288 THE VITAL FUNCTIONS. 



and causes it to remain open : it is a structure very 

 analogous to that of the trachea of insects, or of the 

 vessels of the same name in plants. 



The lungs of Reptiles consist of large sacs, into 

 the cavity of which the bronchia, proceeding from 

 the bifurcation of the trachea, open at once, and 

 without further subdivision. Cells are formed 

 within the sides of this great cavity, by fine mem- 

 branous partitions, as thin and delicate as soap 

 bubbles. The lungs of serpents have scarcely any 

 of these partitions, but consist of one simple pul- 

 monary 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 Chameleon the lungs have numerous processes 

 which project from them like caeca. In theSauria, 

 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 

 indicate 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, how- 

 ever, that its throat is in frequent motion, as if the 

 frog were economising its mouthful of air, and 

 transferring it backwards and forwards betv/een its 



