STRUCTURE OF VERTEBRAE. 



393 



all bones, the vertebrae take their rise from cer- 

 tain determinate points, or centres of ossifica- 

 tion, where, at first, detached pieces of bone are 

 formed, destined to unite together so as to com- 

 pose the entire bone. An accurate knowledge 

 of the general forms and relative situations of 

 these elementary pieces is of much importance, 

 because we find that particular circumstances 

 determine the developement 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 periods of their growth, although their 

 origin and composition are essentially the same. 

 The number of elements which enter into the 

 composition of a vertebra has been differently 

 estimated by different phy- 

 siologists : but the following 

 are certainly entitled to that 

 character. They are repre- 

 sented in their relative situa- 

 tions in Fig. 180. The first 

 is the part which forms the 

 nucleus^ or body (b) of the 

 \\ vertebra ; and its ossification 

 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 si- 



