26 ANIMAL MECHANICS 



We now see the importance of the spine, and 

 we shall next explain how the various offices are 

 provided for. 



If the protection of the spinal marrow had 

 been the only object of this structure, it is natural 

 to infer that it would have been a strong and 

 unyielding tube of bone; but, as it must yield 

 to the inflections of the body, it cannot be con- 

 stituted in so strict an analogy with the skull. 

 It must, therefore, bend; but it must have no 

 abrupt or considerable bending at one part ; for 

 the spinal marrow within would in this way suffer. 



By this consideration we perceive why there are 

 twenty-four bones in the spine, each bending a 

 little ; each articulated or making a joint with its 

 fellows ; all yielding in a slight degree, and, con- 

 sequently, permitting in the whole spine that flex- 

 ibility necessary to the motions of the body. It 

 is next to be observed that, whilst the spine by 

 this provision moves in every direction, it gains a 

 property which it belongs more to our present 

 purpose to understand. The bones of the spine 

 are called vertebras ; at each interstice between 

 these bones, there is a peculiar gristly substance, 

 which is squeezed out from betwixt the bones, 

 and, therefore, permits them to approach and play 

 a little in the motions of the body. This gristly 

 substance is inclosed in an elastic binding or 

 membrane of great strength, which passes from 



