274 GENERAL PHYSIOLOGY 



A. THE GENERAL EXTERNAL CONDITIONS OF LIFE 



1. Food 



The presence of food is required by the fact of metabolism. If 

 living substance is continually undergoing spontaneous destruction, 

 then, in order that it may continue to live, a stream of substances 

 must come into it from the outside, which comprise all those 

 chemical elements that are necessary to its construction. Such 

 chemical substances constitute food. Accordingly, water and 

 oxygen belong to the general conception of food ; it is not customary, 

 however, to include them therein. Following the usage, we shall 

 consider them separately, and shall take up, first, food in the 

 more special sense. 



The twelve organic elements of which all living substance is 

 composed (p. 100) must come into the body of the organism in 

 some form as food. In this lies the general significance of food. 

 But the chemical compounds in which these elements are intro- 

 duced into the body are as manifold for the various forms of 

 organisms as the organisms themselves. A general food for all 

 organisms does not exist ; and it has already been seen ^ that 

 according to the kind of food-stuffs and the manner in which living 

 substance is constructed from them, organisms may be divided into 

 several large groups, such as green plants, fungi and animals. While 

 the green plants are able to construct their living substance out of 

 inorganic material only, carbonic acid and solutions of various salts, 

 animals without exception require organic food, and cannot live 

 without complex organic compounds, such as proteid, carbohydrate, 

 fat, etc. The fungi stand in a certain measure between these 

 two groups, since they can supplj' their need of nitrogen from 

 inorganic salts, although they require organic compounds for their 

 carbon. An exception to this condition is shown by the interesting 

 nitrogen-bacteria only, which derive both their nitrogen and carbon 

 from ammonium carbonate, and thus like the green plants live 

 exclusively upon inorganic food-stuffs. But, however in individual 

 cases food may be procured, without food of some kind no living 

 body can continue to live. 



Regarding quantitative conditions of food, the maximum and 

 the minimum of food that the living body requires, which is 

 different for every form of organism, only a few special cases have 

 been investigated thus far, and these are among the higher 

 vertebrates exclusively. These are questions that still require 

 detailed answer, and, if treated from the cell-physiological stand- 

 pijint, are capable of yielding results equally important theoretic- 

 ally and practically. Thus far individual values for the whole 



' Of. p. 138. 



