134 TEXT-BOOK OF PHYSIOLOGY. 



The foods which are consumed daily in response to the sensations 

 of hunger and thirst are complex in composition and contain, though 

 in varying amounts, proteids, fats, carbohydrates, water, and inor- 

 ganic salts, which, in contradistinction to foods, are termed food 

 principles or nutritive principles. In these compounds is also to be 

 found the potential energy necessary to maintain the dynamic equi- 

 librium of the body and which will become manifest as heat and 

 mechanic motion in the transformations of the material underlying 

 the nutritive processes. 



The animal body may be therefore regarded as a machine capable 

 each day of performing a certain amount of work by the expendi- 

 ture of a definite amount of energy. In the performance of its work, 

 whether it be the raising of weights against gravity, the overcoming 

 of friction, cohesion, or elasticity, the machine suffers disintegration 

 and loses a portion of its available energy. Unlike other machines, 

 however, it possesses the power, within limits, of self-renewal, self- 

 adjustment, when supplied with foods in proper quantity and quality. 



QUANTITIES OF FOOD PRINCIPLES REQUIRED DAILY. 



In order that the body may continue in the performance of its 

 work and yet retain a given weight, it is essential that the loss to the 

 body daily shall be exactly compensated by the introduction and 

 assimilation of a corresponding amount of food principles. If this 

 condition is realized, the body neither gains nor loses, but remains in 

 a condition of nutritive equilibrium. The determination of the exact 

 quantities of the different food principles required daily and their 

 ratio one to another is made from an examination of the quantity and 

 composition of the daily excretions.^ Sinceth^jQroteids disintegrated 

 are represented in the excretions by ur^^oTsirriita? nitrogen-holding 

 compounds and the fats_and carboHydrates _by^_carbon dioxid^it 

 becomes possible to determine from them the quantities required to 

 restore equilibrium under any given condition. But as the activity 

 of the nutritive changes will vary in accordance with climatic condi- 

 tions, work done, etc., and as the excreted products will vary in the 

 same ratio, it is obvious that the required amounts of food will vary 

 in accordance with these varying conditions, if equilibrium is to be 

 maintained. 



Various estimates have been made by different investigators as 

 to the amounts of the excreted products and the food principles re- 

 quired daily, which, though differing to some extent, have, neverthe- 

 less, an average nutritive and energy-producing value. The follow- 

 ing table shows the diet scale of Vierordt and the excretions to which 

 it would give rise. As the income and outgo practically balance, 

 there would be no change in the weight. 



