394 THE MECHANICAL FUNCTIONS. 



tuated between them. The fourth essential ele- 

 ment is the spinous process (s), which unites the 

 two leaves, and thus completes the superior arch, 

 of which it may be regarded as the key stone, 

 for the protection of the spinal marrow. Then 

 come the two transverse processes (T, T) which 

 extend outwards from the sides, and with which 

 the arches of bone, that constitute the ribs 

 (R, R) are generally connected. These are the 

 six parts which may be considered as the ele- 

 ments that are most essential, and most con- 

 stantly present in the composition of the ver- 

 tebrae. 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 vertebrata become articular. There 

 is frequently, also, a developement of processes 

 (F), forming arches and spines at the lower sur- 

 face 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 aorta, or 

 the main artery running along the back, imme- 

 diately under the spinal column. 



There are still other processes, less constantly 



