vin THE VOCAL AND RESPIRATORY ORGANS 171 



follows almost immediately afterward. Much of the air 

 expelled from the lungs in expiration does not escape from 

 the buccal cavity, but is forced back into the lungs again at 

 the next inspiration. It is mixed, however, with the com- 

 partively pure air previously in the mouth cavity. The val- 

 vular arrangement for closing thenares is an essential part of 

 the mechanism for filling the lungs. It was formerly thought 

 that the nares were closed by special muscles attached to 

 the valves, but it was shown by Gaupp that this function is 

 performed through raising the tip of the lower jaw, thus 

 elevating the premaxillaries and thereby closing these open- 

 ings. It may be readily shown that the closure of the nares 

 can be brought about in this way by pressing upward against 

 the premaxillaries with the finger. So long as its mouth is 

 kept open the frog is unable to close its nares ; being unable 

 to force air into the lungs, such a frog will sooner or later 

 die of asphyxiation. During all of the respiratory movements 

 the mouth of the frog is held tightly closed through the 

 tonic contraction of the muscles of the lower jaw. As has 

 been explained in a previous chapter, the tip of the lower 

 jaw is independently movable, owing to the existence of the 

 small mento-meckelian bones, which are opposed to the 

 premaxillaries. The contraction of the small submentalis 

 muscle, which runs transversely across the tip of the jaw, 

 causes this part to be raised above the general level and 

 by pressing upward against the premaxillaries closes the 

 nares. 



As air is forced into the lungs, the pressure in the buccal 

 cavity is indicated by the slight protrusion of the eyes and 

 tympanic membranes. Sometimes, however, when the frog 

 is making strong inspiratory efforts, the eyes are drawn inward 

 during each gulp of air, thus aiding the process by diminish- 

 ing the size of the buccal cavity. 



