76 THE UTILIZATION OF FOOD 



Now, active motion, whether achieved by an automobile 

 or by an animal, depends on chemical transformations 

 which take place in the fuel substance or substances. 

 In living organisms these transformations come under 

 the head of metabolism, and to distinguish them from 

 the chemical processes which are concerned primarily 

 with the fundamental life processes, or with growth, they 

 are classified as lunctional metabolism. 



Nervous Activity. — In practically all animals there 

 is still another kind of functional activity, the activity 

 of the nervous system. When w^e realize that all our 

 mental processes as well as our feelings — the things that 

 make up the really worthwhile parts of life — depend 

 upon processes going on in our nervous systems, we can 

 see that this is a very important part of our bodily func- 

 tioning ; and, as we should expect, it depends upon trans- 

 formations within protoplasm. Although of utmost im- 

 portance, this metabolism consumes little fuel in 

 comparison with the amount necessary for carrying on 

 motion. 



Metabolism as a Source of Energy can be distin- 

 guished from metabolism which gives rise to materials. 

 For example, in growth or in the secretion of saliva or 

 gastric juice, or in the deposition of fats or oils in the 

 bodies of plants or animals, there are chemical trans- 

 formations as the result of which food materials are 

 worked over into special kinds of substances which may 

 be either deposited in place, as in the hard parts of bones 

 or in the cellulose walls of plant cells, or may be secreted, 

 as in saliva or gastric juice. For the fundamental life 

 process, that is, for basic metabolism, and for the making 

 of motions on the part of animals, the purpose of metab- 

 olism is as a source of energy for keeping the machine 

 going and doing its work. This kind of metabolism — 

 the metabolism for energy — consists almost altogether, 

 in plants and animals, of a particular kind of chemical 

 process, burning, or as it is known to the chemist, oxida- 



