CHAPTER XV. 



THE EXCHANGE OF MATERIAL WITH VARIOUS FOODS AND 

 THE DEMAND FOR FOOD IN MAN. 



THE conversion of chemical tension into living energy, which 

 characterizes animal life, leads, as previously stated in Chapter I, 

 to the formation of the relatively simple compounds, carbon 

 dioxide, urea, etc., which leave the organism and which moreover, 

 being very poor in chemical tension, are for this reason of no or 

 very little value for the body. It is therefore absolutely necessary 

 for the continuance of life and the normal course of the functions 

 of the body, that the organism and its different tissues should be 

 supplied with new material to replace that which has been exhausted. 

 This is accomplished by means of food. Those bodies are de- 

 signated as food which have no injurious action upon the 

 organism and which replace those constituents of the body that have 

 been consumed in the exchange of material or that can prevent 

 or diminish the consumption of such constituents. . 



Among the numerous dissimilar substances which man and 

 animals take with the food all cannot be equally necessary or have 

 the same value. Some perhaps are unnecessary, while others may 

 be indispensable. We have learned by direct observation and a 

 wide experience that besides the oxygen, which is necessary for 

 oxidation, the essential foods for animals in general, and for man 

 especially, are water, mine-mi bodies, proteid bodies, carbohydrates, 

 and fats. 



It is also apparent that the various groups of food-stuffs neces- 



1 The translator will use in the following pages for the German word 

 " Stoffwechsel" Dr. Burdon-Sanderson's (Syllabus of Lectures, 1879) transla- 

 tion, exchange of material, and at the same time the more general term 

 "metabolism." 



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