440 THE MECHANICAL FUNCTIONS. 



developed lungs, and afterwards discharging it 

 in the form of a small bubble. When the neces- 

 sary internal changes are at length completed, 

 preparations are made for getting rid of the tail, 

 which is now a useless member, and which, 

 ceasing to be nourished, diminishes by degrees, 

 leaving only a short stump, which is soon re- 

 moved. The gills are by this time shrunk, and 

 rapidly disappear, their function being super- 

 seded by the lungs, which have been called into 

 play ; and the animal now emerges from the 

 water and begins a new mode of existence, having 

 become a perfect frog (Fig. 199). ' It still, how- 

 ever, retains its aquatic habits, and swims with 

 great ease in the water by means of its hind 

 feet, which are very long and muscular, and of 

 which the toes are furnished with a broad web de- 

 rived from a thin extension of the integuments. 



No less curious are the changes which take 

 place in all the other organs, for the purpose of 

 effecting the transformations rendered necessary 

 by this entire alteration in all the external cir- 

 cumstances of that animal, this total reversal 

 of its wants, of its habits, of its functions, and of 

 its very constitution. I shall have occasion to 

 notice several of these transitions when review- 

 ing 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 progressive motion. In order to form a cor- 

 rect idea of these relations it will be necessary 



