FOOD AND FEEDING 683 



In the course of the metabohe processes in the muscles 

 there are intermediate products set free— as kreatin, leucin, 

 glyeocoll, sarcolactic acid, etc. These products are finally 

 split up (katabolized) or synthetized (anabolized) in the 

 liver with the formation of urea, uric acid, phosphates, sul- 

 phates, etc., as end-products, with the liberation of heat. From 

 its absorption to the final urea stage each gram of protein 

 liberates energy or heat equal to 4.1 calories. 



This heat formation is of extreme importance in stimu- 

 lating and sustaining nervous action. 



The following role is played by protein as a nutritive :— 



1. It is the building material for cell protoplasm of tissues 

 of all kinds. 



,2. It forms energy directly by undergoing destruction 

 without tissue-building. 



_ _ 3. It may be so transformed (through its H and C), that 

 it is deposited as fat. 



A dog can live on lean meat (pure protein) for months 

 and gain in weight. 



Carbohydrates in the food, as sugar and. starch, are di- 

 gested in part by the salivary ferment (ptyalin), but chiefly 

 by the pancreatic ferment (trypsin) in the intestines, and are 

 converted by them into glucose. The glucose is then absorbed 

 hy the small intestines, passes thence as glucose into the entero- 

 hepatic circulation and is oxidized in all the cells of the body 

 but chiefly in the liver, muscles and pancreas — in the form of 

 glucose and glycogen and with the production of heat equal to 

 4.18 calories for each gram of carbohydrates. Carbon dioxide 

 and water are formed as end-products. Sugar metabolism is 

 under the control of a medullary centre. The heat production,, 

 as in the case of the combustion of proteids, is of great im- 

 portance in sustaining nerve action. 



Fat in the food is emulsified by the bile and by the pan- 

 creatic and intestinal secretions and is absorbed as fatty acids, 

 glycerol and soaps by special cells of the intestinal mucous 

 membrane and passes into the lymphatics as emulsions of fat; 

 thence into the thoracic duct, and is finally oxidized into car- 

 bonic dioxide and water with production of heat and energy. 

 In what part of the body oxidation of fat occurs is unknown. 

 In the combustion of 1 gram of fat heat equal to 9.4 calories 

 is liberated. 



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