22 HUMAN PHYSIOLOGY CHAP. 



build tissue nor furnish heat, and yet we must eat con- 

 siderable of it to keep our bodies in good health. Each 

 cell must have in it a certain amount of common salt to 

 do its work properly. Just what it does we do not know 

 with certainty, but it seems to stimulate the cells to 

 greater activity. It has been estimated that the total 

 amount of salt in the body is less than one half pound. 



26. Oxygen. Did you ever think of the air we 

 breathe as being a food ? Many do not call it a food, 

 but we shall consider it in connection with foods. We 

 know that oxygen breathed into the lungs gets into the 

 blood and is then carried to all the cells of the body by 

 the red corpuscles. Does a fire burn well if you close 

 the drafts that furnish it air? Well, the foods cannot 

 be oxidized in the cells without an abundant supply of 

 oxygen. When solid foods, water, and oxygen are car- 

 ried to the cells, they undergo certain chemical changes 

 there, i.e. they are slowly burned or oxidized. This oxi- 

 dation produces heat that is used to keep up the body 

 temperature, and energy that is used in doing work. 

 Oxygen is also used to transform the foods we eat into 

 tissues of the body. Oxygen is so important that we 

 could live only a few minutes without it. 



Put a lighted candle into an empty fruit jar. The candle- burns 

 brightly for a few moments, but as soon as the oxygen in the jar is 

 used up the flame dies. If the candle is taken out, lighted, and 

 lowered into the jar again, it will be extinguished at once because 

 there is no oxygen left in the jar. 



27. Substances taken with Foods. There are many 

 things eaten simply because an appetite has been 



