vii THE DIGESTIVE SYSTEM AND ITS FUNCTIONS 163 



in the size of the livers are thus shown to correspond in 

 general to the variations in the glycogen content. In 

 November the liver may become between two and three 

 times as large as it is in June. 



Fate of the Different Kinds of Food. — The functions of 

 food, as we have seen, are to build up tissue and to supply 

 the organism with the energy for carrying on its vital pro- 

 cesses. Only the proteids are capable by themselves of form- 

 ing living tissue, as they alone possess all the necessary 

 elements. The fats and carbohydrates, however, are also to 

 a certain extent tissue builders, but they can supply only 

 three of the elements of living matter; namely, carbon, 

 oxygen, and hydrogen. 



The fat stored in the cells of adipose tissue may be ob- 

 tained from fat contained in the food, but it may also be 

 derived from carbohydrates and even from proteids. 



The principal functions of both fats and carbohydrates is 

 the production of energy. These compounds are split up 

 and oxidized to carbon dioxide and water, yielding energy 

 in this way for the performance of bodily movements and 

 the maintenance of the temperature of the animal. Energy 

 is also derived from the breaking down of proteids, so that 

 it may be said that all of the principal classes of foods are 

 tissue builders and also energy producers. After the food 

 stuffs have played their part and become broken down into 

 simpler compounds, they are eliminated from the body 

 through the organs of respiration and excretion. 



REFERENCES 



Contejean, C. Sur la digestion stomachale de la grenouille. C. R. 

 Ac. Sci., Paris, T. 112, 1 891. 



Dewevre. Note sur la fonction glycogenique chez la grenouille 

 d'hiver. C. R. hebd. Soc. Biol., Paris, 1892. 



