CHAPTER IX. 

 FOODS. 



The functional activity of every organ and tissue of the body is accom- 

 panied by a more or less active disintegration of the living material, the bio- 

 plasm, of which it is composed, as well as of the food materials circulating 

 in its interstices. The complex molecules of the living material and of the 

 non-living food materials are continually undergoing disruption and falling 

 into less complex and more stable compounds; these, through oxidative 

 processes, are eventually reduced through a series of descending chemic 

 stages to a small number of simpler compounds which, being of no further 

 apparent value to the organism, are eliminated by the various eliminating 

 or excretory organs, the lungs, skin, kidneys, and liver. Among these 

 excreted compounds derived from tissue and from food metabolism the 

 most important are urea, uric acid, and carbon dioxid. Many other com- 

 pounds, organic as well as inorganic, are also eliminated from the body in 

 the various excretions, though they are present in but small amounts. Coin- 

 cident with this metabolic process there is a transformation of potential 

 into kinetic energy, which manifests itself for the most part as heat and 

 mechanic motion. 



In order that the organs and tissues may continue in the performance 

 of their functions, it is essential that they be supplied with nutritive mate- 

 rials similar to those which enter into their own composition: viz., proteins, 

 fat, carbohydrates, water, and inorganic salts. These compounds, though 

 originally derived from the food, are immediately derived from the blood 

 as it flows through the capillary blood-vessels. The blood is therefore to be 

 regarded as a reservoir of nutritive material in a condition to be absorbed 

 and transformed into utilizable and living material. Inasmuch as the 

 materials which are lost to the body daily, through processes of disintegra- 

 tion and oxidation, are supplied by the blood, it is evident that this fluid 

 would diminish rapidly in volume, with a corresponding decline in func- 

 tional activity, were it not replenished by the introduction into the body of 

 new material in the food. With the diminution of the volume of the blood and 

 an insufficient supply to the tissues, there arise the sensations of hunger and 

 thirst, which lead to the consumption of food and the subsequent restoration 

 of the physiologic condition of the tissues. These two sensations are also 

 partially dependent on the empty condition of the stomach and the dryness 

 of the mucous membrane of the mouth and throat. 



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

 and thirst are complex in composition and contain, though in vary- 

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

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

 maintain the nutrition, nutritive principles. These compounds also contain 

 the potential energy necessary to maintain the energy equilibrium of the 



