CHAPTEE II. 



THE CHEMICAL COMPOSITION OF FOODS. 



The animal body, though containing many complex 

 compounds, is found on analysis into its simplest com- 

 ponents to be made up of the following elements : 

 carbon, hydrogen, oxygen, nitrogen, sulphur, phosphorus, 

 potassium, sodium, calcium, magnesium, chlorine, silicon, 

 and iron, with traces of others. Throughout life the 

 body is constantly being built up by means of food, and 

 as constantly worn away by work. It has been said 

 that the body of a man is completely changed every 

 seven years, so that no part of what constitutes the 

 living body to-day will be in the body in seven years' 

 time, but all will have been worn away and replaced 

 by material derived from the food. It is obvious, then, 

 that food must contain all the elements enumerated 

 above, in a form capable of assimilation and elaboration, 

 using the term food to include the air an animal breathes 

 and the water it drinks, as well as the solids eaten. 



All the higher animals depend for their food on plant 

 life, and the elements found in the animal body, or 

 simple combinations of them, are abstracted from the 

 air, water, and soil, by growing plants, and built up 

 into the plant substance in such forms as are able to 

 be assimilated by animals. This building-up process 

 carried on by plants requires a considerable amount 



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