142 HUMAN PHYSIOLOGY CHAP. 



The sugars and fats enter the cells and are made to 

 combine with oxygen, i.e. to burn like coal in a furnace, 

 and produce heat to keep us warm. When coal is burned 

 it produces a certain amount of ash, and so these foods 

 leave wastes that must be removed from the cells. In 

 some strange way part of the heat derived from the 

 oxidation of the foods is changed to other forms of 

 energy and enables us to play, to work, and to think. 



We now understand why oxygen is so necessary to 

 life. Coal will not burn when the drafts of the stove 

 are closed so as to shut off all the air. The foods could 

 not be burned in the cells if there were not a sufficient 

 quantity of oxygen present. The oxygen is made to 

 unite with sugars, and as a result we get heat, water, 

 and carbon dioxid. In a similar way it will unite with 

 the carbon of the fats and yield heat, water, and carbon 

 dioxid. The process of growth and cell repair requires 

 an adequate supply of this important gas. When we 

 do not breathe a sufficient amount of good, pure air, the 

 foods cannot be properly oxidized. Instead of the wastes 

 that should be formed there are produced partially oxi- 

 dized products of a very harmful nature. 



Water is indispensable not only in the blood and 

 lymph, where it serves as a solvent, but also in the cell 

 itself. The protoplasm of the cell has a large percent- 

 age of water. The normal activity of the cell cannot 

 be carried on without a liberal supply of water. Hence 

 it is a good thing to drink pure, fresh water freely. Salt 

 is not oxidized, but it seems to be important in stimulat- 

 ing and regulating the activity of the cells. 



