FATS, CARBOHYDRATES, AND ALBUMINS. 337 



holism, by no means tells the whole story. The calorific values serve 

 merely as a skeleton, and give us an outline of the changes which take 

 place in metabolism. These changes are always to be traced back to the 

 individual cells. It is not the foods as such which determine in general 

 the metabolism, but the cells themselves. These, naturally, require a 

 certain amount of energy. We shall see later on, that metabolism varies 

 in different individuals, and that the consumption of material, to a large 

 extent, is regulated by the functional activity of the separate organs. 

 The same work e.g., a definite amount of muscular work will require 

 a greater quantity of energy the first time it is performed than when it is 

 repeated. By practice, the organism adapts itself to its requirements. 

 It learns how to perform a given amount of work with the least expenditure 

 of energy. We must call attention even here to this fact in order to show 

 that experiments in metabolism, and especially experiments dealing with 

 the energy required for a definite amount of work, are not likely to give 

 true values, unless they be carried out through an extended period of 

 time. It will only then be possible to compare the fluctuations and 

 irregularities of the separate daily periods, and it is only in this manner 

 that we shall be able to obtain values which will be comparable with others 

 which have been secured under different circumstances. In practical 

 work, as we shall see later, we do not study the fats, carbohydrates, and 

 proteins by themselves, but make use of those mixtures which are present 

 naturally in meat and vegetables. The impracticability of laying too 

 much stress upon the calorific values is very well shown by the significant 

 discovery that the whole work of the digestive glands, and consequently 

 digestion itself, is dependent to a great extent upon the nature of the 

 ingested food material. It is only by combining the knowledge gained 

 from investigations on the transformation of energy with that obtained 

 in the study of cell activity that we are enabled to get a complete conception 

 of metabolism in general. 



We are first of all interested in the questions: What relations do the 

 foods bear to one another, and what proofs do we have that certain organs 

 are able to perform definite functions with all three classes of nutrients ? 

 Let us first take up the last question. In discussing the carbohydrates, 

 we have already drawn the conclusion, from many experiments, that they 

 form an exceptionally important source of muscular activity. Now, are 

 the muscles also capable of performing their functions by utilizing the 

 energy contained in representatives of the two other kinds of organic 

 food, the fats and proteins? 



The fact that protein may serve as a source of muscular energy was 

 proved by Pfluger. * For over seven months he fed a dog exclusively on 

 meat which contained only small amounts of fat and carbohydrates, in 

 fact, not enough to satisfy the requirements of the heart's work. Pfluger, 



1 E. Pflt^er Pfliiger's Arch. 50, 98, 330, 396 (1891). 



