202 PHYSIOLOGY. 



Glycogen. As stated above, glycogen is formed in the 

 liver. This is indicated by the fact that there is more 

 sugar 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 re- 

 serve. 



Nutrition. All the changes that take place between 

 the reception of food and the excretion of waste are 



ANIMAL 

 PROTOPLASM 



INORGANIC WORLD 



Fig. 74. Animal and Vegetable Protoplasm. 



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 combustion and the processes of nutrition 

 the final result is oxidation, more or less direct. 



Muscular Exertion and Excretion of Urea. 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- 



