RESPIRATORY MOVEMENTS 



143 



closed. Thus when the floor of the mouth is raised the air 

 contained in it can escape in one way only, viz., through the 

 glottis into the lungs. 



Thus inspiration, or breathing in, is produced by the 

 buccal cavity acting as a force-pump : the lowering of its 

 floor draws in air through the nostrils, the raising of its floor 

 forces the imprisoned air into the lungs. Expiration, or 



Fig. 45. — Diagrams illustrating the respiratory movements of the frog. In .-\ the 

 floor of the mouth is depressed and air is being drawn in through the nostrils ; 

 in B the floor of the mouth is raised, the nostrils are closed, and air is being forced 

 into the lungs. 

 t.«. external nostril; gl. glottis ; ^?</. gullet; i. u. internal nostril; /;/^. lung ; 

 olf. s. olfactory sac ; pinx. premaxilla ; tng. tongue. 



breathing-out, is due to a contraction of the elastic lungs, 

 accompanied by a slight lowering of the tij) of the lower 

 jaw • the latter movement releases the premaxillre and thus 

 opens the external nostrils. 



