SPINO-CEUEBUAL AXIS. 345 



§ 4. Skeleton oftlie Vertebrata. 



The purposes to be answered by the Skeleton, in 

 vertebrated animals, resolve themselves into the 

 three following ; first, to afford mechanical support 

 to the body generally, and also to different portions 

 of the body ; secondly, to provide a solid basis for 

 the attachments of the muscles which are to effect 

 their movements ; and thirdly, to give protection to 

 the vital organs, and more particularly to the cen- 

 tral parts of the nervous system. Of these the 

 last is the circumstance that has the greatest influ- 

 ence in determining the principles on which the 

 osseous framework 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 substance,) are the most important parts 

 of that system, 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 

 denominated the spino-cerebral axis, in contradis- 

 tinction 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 

 nervous system, characterising these two great di- 

 visions of the animal kingdom. In the embryo 

 state of the vertebrata the central parts of that 

 system consist of two separate filaments, running 



