322 



THE MECHANICAL FUNCTIONS. 



of the spinous processes of the vertebrae and ribs on the one 

 side, and of the usual pieces which compose the sternum on 

 the other. The upper and lower plates thus formed are 

 united at their edges by expansions of the sternocostal ap- 

 pendices, which become ossified. Thys, no new element 

 has been created; but advantage has been taken of those al- 

 ready existing in the general type of the vertebrata, to mo- 



dify their forms, by giving them different degrees of rela-' 

 tive development, and converting them, by these trans- 

 formations, into a mechanism of a very different kind, and 

 subservient to other objects than those to which they are 

 usually applied. It is scarcely possible to have stronger" 

 proofs, if such were wanting, of the unity of plan which has 

 regulated the formation of all animal structures, than those 

 afforded by the skeleton of the tortoise. 



The first step taken to secure the relative immobility of 

 the trunk, is to unite in one rigid bony column all its verte- 



