354 THE MECHANICAL FUNCTIONS. 



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, has evidently 

 a relation to some particular circumstance in the 

 living habits of the animal, and is subordinate 

 to the general plan of its economy. According as 

 each of these elements of ossification receives dif- 

 ferent degrees of developement, so 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 difficulty identified : 

 on some occasions, as we have just seen in the spi- 

 nous bones of fishes, its accessory structures are 

 multiplied, as if continued efforts 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 importance, and which are therefore consi- 

 dered as constituent parts of the primordial type of 

 the class to which the animal belongs. 



The spinal column is generally prolonged at its 

 posterior extremity into a series of vertebrae, which 

 are sometimes exceedingly numerous ; decreasing 

 in their size as they extend 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 



