THE ANIMAL ORGANISM 



29 



This spot shows the location, beneath it, of the pineal organ, 

 which is a dorsal outgrowth of the diencephalon of the brain. In 

 the embryos of some amphibians and reptiles the tip of the pineal 

 organ enlarges to form an eyelike structure beneath the integu- 

 ment. The pineal organ with its distal enlargement is regarded 

 as a vestigial organ which in modern amphibians has lost its 

 original function and much of its structure. On either side 

 of the head behind the eye is the tympanic membrane or eardrum, 

 a flattened circular area flush with the surface of the head. 



A live frog at rest exhibits pulsating movements in the region 

 of the throat which are concerned with respiration. On the 



Fig. 9. — Two views of the oral cavity of Rana pipiens. A, tongue in place in 

 floor of mouth; B, tongue pulled forward, e.t., Eustachian tube; g, glottis; i.n., 

 internal naris; t, tongue; v.s., vocal sac openings; v.t., vomerine teeth. 



dorsal side of the snout not far from the tip are located the two 

 external nares, leading to respiratory and olfactory passages 

 which open into the anterior end of the oral or buccal cavity. If 

 the frog's mouth is held open (Fig. 9) several structures can be 

 seen. In the roof of the mouth on either side near the anterior 

 end is a small opening, the internal naris, and between the two 

 nares is a pair of white vomerine teeth. Small bluntly pointed 

 teeth are also found in the upper jaw which forms a frame for the 

 roof of the mouth (Fig. 9, A) . On either side, near the angle formed 

 by the articulation of the upper and lower jaws, is the opening 

 of the Eustachian tube, which connects with the tympanic cavity or 

 middle ear. The tympanic cavity is closed to the outside by 

 the tympanic membrane. In the mid-line between the two 

 openings of the Eustachian tubes is the constricted opening into 

 the esophagus. The most conspicuous structure in the floor 



