THE FROG AND VERTEBRATES IN GENERAL 189 



lactase, all of which are concerned with the final stage of carbo- 

 hydrate digestion. (W. f. 113.) 



The Functions of the Liver. The liver, which is the largest 

 gland in the body, has several important functions particularly 

 in connection with nutrition. In the first place, it secretes the bile 

 which together with alkaline pancreatic juice aids in overcoming 

 the acidity of the food from the stomach. It is known that certain 

 substances in the bile play additional roles in converting starch into 

 sugar, and also in emulsifying fats so that they can be absorbed 

 from the intestine without being chemically changed. Undoubt- 

 edly, bile also contains a large amount of excretory materials. 



Furthermore, the liver has the power of synthesizing glycogen, 

 or 'animal starch.' This carbohydrate is normally formed from 

 sugar, but protein material can also be utilized. It will be shown 

 below in the study of the vascular system that the liver receives 

 the blood containing all absorbed food materials, except the fats, 

 directly from the digestive tract through the hepatic portal vein, 

 and it is thus enabled to abstract the materials for the formation 

 of glycogen. In a well-fed animal, the liver contains considerable 

 amounts of glycogen which can be supplied to the cells of the body 

 as needed. Before being released into the blood stream, the glyco- 

 gen is changed into dextrose by the action of a specific enzyme in 

 the liver. Finally, the liver aids in the process of excretion by 

 changing the nitrogenous waste products in the blood into urea, 

 which is later taken from the blood by the kidneys. (W. f. 115.) 



2. Respiratory System 



In the adult Frog the chief organs for the interchange of the 

 gases concerned with the respiratory function are the skin and 

 lungs. It is a question which one is the more important, but it is 

 probable that more carbon dioxide is given off through the skin 

 than through the lungs, and in the winter, when the Frogs are 

 hibernating, the entire burden of the reduced respiratory exchange 

 is carried on by the skin. The wet, mucus-covered skin of the Frog 

 is highly vascularized, and the blood is brought into close contact 

 with the oxygen which may be present either in the surrounding 

 air or water as the case may be. The excess carbon dioxide is 

 given off, the essential oxygen picked up and transported to the 

 cells of the body where respiration takes place. ( W. pp. 161-167.) 



The lungs of the Frog consists of a pair of thin-walled distensible 



