352 TilE MECHANICAL FVNCTIONS. 



the one opposite to that which gives rise to the 

 superior arches already mentioned. This structure 

 is very generally met with in Fishes, and it is ob- 

 served also in the Cetacea. The arches thus formed 

 enclose a large artery, which is the continuation of 

 the aorta, or the main artery running along the 

 back, immediately under the spinal column. 



There are still other processes, less constantly 

 present, and more variable in their shape. They 

 form articular surfaces for the purpose of being 

 connected with the surfaces of corresponding pro- 

 cesses in the contiguous vertebra. Of these there 

 are four (a, a, a, a) belonging to each vertebra, two 

 in front, and two behind. These, however, do not 

 properly rank among the primary elements of the 

 vertebrae, because we find them, in different in- 

 stances, occupying different positions, and formed 

 sometimes by extensions of the bodies, and at other 

 times of the leaves. In following them through 

 the several tribes of animals, we observe them 

 shifting their places, in various ways, and not even 

 preserving any constancy in their number. They 

 are wholly absent in Fishes : in the Crocodile, and 

 other Reptiles, they approximate so as to form 

 three articular surfaces, namely, two close to one an- 

 other, and a third posterior to these. In the Onii- 

 tkorJiynchuSy while the latter retains its situation in 

 the middle, the other surfaces have separated from 

 each other, and have travelled outwards, taking 

 their stations upon the leaves. In the Mammalia, 

 the middle surface has wholly disappeared, and 

 the outer surfaces have risen into what are termed 

 the oblique processes. 



In addition to these, accessory bones are often 



