XIIL] THE FROG. 163 



into the tympanic cavities, which are closed externally by the 

 tympanic membranes. In the male Rana escuknta the small 

 apertures of the vocal sacs are seen on the inner side of each 

 ramus of the jaw, close to the angle of the gape and nearly 

 opposite the Eustachian recesses. In the middle of the back 

 of the throat is the opening of the oesophagus, closed by the 

 approximation of its sides except during deglutition, while in 

 the median line of the hinder part of its floor lies a longitu- 

 dinal slit, the glottis. A fleshy tongue, bifurcated and free 

 at its posterior end, is attached anteriorly to the middle part 

 of the lower jaw. In a state of rest, therefore, it lies on the 

 floor of the mouth with its free end turned backwards, and 

 one point on each side of the glottis. 



The gullet, after traversing the diaphragm, passes into the 

 elongated stomach. At its posterior end this narrows and 

 joins the slender small intestine. Though short, this is too 

 long relatively to the length of the abdominal cavity to lie 

 straight in it. It is, therefore, thrown into sundry folds 

 which are suspended to the dorsal wall of that cavity in the 

 manner before described. Finally, the small intestine enters 

 the suddenly dilated short large intestine, and this opens into 

 a chamber with muscular walls, the cloaca^ the external aper- 

 ture of which has been already mentioned. 



Thus the alimentary canal is a tube which traverses the 

 body from the oral to the anal apertures; and the heart, en- 

 closed in the pericardium, is situated in the middle line on 

 the ventral side of the alimentary canal. 



Separated from the pleuroperitoneal and oral cavities by 

 the bodies of the vertebrae and the hard roof of the oral 

 chamber which continues the direction of these forwards, is 

 an elongated cavity, widest in the head but becoming very 

 narrow posteriorly, which is closed on all sides by the bony 

 and other elements of the head and spinal column. This is 



