RESPIRATION IN REPTILES. 289 



mouth and lungs ; but if we direct our attention to 

 the nostrils, we may observe in them a twirling 

 motion, at each movement of the jaws; for it is, in 

 ftict, 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 re- 

 laxation 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, suffo- 

 cated. 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 of the parts 

 which surround the cavity of the lungs ; for even 

 the ribs of serpents contribute but little, by their 

 motion, to this effect, being chiefly useful as organs 

 of progression. 



The Chelonia have lungs of great extent, passing 

 backwards under the carapace, and reaching to 

 the posterior part of the abdomen. Turtles, which 

 are aquatic, derive great advantages from this 

 structure, which enables them to give buoyancy to 

 their body (encumbered as it is with a heavy shell), 

 by introducing into it a large volume of air ; so 

 that the lungs, in fact, serve the purposes of a 

 large swimming bladder. That this use was con- 

 templated in their structure is evident from the 

 volume of air received into the lungs being much 

 greater than is required for the sole purpose of 



VOL. 11. u 



