710 PHYSIOLOGY 



solutions, especially under the influence of bacteria. It is apparently 

 oxidised with extreme ease. A similar, or more probably the same, body 

 may be produced from intestinal mucous membrane by treating this with 

 solutions of soap. 



In this secreting mechanism we have a very striking example of a 

 correlation between the activities of two different portions of the body 

 effected by chemical means. The strongly acid chyme 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 con- 

 traction 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 pre- 

 vented 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. 711 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 adaptation 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, 



