STRUCTURE OF VERTEBRA. 275 



is the spinous process, (s,) which unites the two leaves, and 

 thus completes the superior arch, of which it may he re- 

 garded as the key stone, for the protection of the spinal 

 marrow. Then come the two transverse jyrocesses (t, t) 

 which extend outwards from the sides, and with which the 

 arches of bone, that constitute the ribs (r, r,) are «renerally 

 connected. These are the six parts which may be consi- 

 dered as the elements that are most essential, and most con- 

 stantly present in the composition of the 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 sur- 

 faces 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 deve- 

 lopment of processes, (f,) forming arches and spines at the 

 lower surface of the vertebrae, or 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 observed also in the cetacea. The arches thus formed 

 enclose a large artery, which is the continuation of the aor- 

 ta, 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 surfiices 

 for the purpose of being connected with the surfaces of cor- 

 responding processes 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, should not be 

 included among the primary elements of the vertebrae, be- 

 cause we find them, in different instances, occupying differ- 

 ent positions, and formed sometimes by extensions of the 

 bodies, and at other times of the leaves. In following tliem 

 through the several tribes of animals, we observe them shift- 

 ing 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 



