DIGESTION AND ABSORPTION IN THE INTESTINES. 805 



operation a small portion of the intestine is isolated by cutting 

 through the intestinal wall at two points a certain distance apart. 

 The continuity of the canal is re-established by appropriate 

 suture, while the piece cut out, with its blood and nerve-supply 

 intact, is given an opening to the exterior at one or both ends by 

 suturing to the abdominal wall. In this way a small pouch or loop 

 of the intestine is separated from the rest of the alimentary canal 

 and is so arranged that its secretion can be obtained through the 

 fistulous openings, or material of any kind can be introduced 

 into the loop and be removed after a given time to determine what 

 absorption has taken place. The secretion from these loops is 

 usually said to be small in quantity, especially in the jejunum. 

 Pregl estimates that as much as three liters may be formed in the 

 whole of the small intestine in the course of a day, but this esti- 

 mate does not rest upon very satisfactory data. The liquid gives 

 an alkaline reaction, owing to the presence of sodium carbonate. 

 Experiments have shown that this liquid has little or no digestive 

 action except upon the starches, and it may perhaps be doubted 

 whether it is a true digestive secretion. Extracts of the walls of 

 the small intestine or the juice squeezed from these walls have been 

 found, on the contrary, to contain four or five different enzymes 

 and to exert a most important influence upon intestinal digestion. 

 These enzymes belong probably to the group of endo-enzymes, and 

 are not actually secreted into the lumen of the intestines. While 

 they are not, strictly speaking, constituents of the intestinal juice, 

 nevertheless it is their action on the food which forms the charac- 

 teristic contribution to the process of digestion made by the glands 

 of the intestinal wall. These enzymes and their actions are as 

 follows : 



1. Enterokinase (see p. 797), an enzyme which in some way acti- 

 vates the proteolytic enzyme of the pancreatic juice, by converting 

 the trypsinogen to trypsin. 



2. Erepsin. This enzyme, discovered by Cohnheim,* acts 

 especially upon the proteoses and peptones, causing further hydro- 

 lysis. Its splitting action upon the peptones is supposed to be 

 complete, and the natural suggestion regarding this enzyme is that 

 it supplements the work begun by the trypsin and pepsin. Erepsin 

 occurs not only in the intestinal mucosa, but also, it is claimed, in 

 the liver, kidney, and pancreas, and perhaps in other tissues. Its 

 characteristic is that it is adapted especially to hydrolyze the pro- 

 teoses and peptones. On the theory that proteins during digestion 

 are broken down completely to their constituent amino-acids, the 

 importance of this enzyme in the normal digestion of proteins has 



* Cohnheim, "Zeitschrift f. physiol. Chemie," 33, 451, 1901; also 35, 134, 

 et seq.; Kobzarenko, "Biochemische Zeitschrift," 66, 344, 1914. 



