l6 THE COMMON FROG. 



but in the mouth. Behind the root of the tongue is 

 the slit-like opening of the trachea : and at the front 

 of the upper part of the head are two nostrils, 

 through which the animal always draws the air, 

 never opening its mouth for this purpose. Indeed 

 the jaws during this action are kept closely locked 

 into each other by grooves ; for if the mouth is kept 

 open it cannot respire at all, and the animal will 

 presently be seen struggling for breath. When we 

 observe it carefully, we perceive a frequent dilata- 

 tion and contraction in the skinny bag-like part of 

 the mouth w T hich covers the under jaw. From this 

 it would appear, at first sight, as if the creature lived 

 all the while on one mouthful of air, which it seems 

 to be playing backwards and forwards betwixt its 

 mouth and lungs. But for each movement in the 

 jaw a corresponding twirling movement may be 

 observed in the nostrils. The mouth seems there- 

 fore to form a sort of bellows, of which the nos- 

 trils are the air-holes, and the muscles of the jaws 

 by their contraction and dilatation make the draught. 

 The nostrils are so situated that the least motion on 

 them enables them to perform the office of a valve. 

 Bv the tw r irl of the nostril the air is let into the 

 mouth, when a dilatation of the bag takes place : it 

 is then emptied from the mouth, through the slit 

 behind the tongue, into the lungs, when there is a 

 slight motion in the sides of the animal, and the 

 muscles of the abdomen again expel it ; and soon 

 afterwards a second twirl in the nostrils takes place, 

 and the like motions follow. Thus it appears that 



