704 PHYSIOLOGY OF DIGESTION AND SECRETION. 



attention to another more probable explanation. These authors find 

 that if the mucous membrane of the duodenum (or jejunum) is 

 scraped off and treated with acid (0.4 per cent. HC1) the extract 

 thus made when injected into the blood sets up an active secretion of 

 pancreatic juice. They have shown that this effect is due to a special 

 substance, secretin, which is formed by the action of the acid upon 

 some substance (prosecretin) present in the mucous membrane. 

 Secretin is not an enzyme, since its activity is not destroyed by boil- 

 ing or by the action of alcohol. The experimental evidence at 

 present favors the view that the normal sequence of events is as 

 follows: The acid of the gastric juice upon reaching the duodenum 

 produces secretin; this in turn is absorbed by the blood, carried to 

 the pancreas, and stimulates this organ to activity. Whether the 

 acid in addition causes reflex stimulation of the secretory nerves is 

 not certain, although this statement is made. It is not clear, also, by 

 what means the secretion of pancreatic juice is maintained through 

 the six or seven hours or more of gastro-intestinal digestion. The 

 increased flow during the first hours is connected with the increas- 

 ing discharge of food from stomach to intestine, but whether the 

 effect upon the pancreas is traceable entirely to a continual forma- 

 tion of secretin, or partly to secretin and partly to nerve stimulation, 

 are matters not yet settled. 



Enterokinase. It was discovered in Pawlow's laboratory 

 (Chepowalnikow) that the pancreatic juice obtained from a fistula 

 may have little or no digestive action on proteids, but if brought into 

 contact with the duodenal membrane or an extract of this membrane 

 it shows at once powerful proteolytic properties. This discovery has 

 been confirmed repeatedly. Evidently the proteolytic enzyme of the 

 juice is secreted in a zymogen or pro-enzyme form (typsinogen), which 

 is activated or converted to trypsin by something contained in the 

 mucous membrane of the small intestine (duodenum, jejunum). 

 This something Pawlow supposes is an enzyme, and since its action is 

 on another enzyme, "a ferment of ferments/' he designated it as a 

 kinase or enterokinase. The action of the enterokinase seems to be 

 quite specific. According to Bayliss and Starling, trypsinogen is a 

 stable body which cannot be changed to trypsin otherwise than by 

 the action of the kinase; but a very small amount of the latter 

 suffices to convert a large quantity of trypsinogen. The active 

 trypsin itself, on the other hand, is very easily destroyed, especially 

 in alkaline solutions. The physiological value of this very interesting 

 relation is not clear, but it seems possible that it may serve to protect 

 the living tissues from the powerful digestive action of the trypsin. 

 The other enzymes of the pancreatic juice, the diastase and the lipase, 

 are secreted apparently in active form. 



