CHEMICAL MECHANISMS OF SECRETION 185 



after severing the mesenteric nerves. Such a procedure was im- 

 possible for anatomical reasons in the duodenum, but was success- 

 fully carried out with a positive result on a loop of jejunum. 



In this crucial experiment the loop of intestine was completely 

 cut off from all nervous connection with the pancreas, and hence 

 the conclusion is an inevitable one that the effect must be produced 

 by some chemical substance finding its way into the circulation, 

 and then either directly or indirectly stimulating the pancreatic 

 cells. 



It must be admitted here that the process of severing all the 

 network of sympathetic nerve-fibres surrounding the blood-vessels 

 passing to the intestinal loop is a difficult one, and it is hard to 

 make certain that it has been effectually carried out, so that it 

 would have been well to insert in this experiment small cannulse 

 into the completely severed artery and vein of the loop. But, 

 as Bayliss and Starling point out, the experiment was that which 

 led to the discovery of secretin, the specific chemical excitant, or 

 hormone, of the pancreatic secretion. Also the effects about to 

 be described of injection of extracts of the duodenal or jejunal 

 mucous membrane prepared by the action of dilute acid clearly 

 demonstrate a local action of the secretin upon the pancreas. 



The positive result obtained in the experiment with the ener- 

 vated loop of intestine, taken in conjunction with the result 

 obtained by Wertheimer and Lepage, that acid itself introduced 

 into the circulation has no effect upon the pancreatic secretion, 

 led Bayliss and Starling to the view that the acid must give rise 

 to some active substance in the cells of the mucosa which is taken 

 into the circulation and produces the specific effect. This view 

 was then abundantly confirmed by the results of experiment. 

 The loop of jejunum from which the positive result was obtained 

 was cut out, the mucous membrane scraped off, rubbed up with 

 sand and 0-4 per cent, hydrochloric acid in a mortar, filtered 

 through cotton wool, and the extract injected into a vein. The 

 result was a flow of pancreatic juice at more than twice the rate 

 produced at the beginning of the experiment by introduction of 

 acid into the duodenum. Two further results were obtained 

 in the same experiment : first, it was shown that the acid extract 

 could be boiled without losing its activity, so that the active sub- 

 stance (secretin) was shown not to be a ferment ; and secondly, it 

 was shown that the activity of extracts of portions of the small 



