142 THE BIOLOGY OF THE FROG chap. 



the glands of the cardiac end of the stomach. The mouths 

 of the glands are lined with a short cylindrical epithelium 

 with occasional ciliated cells. 



Gastric Digestion. — In the stomach the food is subjected 

 to the action of the gastric juice, which is secreted by the 

 glands of the mucosa. Gastric juice is acid in reaction from 

 the presence of a small quantity of free hydrochloric acid, 

 and it contains also a ferment, pepsin,, which acts upon the 

 proteids, converting them into soluble peptones. Neither the 

 fats nor the carbohydrates undergo digestion in the stomach. 

 By digesting out the proteid portion of foods in which fats 

 and carbohydrates are contained the gastric juice helps to 

 render these substances more readily digestible by other 

 fluids. 



The action of the gastric juice of the frog may be readily 

 demonstrated by siphoning off some of this fluid from the 

 stomach by means of a bent glass tube and placing in it a 

 small bit of the white of a hard-boiled ^gg. The piece of 

 egg after a tiaie will be seen to be corroded, and finally it 

 will become entirely dissoh^ed. 



The secretion of the esophagus has a strong digestive 

 power, but its reaction is alkaline instead of acid, and it is 

 capable of acting only after it has been rendered acid through 

 mixture with the fluid of the stomach (Nussbaum). 



When gastric digestion is completed, the food passes 

 through the pylorus into the small intestine. 



Changes in the Glands during Digestion. — The changes 

 undergone by the glands of the esophagus and stomach have 

 been studied by Partsch, Swiecicki, Nussbaum, Griitzner, 

 Langley, and Sewall. In frogs which have been kept for 

 several days without food Langley found the cells of the 

 body of the gland to be enlarged so as to practicafly obliter- 

 ate the central canal. The contents of the cell are uniformly 



