528 LECTURE XXII. 



as such in the intestinal membrane, but that its antecedent, prosecretin, 

 is there, and becomes changed into secretin on being acted upon by acid; 

 i.e., it may be set free in this way from some other compound. It might 

 also be assumed, of course, that the prosecretin undergoes an atomic 

 rearrangement in the molecule. Now how shall we regard the action of 

 the secretin under normal conditions? We must remember that Pawlow 

 found that the hydrochloric acid from the stomach stimulated the secre- 

 tion of pancreatic juice. We have already shown how hydrochloric acid, 

 directly or indirectly, by altering the alkalinity of the blood, can excite 

 into activity the pancreas, and have left it open as to how the acid acts 

 upon the intestinal membrane. 'The work of Bayliss and Starling may 

 perhaps serve to explain how the hydrochloric acid can influence the pan- 

 creatic gland. Evidently it is constantly changing prosecretin into secretin. 

 As quickly as the latter is formed, it is taken into the circulation, and 

 now acts in some way upon the gland. It seems most probable from cer- 

 tain observations that secretin affects the blood-vessels in the pancreas 

 and increases the circulation. This does not necessarily imply that there 

 is not some specific action as well. This would seem quite likely from 

 the fact that secretin stimulates only the pancreas to any extent. Now 

 this fact gives to the hydrochloric acid of the stomach an entirely new 

 significance. Not only does this aid us in our knowledge concerning the 

 action of acid upon the intestinal membrane, but we obtain, at the same 

 time, new prospects for further investigations concerning the cell-work of 

 the glands and tissues. Even though we may be a long way from being 

 able to understand the entire chain of processes, from the formation of 

 the hydrochloric acid in the stomach to the production of the secretion 

 on the part of the pancreas, and understand the phenomena only approx- 

 imately, still we are justified in hoping from the work of Pawlow and of 

 Bayliss and Starling that in following the paths now broken it will not 

 be very long before one link after another will be added until finally the 

 complete chain is forged. To be sure, there remain countless problems 

 to be solved. We should like to know exactly what prosecretin is, and 

 to what class of chemical compounds it belongs. 1 The fact that it is not 

 a ferment is shown by its being unchanged by moderate heat. The intes- 

 tine, therefore, concerns itself not only with the absorption and assimilation 

 of the food, but takes part to a considerable extent in the digestion itself. 

 The anatomical evolution of a unit from the intestine and its accessory 



1 Popielski, Zentr. Physiol. 19, 801 (1906), has recently proved that unquestionably 

 HC1 also reflexively influences the secretion of the pancreas by its action upon the intes- 

 tinal membrane. He believes that secretin belongs to the group of peptones. If this 

 be true we have here a case of one of the products of digestion acting upon this pancre- 

 atic secretion. The discovery of Bayliss and Starling will of course only receive its full 

 value when it is possible to isolate the secretin, and study by itself the action of HC1 

 upon it. 



