CHAPTER III 



THE FROG: VISCERA AND VASCULAR SYSTEM 



Alimentary 

 System. 



membrane. 



,-cr. 



The food of the frog is received and digested by a 

 winding tube, known as the gut or alimen- 

 tary canal, which runs from mouth to cloacal 

 opening and is lined by a soft, glandular mucous 

 The gape of the mouth lies between two jaws, 

 of which the upper is not movable, but the lower is hinged. 

 There are no teeth in the lower jaw, but the upper bears a 

 row of maxillary teeth, and a patch of vomerine teeth is 

 found on each side of the roof of the 

 mouth. The teeth are small, sharp- 

 pointed structures, consisting of a 

 base and a spike or crown. The 

 greater part of the crown is com- 

 posed of ivory or dentine, but the 

 base is formed of bone, and the 

 crown is covered by a cap of very 

 hard substance known as enamel, and 

 both are pierced by a core of soft 

 tissue called the pulp (Fig. 23). The 

 teeth are all alike, and all fused to 

 the surface of the bones that carry 

 them. As they are destroyed by use 

 they fall out and are replaced one by one. On the front 

 part of the roof, at the side of the vomerine teeth, open 

 the internal nares. The tongue is a muscular structure 

 arising from the front part of the floor of the mouth and 

 forked at its free end, which is directed backwards when it 

 is at rest. In taking food the tongue is turned over and its 

 free end thrown out of the mouth, wiping up, as it goes, a 



sticky substance secreted by glands in the roof of the mouth, 



46 



Fig. 22. — Two of the 

 maxillary teeth of a 

 frog, seen from the 

 outside of the jaw. 



b., Base of the tooth; cr., 

 crown; in., edge of the 

 maxilla. 



