STRUCTURE OF THE SPINE. 397 



is the condition of the spine of fishes, where this 

 latter kind of motion is the one principally 

 wanted. In dolphins, and other cetacea, on the 

 contrary, where the actions are required to be 

 vertically upwards and downwards, the spinous 

 processes are small, and the transverse processes 

 very long and broad. 



Every instance of variation in the forms of 

 these important parts of the osseous system, will, 

 in like manner, be found to have a relation to 

 some particular circumstance in the living habits 

 of the animal, and to be subordinate to the 

 general plan of its economy. But in order to 

 understand the mode in which nature has effected 

 these changes, it is necessary to study the ele- 

 ments of each part of the osseous system ; for 

 these constitute the alphabet by which the com- 

 binations she presents to us become legible, and 

 their origin and progress are unfolded to our 

 comprehension. According as each of these 

 elements of ossification receives different degrees 

 of developement, so do the different bones they 

 compose acquire their particular shapes and 

 relative dimensions. Sometimes, indeed, we find 

 that one or other of these elements has disap- 

 peared ; or at least we can discover no trace 

 of its developement ; in other cases, we see it 

 exceedingly expanded, and appearing under 

 forms of greater complication, so as to be with 



