STRUCTURE OF THE SPINE. 277 



Every instance of variation in the forms of these impor- 

 tant 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 elements 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 ori- 

 gin and jDrogress are unfolded to our comprehension. Ac- 

 cording as each of these elements of ossification receives dif- 

 ferent degrees of development, so do the different bones 

 they compose acquire their particular shapes and relative di- 

 mensions. Sometimes, indeed, we find that one or other of 

 these elements has disappeared; or, at least, we can discover 

 no trace of its development; in other cases, we see it ex- 

 ceedingly expanded, and appearing under forms of greater 

 complication, so as to be with difficulty identified: on some 

 occasions, as we have just seen in the spinous bones of fishes, 

 its accessory structures are multiplied, as if continued eflbrts 

 were made by the system to repeat the same structures. 

 Amidst all these modifications, the parts that preserve the 

 greatest constancy of form are those which are of most im- 

 portance, and wdiich are constituent parts of the primordial 

 type of the class to which the individual animal belongs. 



The spinal column is generally prolonged at its posterior 

 extremity into a scries of vertebra}, which are sometimes 

 exceedingly numerous; decreasing in their size as they ex- 

 tend backwards, and having continually smaller processes, 

 the one disappearing after the other, till all of them are lost, 

 and nothing remains in those at the extremity of the series 

 but the cylindrical bodies of the vertebrae. Even these be- 

 come stinted in their growth and ossification, until w^e find 

 the terminal pieces generally remaining in the state of car- 

 tilage. Such is the structure of the osseous support of the tail, 

 as seen in many quadrupeds in its most developed forms. It 

 illustrates the law, that when in any system there occurs a 

 frequent repetition of the same structure, the evolution, in 



