798 PHYSIOLOGY 



enters the first part of the duodenum. Immediately a certain amount 

 of secretin is produced by the acid in the cells of the mucous membrane. 

 The secretin is carried by the blood stream to the cells of the pancreas 

 and excites there the secretion of strongly alkaline pancreatic juice. 

 As soon as sufficient juice has been secreted to neutralise the acid 

 chyme the formation of secretin, and therefore the further secretion 

 of pancreatic juice, comes to an end. If the stomach still contains 

 food the process is, however, renewed in virtue of the local reflex 

 mechanism which we have just studied regulating the opening and 

 closure of the pylorus. So long as the contents of the duodenum 

 are acid the pylorus remains firmly closed. As soon as these are 

 neutralised the pylorus relaxes and allows the entrance of a further 

 portion of acid chyme. Thus, the formation of secretin proceeds 

 afresh, and the whole chain of processes goes on until the stomach 

 is empty and all its contents have passed into the intestine. 



In view of the efficacy of this chemical reflex mechanism, the 

 question arises whether the results first obtained by Pawlow were 

 really due in some way to the formation of secretin. Stimulation 

 of the vagus may cause contraction of the stomach, opening or closing 

 of the pylorus, and it seems possible that under its action there 

 might have been an escape of acid gastric contents into the intestine, 

 and therefore the formation of secretin, which would suffice to arouse 

 the pancreatic secretion. Later experiments by this observer, in 

 which the escape of any gastric contents was effectively prevented 

 by ligature of the pylorus while the stomach itself contained an 

 alkaline solution, have shown that even with these precautions a 

 flow of juice may be obtained on stimulation of the vagus nerve. 

 The flow, however, is very small in comparison with that obtained 

 by injection of secretin, and one must conclude that although the 

 nervous system may play a small part in the excitation of the activity 

 of this gland, the main factor involved is the chemical mechanism 

 which has just been described. 



The amount of pancreatic juice obtained after a meal varies 

 with the nature of the latter. The Table on p. 799 represents the 

 results obtained on an animal fed with 600 c.c. of milk, 250 grm. 

 of bread, and 100 grm. of meat respectively. 



The differences between these results seem, however, largely 

 determined by the duration of gastric digestion, and therefore the 

 amount of acid secreted in the stomach and passed on to the duodenum. 

 It was suggested by Walther that apart from this quantitative adapta- 

 tion there was a qualitative alteration in the constitution of the 

 juice according to the nature of the food ingested, that, e.g., excess 

 of protein causes an increase of the trypsin, while excess of carbo- 

 hydrate would cause an increase in the amylase of the juice. Later 



