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mechanism of the spine . 
right to mention one more peculiarity, apparently connected 
with the same mechanism. Contrary to the usual course in 
other animals, the nerves that are given off from the cervical 
portion of the spinal marrow, pass obliquely upwards at a 
considerable angle, through an opening between the root of 
the inferior articulating process, and the body of the bone ; 
they then divide, and one branch descends through the open- 
ing in the lateral process, and the other branch is distributed 
to the surrounding muscles and integuments. 
One principal object in comparative anatomy, or rather in 
comparative physiology, is to enable us, by examining par- 
ticular structures, w T hich are more developed in some animals, 
and in whom consequently the functions of such structures 
are more apparent, to judge of the probable uses of similar 
structures existing in a diminished proportion in other animals. 
On investigating this subject, and examining the spines of 
several other animals, I have found a similar arrangement, 
varying only in degree, and that exactly in proportion to the 
extent of motion permitted between the vertebrae.* 
In the formation of the spine in man, it was requisite to com- 
bine two very opposite qualities. The solidity and strength of 
a column were required to be united with the flexibility neces- 
sary to the performance of our diversified actions. To attain 
* This rule will be found to hold good, even in those animals which form excep- 
tions, with respect to the general form and arrangement of the different processes. 
Thus, in the mole, whose cervical vertebrae are mere bony rings without any 
spinous processes, and which, consequently, admit of extensive motion, the canal 
is remarkably capacious. In the bat, whose dorsal vertebrae are either wholly with- 
out spinous processes, or have only short tubercles, the canal, at this part, is of 
greater volume than either in the cervical or lumbar vertebra ; and, contrary to the 
general rule, this division of the spine admits of considerable motion. 
