THE ABDOMEN. 147 



Food and Metabolism. Anything serves as food that 

 replaces or hinders the loss to which the component parts 

 of the body are liable. Proteins, carbohydrates, fats, 

 some mineral matters, as salt and perhaps iron, and water 

 are needed. The energy once expended by plants or 

 animals in the formation of the materials which serve as 

 food is set free in the body by the breaking up of these 

 complex substances into their original elements, which 

 are then recombined into the complex materials needed 

 for the body's life and growth. This process of building 

 up complex materials from simple ones is known as 

 anabolism and that of breaking them down as katabolism, 

 while the two combined form the complete cycle of 

 metabolism. Those foods have the best value that give 

 up their energy most readily. For their combustion, 

 heat, oxygen, and water are needed. Hunger indicates 

 that the supply of material for katabolism has been used 

 up and that more is needed, just as thirst indicates the 

 need of the system for more fluids. 



The proteins or nitrogenous foods include all animal 

 foods except fats, fish, crustaceans, eggs, milk and its 

 products, certain vegetables, especially the lentils, that 

 is, peas and beans, and gelatine. The fats include various 

 fats and oils commonly eaten. The carbohydrates are 

 the starchy foods, as cereals, sugars, fruits, and most 

 vegetables, in fact, practically all except the lentils. 

 Various beverages and condiments have no great nutri- 

 tive value but serve to stimulate the appetite and to 

 excite the secretion of the digestive juices. Coffee, tea, 

 and alcohol are stimulants. 



The different classes of foods have different functions 

 in the nourishment of the body. The proteins are pri- 

 marily tissue-builders and also help somewhat in force 

 production. The fats are essentially heat-producers, 

 though they too help in force-production. The carbo- 

 hydrates are chiefly important as force-producers, 

 though they also produce heat and to a certain extent 

 save protein oxidation. Fat is formed by all three but 



