AMPHIBIA. 189 



Bones thicken by the addition of layers to the outside, formed 

 by the periosteum. 



The vertebral centra contain peculiar cellular cores, the last 

 remnant of an important embryonic structure, the notochord. 



4. The digestive organs (Fig. 51) consist of a convoluted tube, 

 the gut or alimentary canal, and of glands connected with this. 

 The gut presents the following regions : mouth-cavity, gullet, 

 stomach, small intestine, and large intestine opening into a cloaca 

 which also receives the excretory and reproductive ducts. The 

 most important annexed glands are the liver and pancreas. 



The wide mouth, which reaches back as far as the tympanic 

 area, and possesses a narrow upper lip, leads into a spacious 

 mouth-cavity, the back part of which is termed the pharynx. 

 More than 100 minute double-pointed teeth are affixed to the 

 inner side of the upper jaw. They are attached to the pre- 

 maxillse and maxillse, and are placed in a furrow bounded by the 

 upper lip externally, and a fold of mucous membrane internally. 



The mucous membrane is a pale, soft, extremely glandular layer which 

 lines the alimentary canal. 



On the roof of the mouth are two small patches of vomerine 

 teeth, forwardly placed, and borne by the vomers. They are 

 similar to the others. No teeth are present in the lower jaw. 

 On the roof of the mouth, in the extreme front, a number of 

 minute pores are present, the openings of the intermaxillary glands. 

 The vomerine teeth are a little way behind this, and external to 

 each patch is a small, transversely oval opening, the internal naris. 

 Still further back two large rounded prominences, caused by the 

 eyes, project into the mouth. Near the angle of the jaw, on each 

 side, is a good-sized opening, that of the Eustachian tube, which 

 leads to the tympanic cavity. On the floor of the mouth is a 

 narrow, elongated muscular tongue attached in front. In the 

 quiescent state its forked end is directed backwards. Numerous 

 small elevations cover its surface, some of which, filiform papillce, 

 are narrow-ended ; others, fungiform papillae, broad-ended. 



Near the angle of the mouth in the male R. esculenta an oval 

 opening is present, which leads into a rounded, dilatable vocal sac. 



Posterior to the end of the tongue there is a longitudinal 

 chink, the glottis, with firm swollen edges. It leads into the 

 respiratory organs. 



