170 THE BIOLOGY OF THE FROG chap. 



cavity after each expiration from the lungs they enable com- 

 paratively pure air to be forced into the lungs again at the 

 next inspiration. The breathing in which the lungs are 

 involved is indicated by movements of the flanks, or regions 

 above and behind the fore legs. Certain small movements, 

 however, occur in these regions which appear to be inciden- 

 tally associated with the oscillatory movements of the floor 

 of the mouth and play no part in lung respiration ; the true 

 flank movements are quite well marked. After each drawing 

 in of the flank or expiration there follows immediately a 

 swelling of the flank due to inspiration, but there may elapse 

 a considerable interval before the next expiration occurs, so 

 that the lungs are always filled with air during the pause 

 between successive respiratory acts. Expiration is effected 

 by the contraction of the muscles of the body wall aided by 

 the elasticity of the walls of the lungs. During the act of 

 expiration the glottis opens and almost immediately after- 

 wards closes. If the sides of the body are cut open so that 

 the lungs cannot be compressed by the muscles of the body 

 wall, air will be expelled, though more slowly, every time the 

 glottis opens. A frog thus operated on is still capable of 

 both inspiring and expiring air, the mere elasticity of the 

 walls of the lungs being sufficient for the latter function. 



In filling the lungs the buccal cavity acts as a sort of force 

 pump. As the floor of the mouth rises, the nares are closed, 

 the glottis opens, and the air in the buccal cavity thus sub- 

 jected to pressure and having no other avenue of escape is 

 forced through the glottis into the lungs. The glottis then 

 closes, and the movements of the floor of the mouth may 

 continue for some time before the next inspiration takes 

 place. The rising of the floor of the throat and the closure 

 of the nares take place almost at the same time that air is 

 expelled from the lungs ; and the expansion of the lungs 



