THE FROG AND ITS RELATIVES 



261 



the air out again. The slight throbbing of the throat is not breai . 

 ing; it merely pumps air in and out of the mouth. When air is 

 really " swallowed'" the sides of the body expand and the floor 

 of the mouth rises. Then the expired air is forced back into the 

 mouth where the constant pumping, above mentioned, gradually 



P.U. FVknoMARY ARTERY 

 FETROOUCTIVL ON 

 T*. TRACHEA 



FIG. 9 1 a. 



replaces it with fresh air, which is then swallowed and the process 

 repeated. 



Considerable blood is aerated by the capillaries in the skin, 

 which act as a sort of gill, obtaining dissolved oxygen when the 

 animal is under water. This is an evident adaptation for its 

 amphibious life. 



Nervous System. The nervous system shows considerable ad- 

 vance over that of the fish. The cerebrum is larger compared with 

 the other brain parts. The brain as a whole is more specialized 



