170 AXIMAL PHYSIOLOGY. [LESSON 48. 



teeth. The teeth (a, a) are seen in their sockets, but so slightly at- 

 tached that many of them are missing; neither is their sharp, pointed 

 character conspicuous, being concealed by Canada balsam. The 

 injection has run so minutely, that the pulps (6, b) of many of the 

 teeth are injected. Two eminences may be seen to the right; these 

 are the palatine bones, and to each, three and four teeth are arti- 

 culated ; in the frog the teeth are confined to these bones and the 

 upper jaw. The beauty of the vascular (vessels) arrangement of the 

 mucous membrane of the palate, need not be insisted on. 



746. The strong muscular oesophagus is short, dilatable, and lon- 

 gitudinally folded ; it leads to a narrow, elongated stomach, placed 

 transversely from left to right, with thick, fleshy sides, especially at 

 the pyloric extremity, and covered above by the two lobes of a large 

 liver, always provided with a free and distinct gall bladder. 



747. Injected, opened, and microscopically examined, the stomach 

 of a frog presents all the elements of a true digestive cavity. Its 

 mucous membrane is traversed with capillary blood-vessels, which 

 ramify on the surface of the cell-walls, everywhere conspicuous (Fig. 

 270). Mucus issues from the cells, and a gastric fluid is secreted 

 by the capillaries of the mucous membrane, so that the resolution of 

 the food is accomplished on precisely the same principle in a frog as 

 in a man. 



748. The duodenum is remarkable for the entire absence of pa- 

 pillae (papilla, a teat or nipple), but it is provided with transverse 

 folds of the intestine, called valvulcs conniventes (valvular folds). 

 These valvular folds have edges like papillae, but, instead of being 

 distinct bodies, stretch across the intestine as one single fold. 



FIG. 270. FIG. 271. 



Stomach of Frog. Duodenum, Frog. 



749. This is shown from the injected duodenum of a frog (Fig. 

 271). The villi (or papillae) of the duodenum, that we look in vain for 

 in the Frog, present themselves with all their characteristics well de- 

 veloped, in the duodenum of the Toad (Fig. 272). 



