FOODS 117 



sensation which is then referred to the stomach region. With the introduc- 

 tion of food these contractions disappear and the sensation subsides. The 

 characteristic peristaltic contractions occurring during digestion do not give 

 rise to the hunger sensation. The sensation of thirst arises when there is a 

 deficiency of water in the body and is mainly referred to the mouth and fauces. 

 With the introduction of water into the stomach and intestine and its 

 absorption into the blood and tissues the sensation speedily disappears. 

 The nerve mechanism by which these various results are brought about is 

 unknown. 



The foods which are consumed daily in response to sensations of hunger 

 and thirst are complex in composition and contain, though in varying 

 amounts, proteins, fats, carbohydrates, water, and inorganic salts, which, 

 in contradistinction to foods, are termed food principles or, as they main- 

 tain the nutrition, nutritive principles. These compounds also contain the 

 potential energy necessary to maintain the energy equilibrium of the body 

 which becomes manifest as heat and mechanic motion in the transforma- 

 tions of the material used in the nutritive processes. 



It has been stated in a previous chapter that the animal body may be 

 regarded as a machine capable of performing each day a certain amount of 

 work by the expenditure of a definite amount of energy. In the performance 

 of its work, whether it be the raising of weights against gravity, or the over- 

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

 and metabolizes a portion of the food materials and loses a portion of its 

 available energy. Unlike other machines, however, it possesses the power 

 within limits of self -renewal, 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 corre- 

 sponding amount of food principles. If this condition is realized, the body 

 neither gains nor loses in weight, but remains in a condition of nutritive 

 equilibrium. The determination of the extent of the metabolism is made 

 from an analysis of the daily excretions. If therefore these are collected 

 and analyzed, it will become possible to determine from their chief constitu- 

 ents the extent and character of the tissue and food metabolized, and hence 

 to calculate the relative quantities of the different food materials necessary 

 to replace the materials metabolized. Thus of the constituents the urea and 

 other nitrogen-holding compounds contained in the urine and feces represent 

 the proteins metabolized; the carbon dioxid and water represent the fat and 

 carbohydrates metabolized. Therefore it becomes possible to determine 

 approximately at least, from the amounts of the urea and carbon dioxid 

 eliminated, the different amounts of the food principles required to restore 

 the nutritive equilibrium under any given condition. As the activity of the 

 nutritive changes varies in accordance with age, weight, climatic conditions, 

 work done, etc., and as the excreted products vary in the same ratio, it is 

 obvious that the required amounts of food will vary in accordance with 

 these varying conditions, if nutritive equilibrium is to be maintained. 



