METABOLISM 147 



olism contain but 3 atoms each. Even the cleavage products 

 of protein which are resorbed from the digestive tract are, with 

 few exceptions, much more complex than the final products 

 which result from their metabolism. 



206. Metabolism a gradual process. While metabolism 

 has just been characterized as an oxidative process, and is often 

 loosely spoken of as a burning of the feed or tissue ingredients, 

 it is in fact radically different from what is commonly under- 

 stood by these terms. The building up and breaking down of 

 materials in metabolism is a gradual, i.e., a step by step, process. 



Metabolism is the sum of the chemical reactions through which 

 the life of the cells is manifested. These reactions, however, 

 differ from tissue to tissue and from cell to cell, and even in the 

 same cell from time to time, and the resulting products are 

 correspondingly numerous and varied. Between the nutrients 

 supplied to the cells by the blood and the final products of 

 metabolism as excreted from the body there are innumerable 

 intermediate products, comparatively few of which, in all proba- 

 bility, have been recognized. We know the first and last terms 

 of the series and thus are able to measure, as it were, the alge- 

 braic sum of the changes, but of the single factors making up 

 the so-called intermediary metabolism as well as of the specific 

 tissues concerned in the changes, we are largely ignorant, al- 

 though we know that they are numerous. 



Furthermore, while metabolism results in the formation of 

 highly oxidized products, it does not consist primarily in the 

 direct union of oxygen with feed materials, i.e., the step by 

 step processes of which it is made up do not consist of a series 

 of partial oxidations. The primary processes of metabolism 

 are of the nature of cleavages and hydrations and it is only 

 the comparatively simple molecules resulting from these which 

 unite directly with oxygen. Correspondingly, the extent of 

 metabolism is determined by the amount of functional activity 

 of the various cells and not, as in the case of direct oxidation 

 in a fire, by the supply of oxygen (193) . The somewhat com- 

 mon notion that an increased proportion of oxygen in the air or a 

 voluntary increase in the rate and depth of breathing may cause 

 more material to be oxidized in the body is without foundation, 

 except -so far as increased breathing involves increased mus- 

 cular exertion. 



