290 CHEMICAL MECHANISMS OF SECRETION 



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 cannulae 

 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 nerve- 

 less 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 04 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 

 intestine taken at different levels showed a decreasing amount 

 of activity as the intestine was descended, corresponding to the 

 known effects upon the pancreatic secretion of injection of acid 

 into these various portions. Thus injection of acid into a loop 

 from the lower end of the ileum gives rise to no pancreatic secretion, 

 and, corresponding to this, an acid extract from the mucous 

 membrane of the lower end of the ileum possesses when in- 

 travenously injected no exciting effect upon the pancreatic secretion. 



With regard to the seat of action of secretin, Bayliss and Starling 



