STRUCTURE OF VERTEBRA. 



351 



by different physiologists ; but 

 the following are more parti- 

 cularly recognised as being en- 

 titled to that character. They 

 are represented in their relative 

 situations in Fig. 180. The first 

 is the part which forms the nu- 

 cleus, or body (b) of the vertebra; 

 and its ossification begins at the 

 centre. Next in importance are 

 the two bony plates, or haves, 

 as they may be called (l, l), 

 which proceed from the sides of the body, and em- 

 brace the spinal marrow which is situated between 

 them. The fourth essential element is the spinous 

 process (s), which unites the two leaves, and thus 

 completes the superior arch, of which it may be re- 

 garded as the key stone, for the protection of the 

 spinal marrow. Then come the two transverse pro- 

 cesses (t, t), which extend outwards from the sides, 

 and with which the arches of bone, constituting the 

 ribs (r, r), are generally connected. These are the 

 six parts which have been considered as the ele- 

 ments that are most essential, and most constantly 

 present in the composition of tlie vertebrae. But 

 some other parts may also be noticed as of very 

 frequent occurrence : such are the bony plates 

 which cover the two flat portions of the bodies of 

 the vertebrae, forming the surfaces immediately 

 contiguous to the intervertebral ligament ; which 

 surfaces, in some of the lower orders of the verte- 

 brata, become articular. There is frequently, also, 

 a developement of processes (i), forming arches 

 and spines at the lower surface of the vertebrae, or 



