306 



THE MECHANICAL FUNCTIONS. 



its wants, of its habits, of its functions, and of its very con- 

 stitution. I shall have occasion to notice several of these 

 transitions when reviewing the other functions of the animal 

 economy: but at present our concern is chiefly with the 

 structure of the frame in its mechanical relations to progres- 

 sive motion. In order to form a correct idea of these re- 

 lations, it will be necessary to notice the leading peculiari- 

 ties of the skeletons of this tribe of animals. 



The skeleton of the adult frog is shown in Fig. 200; from 

 which it will be seen that the spinal column is comparatively 



much shorter than that of fishes, or, indeed, of any other 

 class of animals; for it consists of only eight vertebrae, ex- 

 clusive of those which have united to form the os coccygis. 

 It was evidently the intention of nature to consolidate the 

 frame-work of the trunk, in which flexibility was not re- 

 quired for progressive motion: the performance of that func- 

 tion being transferred to the hind extremities, which are ex- 

 ceedingly large in proportion to the rest of the body. There 

 is a tendency in every part of the skeleton to develope itself 

 in a transverse direction, while the trunk is shortened as 

 much as possible. 



The mode in which the vertebras are articulated together, 



