OR OSSEOUS SYSTEM. 57 



appear most constant and distinct in the composition of 

 a vertebra are the round central body, or cyclo-vertebral 

 element, the two superior laminae or peri-vertebral ele- 

 ments which encompass the spinal chords, the two por- 

 tions of the superior spinous process, or the epi-vertebral 

 elements, the two inferior laminae, or para vertebral ele- 

 ments, which form a cavity for the blood-vessels, and 

 the two portions of the inferior spinous process, or the 

 cata-vertebral elements. The most frequent position of 

 these nine component elements of a perfect vertebra is 

 shown in the annexed diagram, (Fig. 27.) where (a,) repre- 

 sents the spinal cord protected by the two peri-vertebral 



FIG. 27. 



-vertebral 

 elements. 



pieces ; (b,) is the common position of the artery, and 

 (c } ) of the vein beneath the bodies of the vertebrae in 

 most parts of the column, and these are embraced by 

 the two para- vertebral elements. The cyclo-vertebral ele- 

 ments are tubular in the articulated classes of animals 

 where they envelope the whole trunk as hollow seg- 

 ments, they are nearly solid to their centre, and present 

 two concave surfaces in fishes ; they are convexo-concave 

 in reptiles, and have flat surfaces in mammalia. They 

 are the most constant and typical parts of the vertebral 

 column. The other vertebral elements vary their forms 

 and positions chiefly according to the dimensions of the 

 organs they embrace, and the extent of surface required 

 for muscular attachment ; consequently they vary much 

 in different parts of the same column, and in the ske- 

 letons of different classes. Three very different positions 

 of the same vertebral elements are represented in this 



