CHAPTER XII 

 METABOLISM 



General. The body in some respects resembles an engine, 

 it requires fuel to carry out its various activities. This fuel is 

 ' ' burned ' ' or oxidized, and heat or mechanical work results. In 

 the process, waste products are formed. In addition, the in- 

 dividual parts of the structure are continually breaking down 

 and being repaired. All this we know from a comparison of the 

 ' ' fuel ' ' or food substances ingested, and the waste products elim- 

 inated. All evidence points to the fact that extensive chemical 

 activity is going on in the tissues. Materials are torn to pieces, 

 chemically speaking, other substances are built up. The field 

 covering the processes going on in the tissues is known as metab- 

 olism. This does not include digestion, for strictly speaking, 

 the contents of the alimentary tract are not in the body, at least 

 not in the tissues, but only in a passageway or tube running 

 through the body. The study of metabolism covers the history 

 of the foodstuffs from the time of their absorption to the point 

 where they, or the products formed from them are excreted from 

 the body. We may differentiate various fields of metabolism, 

 such as that of proteins, carbohydrates, fats or inorganic ma- 

 terials, and also some other fields, such as energy exchange. In 

 addition, we may study the general metabolism of a localized 

 area, such as metabolism of the muscles, liver, etc. 



It is obviously impossible to enter the tissues and observe what 

 is going on in them. Our observations must be made in indirect 

 ways, and conclusions drawn as well as the case permits. Vari- 

 ous methods for the study of metabolism are employed. For ex- 

 ample, from variations in the constituents of the urine and the 

 amounts of the different substances present, from observations 

 made on excised surviving organs through which blood or some 



194 



