Amphibia. 1 83 



How the Frog Breathes. The adult frog breathes chiefly 

 by means of lungs. In dissecting, the lungs may be found 

 collapsed ; in which case they are small and dark-colored. 

 When inflated they are of considerable size and of a beau- 

 tiful pinkish color, owing to the blood in the capillaries. 

 A frog's lung is not a mere air sac, with transparent walls, 

 as with most fishes. There is blood circulating in the 

 wall, and the wall is somewhat thickened, and has ridges 

 extending on the inside, which, to a limited extent, parti- 



FIG. 117. PLAN OF FROG'S STRUCTURE, VENTRAL VIEW. 



tion off the space into air vesicles, thus increasing the area 

 of the inside of the lungs, and consequently exposing more 

 blood to the action of the air contained within the lung. 



In watching the breathing movements of the frog, three 

 actions are seen : first, the in-and-out movement of the 

 floor of the mouth ; second, occasional movements of the 

 side of the body ; third, opening and closing of the nos- 

 trils. When the floor of the mouth is lowered and the 

 nostrils are opened, air is taken into the mouth cavity ; if, 

 now, the nostrils are closed and the floor of the mouth 



