340 FOOD AND DIGESTION 



solution by pepsin. Food material in a semi-liquid 

 form called chyme passes into the small intestine where 

 intestinal digestion takes place. The intestinal juice, 

 alkaline in character, supplies the enzyme maltase for 

 changing maltose to dextrose, lactase for changing lactose 

 to dextrose and galactose, and invertase for changing 

 sucrose to dextrose and levulose. Pancreatic juice is also 

 an alkaline liquid, and is secreted by the pancreas. It 

 contains amylopsin which hydrolyzes starch to maltose; 

 steapsin which hydrolyzes fat to glycerine and fatty acids, 

 the latter uniting with alkalies to form soaps; and trypsin 

 which changes proteins to peptones and possibly to amino- 

 acids. These enzymes are all more active than the corres- 

 ponding enzymes in the mouth and stomach. The bile is a 

 fluid secreted by the liver and its function is to assist the 

 enzymes in their activities. 



Carbohydrates in the form of dextrose, levulose, and 

 possibly galactose, are absorbed through the outer cells of 

 the villi, which are conical projections in the walls of the 

 small intestine. Practically no absorption takes place 

 from the mouth and stomach. The carbohydrates pass 

 through the capillaries into the portal vein and thence to the 

 liver where they are dehydrated and stored as glycogen 

 until needed by the blood, when glycogen is transformed 

 into dextrose. Fats in the form of glycerine and soaps are 

 absorbed through the walls of the villi, changed into fats 

 again and pass through the lymph vessels into the tho- 

 racic duct and thence into the veins. The proteins in the 

 form of peptones and amino-acids during their absorption 

 through the walls of the villi are changed to serum-albumin 

 and globulin, passing through the blood capillaries to the 

 liver and thence into the circulatory system. Inorganic 

 salts follow the same route as do the carbohydrates and 

 proteins. 



The carbohydrates in the form of dextrose are utilized 

 by the body tissues as a source of energy by combustion 

 with oxygen. Carbon dioxide and water are the final pro- 

 ducts. Excess of dextrose is converted into fat and deposited 

 in the adipose tissue. Fats also serve as a source of energy, 



