vii THE DIGESTIVE SYSTEM AND ITS FUNCTIONS 157 



nothing is known of the routes taken by the different kinds 

 of absorbed food material in the frog, and little enough of 

 the courses followed by peptones and sugar in any form. 



Probably but a small fraction of food is absorbed by the 

 stomach ; most of the cells of the lining of that organ are of 

 the secretory type. The inner surface of the intestine is 

 especially adapted for absorption on account of the large 

 number of folds it contains which give a large amount of 

 surface for contact with the food. The numerous blood and 

 lymph vessels near the epithelium of the mucous layer 

 afford ready means of transport of substances which diffuse 

 into them through their walls. 



The Glycogenic Function of the Liver. — One of the 

 principal functions of the liver is the formation of glycogen, 

 a carbohydrate, having the same empirical formula as starch, 

 C 6 H 10 O 5 . This substance is, in fact, often referred to as 

 "animal starch," and it possesses several points of resem- 

 blance to the starch found in plants. It is soluble in water, 

 forming a milky white solution. When treated with iodine 

 its solution gives a reddish, port-wine color. In its dry state 

 it forms a white powder. 



Glycogen occurs in the cells in the form of granules or 

 even lumps of considerable size. Its presence may be 

 detected by staining with iodine sections of liver prepared 

 by hardening the organ in absolute alcohol and then embed- 

 ding it and cutting without allowing the tissue to pass through 

 water. In this way the glycogen may be prevented from 

 dissolving out. Glycogen may be prepared by throwing the 

 liver of a recently killed frog into boiling water, then grind- 

 ing it up with sand in a mortar, extracting with water and 

 filtering. A milky fluid will thus be produced which can 

 then be evaporated until the residue is obtained, which is 

 largely glycogen. 



