228 THE VITAL FUNCTIONS. 



bubbles. The lungs of serpents have scarcely any of these 

 partitions, but consist of one simple pulmonary sac, situated 

 on the right side, having the slender elongated form of all 

 the other viscera, and extending nearly the whole length of 

 the body. The lung on the left side is in general scarcely 

 discernible, being very imperfectly developed. In the cha- 

 meleon the lungs have numerous processes which project 

 from them like caeca. In the Sauria, the lungs are more 

 confined to the thoracic region, and are more completely 

 cellular. 



The mechanism, by which, in these animals, the air is 

 forced into the lungs, is exceedingly peculiar, and was for 

 a long time a subject of controversy. If we take a frog as 

 an example, and watch its respiration, we cannot readily 

 discover that it breathes at all, for it never opens its mouth 

 to receive air, and there is no motion of the sides to indi- 

 cate that it respires; and yet, on any sudden alarm, we see 

 the animal blowing itself up, as if by some internal power, 

 though its mouth all the while continues to be closed. We 

 may perceive, however, that its throat is in frequent mo- 

 tion, as if the frog were economizing its mouthful of air, 

 and transferring it backwards and forwards between its 

 mouth and lungs; but if we direct our attention to the nos- 

 trils, we may observe in them a twirling motion, at each 

 movement of the jaws; for it is, in fact, through the nostrils 

 that the frog receives all the air which it breathes. The 

 jaws are never opened but for eating, and the sides of the 

 mouth form a sort of bellows, of which the nostrils are the 

 inlets; and by their alternate contraction and relaxation the 

 air is swallowed, and forced into the trachea, so as to inflate 

 the lungs. If the mouth of a frog be forcibly kept open, it 

 can no longer breathe, because it is deprived of the power 

 of swallowing the air required for that function; and if its 

 nostrils be closed, it is, in like manner, suffocated. The 

 respiration of most of the Reptile tribes is performed in a 

 similar manner; and they may be said rather to swallow the 

 air they breathe, than to draw it in by any expansive action 



