248 



NUTRITION. 



in the blood in the hepatic vein than in the portal vein, except 

 during digestion. Glycogen is formed by and stored in the 

 liver, and is doled out to the tissues. That muscles use sugar 

 in their action is indicated in the fact that the arteries bring 

 to the muscles more sugar than is carried away from them by 

 the veins. As fat is a reserve food, so glycogen serves as a 

 temporary carbohydrate reserve. 



INORGANIC WORLD 



Fig. 75. Animal and Vegetable Protoplasm. 



Nutrition. All the changes that take place between the 

 reception of food and the excretion of waste are included 

 under the term Nutrition. The materials taken as food are 

 usually more complex and unstable, the waste products more 

 simple and stable; just as the products of combustion are, as 

 a rule, simpler and more stable than fuels. In both combus- 

 tion and the processes of nutrition the final result is oxida- 

 tion, more or less direct. Since muscles are the engines of 

 motion, and also are largely composed of proteid (nitrogen- 

 containing) material, we would naturally expect that increased 

 muscular exertion would increase the excretion of urea (the 

 only nitrogen-containing waste). But experiment shows that 

 increased muscular action, such as mountain climbing, hardly 

 increases the amount of urea excreted. Such work, however, 

 does largely increase the amount of carbon dioxid excreted. 

 It is thought, therefore, that our energy is largely derived 

 from carbohydrate foods and fats ; and this view is strength- 



