carti- 

 laginous rings, which are destined to shield it during its 

 growth, and, by their subsequent ossification, to protect it, 

 most effectually, from all injurious pressure. It is this part 

 of the skeleton, accordingly, of which the rudiments appear 

 the earliest in the embryo animal. These rings form a co- 

 lumn, 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 %/irticulata^ are 



