482 COLLEGE ZOOLOGY 



which changes proteids into soluble peptones. The food then 

 passes through the pyloric constriction into the intestine. Here 

 it is attacked by the pancreatic juice and the bile. The pan- 

 creatic juice contains three ferments: (i) irypsin, which converts 

 proteids into peptones; (2) amylopsin, which converts starch into 

 sugar; and (3) steapsin, which splits up fats into fatty acid and 

 glycerin. The bile emulsifies fats and converts starch into sugar. 

 The intestinal wall produces a secretion which probably aids in 

 converting starch into sugar. 



Absorption begins in the stomach, but takes place principally 

 in the intestine. The food substances which have been dis- 

 solved by the digestive juices are taken up by the mucosa layer, 

 passed into the blood and lymph, and are then transported to 

 various parts of the body. The undigested particles of food 

 pass out of the intestine into the cloaca and are then discharged 

 through the anus as faeces. 



The absorbed food is used by the frog to build up new pro- 

 toplasm to take the place of that consumed in the various life 

 activities, and to increase the size of the body. Food is stored 

 up in the liver as glycogen, a carbohydrate similar to starch and 

 often called " animal starch." When needed by the body, this 

 glycogen is changed into dextrose by enzymes produced by the 

 liver, and slowly passed into the blood. During the winter 

 the hibernating frog lives largely on the glycogen stored up in 

 the liver in the autumn. 



The Respiratory System. - - Respiration takes place to a con- 

 siderable extent through the skin both in water and in air, but 

 is carried on principally by the lungs. As shown in Figure 412, 

 air passes through the nostrils or external nares (Fig. 412, A, e.n) 

 into the olfactory chamber (olf.s), and then through the internal 

 or posterior nares (Fig. 412, /./?; Fig. 411, N) into the mouth 

 cavity. The external nares are then closed (Fig. 412, B, e.n), 

 the floor of the mouth is raised, and the air is forced through the 

 glottis (Fig. 412, B, gl; Fig. 411, G) into a short tube, the larynx, 

 and thence into the lungs (Ing). Air is expelled from the lungs 



