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A TEXTBOOK OF PHYSIOLOGY 



effect, and it is quite possible that the products of digestion may 

 in some way modify -the enzymic content, soaps calling forth 

 steapsin, dextrins amylopsin, peptones trypsin, and so on. 



It is a question whether the secretion of the pancreatic juice may 

 be reflexly excited by a nervous mechanism. Stimulation of the vagus 

 causes a flow of juice after a long latent period, probably by increasing 

 the movements of the stomach and the flow of acid chyme into the 

 duodenum. However, a secretion has been obtained when the outflow 

 of chyme from the stomach was prevented, and the chemical mechanism 

 thus excluded. In favour of the nervous mechanism is the fact that, 

 in the dog, pancreatic juice begins to flow one to one and a half minutes 

 after the ingestipn of food, and before the entrance of the gastric 

 contents into the duodenum. 



FIG. 198. To SKUW ACTION OF ACID EXTRACT OF Mucous MEMBRANE OF DUODENUM 

 (SECRETIN) DEHYDRATED BY ALCOHOL. (Bayliss and Starling.) 



A, Blood-pressure; B, drops of pancreatic juice. 



The quantity of juice secreted varies in amount with the nature 

 of the nourishment. The period at which the maximum secretion 

 occurs ako varies, as the following figures show : meat, 2 hours ; bread, 

 a little later: milk, 3 to 4 hours. These figures apply to the dog. 

 In man, after a carbohydrate meal, the maximum is reached between 

 3 to 4 hourw : after meat and fat, 4 to 5 hours. Carbohydrates produce 

 most secretion, proteins somewhat less, and fat least of all. 



The Activation of the Pancreatic Enzymes. Opinion seems to 

 vary considerably about the state in which the steapsin and amylopsin 

 of the juice are secreted. Some hold that they are secreted in the 

 active state, others that the pancreas elaborates and secretes their 

 zymogens. 



There is no doubt about trypsin: this is secreted as trypsinogen. 



