iv THE INTERNAL STRUCTURE 69 



lateral subrostral fossce, which are mere deepenings of the 

 sulcus marginalis. The lower jaw is entirely devoid of teeth 

 and is held tightly pressed against the upper jaw, the mandib- 

 ular muscles being normally in a state of tonic contraction ; 

 the tip of the lower jaw is flexible and is capable of being ele- 

 vated or depressed independently of the rest of that structure, 

 the joints of the movable part, pars mentalis, lying under 

 the pulvinars of the upper jaw. The elevation at the extreme 

 tip of the lower jaw {tuberculum prehnguale) fits into the 

 median subrostral fossa, and there is a slight depression on 

 either side of this tubercle corresponding to the pulvinars. 

 The two jaws, therefore, fit together, part for part, with great 

 nicety. In fact, they form an air-tight joint which, as we 

 -shall see later, is a necessary feature in relation to the pecul- 

 iar mode of breathing which the frog is forced to employ. 

 The tip of the upper jaw is likewise movable, its free portion 

 corresponding in extent to that of the lower jaw, so that the 

 two parts can be raised and lowered together. It may be 

 noted that the elevation of the tip of the jaw effects the 

 closure of the nares, a point of considerable importance in 

 relation to the process of respiration. 



If a bristle is passed into one of the anterior nares, it may 

 be seen to emerge into the anterior portion of the mouth 

 cavity by one of a pair of rounded or oval openings, the 

 choance, or posterior nares. Between the choanae is a pair 

 of prominences which bear the vomerine teeth. At the 

 posterior end of the buccal cavity near the angle of the jaw 

 are the large openings, Eustachian tubes, which lead outward 

 to the tympanic membrane, as may easily be demonstrated 

 by means of a bristle. In the male frog another and a 

 smaller pair of openings may be seen on the lower side of 

 the buccal cavity, a little in front of the Eustachian tubes ; 

 these are the openings of the vocal sacs, and their conti- 



