732 PHYSIOLOGY OF DIGESTION AND SECRETION. 



the digestion and absorption of various substances. 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 estimate does not rest upon very satisfactory data. The liquid 

 is distinctly alkaline, 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 secretion. Extracts of the walls of the small 

 intestine or the juice squeezed from these walls have been found to 

 contain four or five different enzymes and to exert a most important 

 influence upon intestinal digestion. Whether these enzymes are 

 actually secreted into the lumen of the intestine is not satisfactorily 

 shown, but since they are contained in the intestinal wall we must 

 regard them as secretory products and consider them as the impor- 

 tant and characteristic feature of the intestinal secretion. These 

 enzymes and their actions are as follows : 



1. Enterokinase (see p. 725), an enzyme which in some way activates the 



proteolytic enzyme of the pancreatic juice, by converting the tryp- 

 sinogen to trypsin. 



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



the deutero-albumoses and peptones, causing further hydrolysis. 

 Whether its splitting action upon the peptones is complete is not 

 as yet known, but, as was said above (p. 728) the natural suggestion 

 regarding this enzyme is that it supplements the work begun by 

 the trypsin. 



3. Inverting enzymes capable of converting the disaccharids into the 



monosaccharids. These enzymes are three in number: maltase, 

 which acts upon maltose (and dextrin); invertase or invertin, 

 which acts upon cane-sugar; and lactase, which acts upon lactose. 

 The maltase acts upon the products formed in the digestion of 

 starches, the maltose and dextrin, converting them to dextrose 

 according to the general formula: 



C^H^A! + H 2 = C 6 H 12 6 + C 6 H 12 6 



Maltose. Dextrose. Dextrose. 



In the same way invertase converts cane-sugar to dextrose and levu- 

 lose, and lactase changes milk-sugar to dextrose and galactose. This 

 inverting action is necessary to prepare the carbohydrate food for 

 nutritive purposes. Double sugars can not be used by the tissues 

 and would escape in the urine, but in the form of dextrose or 

 dextrose and levulose they are readily used by the tissues in their 

 normal metabolic processes. 



4. Lastly, the substance secretin, which, as explained above, plays 



such an important role in the control of the secretion of the pan- 

 creas, is formed in the walls of the small intestine. It is not an 

 enzyme, but a more stable and definite chemical substance which 

 is secreted or formed in the intestinal mucosa in a preliminary form, 

 prosecretin, and under the influence of acids is changed to secretin. 

 In this latter form it is absorbed, carried to the pancreas, and 

 causes a flow of pancreatic secretion. 



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

 et seq. 



