230 THE VITAL FUNCTIONS. 



perfect frog. We have already had occasion to notice seve- 

 ral of these transformations in the organs of the mechanical 

 functions, and also in those of digestion and circulation: but 

 the most remarkable of all are the changes occurring in the 

 respiratory apparatus, corresponding with the opposite na- 

 ture of the elements which the same animal is destined to 

 inhabit in the different stages of its existence. No less than 

 three sets of organs are provided for respiration; the two 

 first being branchiae, adapted to the fish-like condition of the 

 tadpole, and the last being pulmonary cavities, for receiving 

 air, to be employed when the animal exchanges its aquatic 

 for its terrestrial life. It is exceedingly interesting to ob- 

 serve that this animal at first breathes by gills, which pro- 

 ject in an arborescent form from the sides of the neck, and 

 float in the water; but that these structures are merely tem- 

 porary, being provided only to meet the immediate exigen- 

 cy of the occasion, and being raised at a period when none 

 of the internal organs are as yet perfected. As soon as ano- 

 ther set of gills, situated internally, can be constructed, and 

 are ready to admit the circulating blood, the external gills 

 are superseded in their office; they now shrivel, and are re- 

 moved, and the tadpole performs its respiration by means of 

 branchiae, formed on the model of those of fishes, and acting 

 by a similar mechanism. By the time that the system has 

 undergone the changes necessary for its conversion into the 

 frog, a new apparatus has become evolved for the respira- 

 tion of air. These are the lungs, which gradually coming 

 into play, direct the current of blood from the branchiae, and 

 take upon themselves the whole of the office of respiration. 

 The branchiae, in their turn, become useless, are soon obli- 

 terated, and leave no other trace of their former existence 

 than the original division of the arterial trunks, which had 

 supplied them with blood directly from the heart, but whjch, 

 now uniting in the back, form the descending aorta.* 

 There is a small family called the Perenni-branchia, be- 



i 

 * See Fig. 357, p, 197. 



