274 



THE MECHANICAL FUNCTIONS. 



nical circumstances of the system, and to the particular in- 

 tentions of their formation. 



There is scarcely any part of the osseous fabric of which 

 the variations better illustrate the strict unity of plan and 

 the beautiful law of gradation observed by nature in all her 

 operations, than the spine. In studying the various modifi- 

 cations which this part of the skeleton undergoes, it will be 

 useful to bear in mind the principles which appear to regu- 

 late its formation, and which Geoffroy St. Hilaire has de- 

 duced by following the history of its early growth, and no- 

 ticing the order in which its several parts are developed.* 

 In common wath all bones, the vertebrae take their rise from 

 certain determinate points, or centres of ossification, where, 

 at first, detached pieces of bone are formed, destined to 

 unite together so as to compose the entire bone. An accu- 

 rate knowledge of the general forms and relative situations 

 of these elementary pieces is of much importance, because 

 we find that particular circumstances determine the deve- 

 lopment of some of these parts much earlier, and to a greater 

 extent than other parts, and thus lead to great differences in 

 the shapes and proportions of various bones, at different pe- 

 riods of their growth, although their origin and composition 

 are essentially the same. 



The number of elements which enter into the composi- 

 tion of a vertebra has been differently estimated by different 



physiologists: but the following are 

 certainly entitled to that character. 

 They are represented in their relative 

 situations in Fig. 180. The first is 

 the part which forms the nucleus, or 

 body (b) of the vertebra; and its ossi- 

 fication begins at the centre. Next 

 in importance are the two bony plates, 

 or leaves, as they may be called (l, 

 L,) which proceed from the sides of 

 the body, and embrace the spinal 

 marrow which is situated between 

 them. The fourth essential element 

 * Memoii'es du Museum, ix. 79 and 89. 



