RESPIRATION IN REPTILES. 321 



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 per- 

 formed 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 lungs; for 

 even the ribs of serpents contribute but little, by 

 their motion, to this effect, being chiefly useful 

 as organs of progressive motion. 



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 advan- 

 tages 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 contemplated in their struc- 

 ture is evident from the volume of air received 

 into the lungs being much greater than is re- 

 quired for the sole purpose of respiration. The 

 section of the lungs of the turtle (Fig. 375), 



VOL. II. Y 



