DIGESTION AND ABSORPTION IN THE INTESTINES. 793 



converted to dextrose. The starchy food that escapes digestion in 

 the mouth and stomach becomes mixed with this enzj^me in the 

 duodenmn, and from that time until it reaches the end of the large 

 intestine conditions are favorable for its conversion to maltose. 

 Most of this digestion is probably completed, under normal con- 

 ditions, before the contents of the intestinal canal reach the ileo- 

 cecal valve. 



Action of the Lipolytic Enzyme (Lipase, Steapsin). — ^The 

 importance of the jDancreatic secretion in the digestion of fats was 

 first clearly stated by Bernard (1849). We know now that this secre- 

 tion contains an active enzyme capable of hydrolyzing or saponifying 

 the neutral fats. These latter bodies are chemically esters of the 

 trihydric alcohol glycerin. When hydrolyzed they break up into 

 glycerin and the constituent fatty acid. The action of lipase may 

 be represented, therefore, by the following reaction, in the case of 

 palmitin : 



C3H,^C,,H3,COO)3 + 3HP = C3H,(OH)3 + 3(C,,H3,COOH) 



Palmitin. Glycerin. Palmitic acid. 



When lipase from any source is added to neutral oils its splitting 

 action is readily recognized by the development of an acid reaction 

 due to the formation of the JFatty acid. If a bit of fresh pancreas 

 is added to butter, for example, and the mixture is kept at the body 

 temperature the hydrolysis of the fats is soon made evident by the 

 rancid odor due to the butyric acid produced. When pancreatic 

 juice is mixed with oils or liquid fats two phenomena may be 

 noticed: first, the splitting of the fat already referred to, and, second, 

 the emulsification of the fat. The latter process is very striking. 

 An oil is emulsified when it is broken up into minute globules that do 

 not coalesce. Artificial emulsions may be made by vigorous and 

 prolonged shaking of the oil in a viscous solution of soap, mucilage, 

 etc. Milk may be regarded as a natural emulsion that separates 

 slowly on standing, as the fat rises to the top to form the cream. 

 When a little pancreatic juice is added to oil at the body temperature 

 the mixture, after standing for some time, will emulsify readily with 

 very httle shaking or even spontaneously. The emulsification is 

 due to the formation of soaps. The lipase splits some of the fats, 

 and the fatty acid liberated combines with the alkaline salts 

 present to form soaps. The presence of the small amount of soap 

 formed in this way at the beginning of the reaction is then instru- 

 mental in causing the emulsification of the remainder of the 

 neutral fat. The emulsification produced under these conditions 

 is very fine and quite permanent, and it was formerly believed 

 that the formation of this emulsion is the main function of the 

 pancreatic juice so far as fats are concerned. It was thought 

 that in the form of fine droplets the fat may be taken up directly 

 by the epithelial cells of the villi, and this view was supported 



