392 . THE MECHANICAL FUNCTIONS. 



function being superseded by the lungs, which 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, however, 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 derived from a thin extension of the integu- 

 ments.* 



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 circum- 

 stances of that animal, — this total reversal of its 

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

 constitution. 1 shall have occasion to notice seve- 

 ral 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 progressive 

 motion. In order to form a correct idea of these 

 relations it will be necessary to notice the leading 

 peculiarities of the skeletons of this tribe of animals. 



* Duges distinguishes the following six periods in the life of the 

 frog, after its exclusion from the egg : the first being characterized 

 by a lengthened body, and the presence of external temporary 

 branchiae : the second, by the absence of these branchiae, and by 

 the globular form of the body : the third, by the developement of 

 the hind legs : the fourth, by the protrusion of the fore legs, and 

 the gradual disappearance of the tail: the fifth, by the total loss of 

 tail and branchiae, the body being as yet but imperfectly developed : 

 the sixth, and last, by the body and limbs having attained their full 

 dimensions. — Recherches sur I'Osteologie et la Myologie des Ba- 

 traeiens a leurs difFcrens ages, p. 79. 



