CHEMICAL CORRELATION 561 



We succeeded, for instance, in entirely severing all the nervous 

 connections of a loop of intestine in the upper part of the 

 jejunum, leaving it, however, still in vascular continuity with 

 the rest of the body. The introduction of '4 per cent. HC1 into 

 such a loop evoked a flow of pancreatic juice as profuse as that 

 obtained early in the experiment, when the acid was injected 

 into the loop while all its nervous connections were intact. We 

 knew already from W'ertheimer's experiments that the intro- 

 duction of acid directly into the blood-stream was without effect 

 on the pancreas. The only possible conclusion to be drawn 

 from our experiments was that the acid acted on the epithelial 

 cells covering the villi and separating the lumen of the gut from 

 the blood-vessels, and that it was some substance, produced in 

 these cells by the action of the acid, which was absorbed into 

 the blood-stream, and carried to the gland to act as a specific 

 excitant of the secretory cells. This conclusion was easily 

 verified. On scraping off some of the mucous membrane, 

 rubbing it up with acid, and injecting the hastily filtered extract 

 into the jugular vein, we obtained within two minutes a flow of 

 pancreatic juice greater than any we had observed as the result 

 of the introduction of acid into the intestine. It was evident, 

 therefore, that the nexus between the duodenal mucous membrane 

 and the pancreas was not nervous but chemical. Under the 

 influence of the acid, a new substance, which we may call 

 pancreatic secretin, was produced in the epithelial cells to act as 

 the special chemical messenger to call forth the activity of the 

 pancreas. Although our observations have been fully confirmed 

 by later workers on the subject, physiologists have not yet 

 succeeded in isolating secretin. The fact, however, that it is not 

 destroyed by boiling even in a strongly acid medium, that it is 

 unaffected by gastric juice, that it is readily diffusible, and is 

 not precipitated by the ordinary reagents for proteins and 

 peptones, such as tannic acid or phosphotungstic acid, marks it 

 out as a relatively stable body of definite composition, and 

 probably of low molecular weight. It belongs, in fact, to the 

 drug-class of physiological agents which we have designated as 

 hormones. 



Since the co-operation of the three juices — pancreatic juice, 

 bile, and succus entericus — is necessary for the normal carrying 

 out of the digestive processes of the duodenum, it would 

 evidently be an economy of mechanism if the activity of the 



