CLASS AMPHIBIA 



483 



into the mouth cavity by the contraction of the muscles of the 

 body-wall. 



The air in the mouth cavity is changed by throat movements. 

 The glottis remains closed, while the floor of the mouth is alter- 



prn 



e.n 



i.'t 



FIG. 412. Diagram to illustrate the respiratory movements of the frog. 

 In A the floor of the mouth is depressed, the nares are open, and air rushes 

 through them into the buccal cavity. In B, the floor of the mouth is raised, 

 the nares are closed, and air is forced from the buccal cavity into the lungs. 

 e.n, external nares ; gl, glottis ; gul, gullet ; i.n, internal nares ; Ing, lung ; 

 olf.s, olfactory chamber ; pmx, premaxillary bone ; tng, tongue. (From Holmes, 

 after Parker.) 



nately raised and lowered. Air is thus drawn in and expelled 

 through the nares. 



The lungs are pear-shaped sacs with thin, elastic walls. The 

 area of their inner surface is increased by folds which form 

 minute chambers called alveoli. Blood capillaries are numerous 

 in the walls of these alveoli. 



The larynx is strengthened by five cartilages. Across it are 

 stretched two elastic bands, the vocal cords. The croaking of 

 the frog is produced by the vibrations of the free edges of the 



