CHEMICAL COMPOSITION OF THE BODY 35 



METABOLISM 



All the materials of which living bodies are composed come 

 from the soil and from the air. All vegetable foods surely 

 come from these sources; animals eat plants or eat 

 other animals which, in turn, live on plants. After being 

 taken into the body the foods go through certain changes, the 

 final result of which is that part of the food, at least, is 

 transformed into living tissues. These changes constitute 

 what is generally spoken of as a " building up " process, 

 which means that complex substances are made of simple 

 ones. After these tissues have severally fulfilled their 

 functions, serving as muscle, brain, fat, bone or gland, as the 

 case may be, they gradually wear out; as this occurs they 

 are " broken down " from their complex condition into 

 simple forms again. 



In living matter, there occur two types of changes a 

 building up, or anabolism, and a breaking down, or kata- 

 bolism. As a result of this breaking down process the sub- 

 stances which have been alive and have acted at the bidding 

 of our wills become again inactive and non-living. Gradually 

 all parts of our bodies heart, brain and everything else 

 become once more a part of the soil and air, just as they were 

 before they were first taken up by plants. The changes are 

 long and complex. Some take place in the body and some 

 outside the body, for materials are sometimes excreted before 

 they are completely broken down. The changes that take 

 place in the body, the building up and the breaking down 

 taken together, are spoken of as metabolism. Metabolism 

 is thus a name for the chemical changes which are taking 

 place in living tissues. 



Many factors combine to regulate each of these pro- 

 cesses. Katabolism (destructive change), for instance, is 

 increased by excessive work, by poor nutrition, by loss of 



