402 DIGESTION 



into the lower part of the ileum. It is obtained as strongly and as 

 promptly from an isolated loop of intestine when all the nerves 

 passing to it have been cut, and the solar plexus extirpated, and also 

 after the administration of atropine, which paralyzes the endings of 

 secretory nerves elsewhere. The secretion accordingly does not 

 depend upon a local reflex mechanism, with its afferent endings in 

 the intestinal mucous membrane, but upon some substance which is 

 carried to the pancreas by the blood, and acts directly upon its cells. 

 This substance is not the acid, for the injection of 0-4 per cent, 

 hydrochloric acid into the blood produces no effect upon the pan- 

 creas. It has been shown by Bayliss and Starling that the exciting 

 substance is a diffusible body of low molecular weight, probably of 

 organic nature, but not a protein, which they call secretin. It is 

 soluble in alcohol or alcohol and ether, and is not destroyed by 

 boiling. It is produced in the mucous membrane of the jejunum or 

 duodenum on exposure to dilute hydrochloric acid. Extracts of 

 mucous membrane so treated cause a copious pancreatic secretion, 

 and a smaller secretion of bile, when injected in small quantities into 

 the blood of animals in which no such secretion is taking place, but 

 have no influence on any other gland. At the same time the arterial 

 blood-pressure falls somewhat. The substance which produces the 

 fall of blood-pressure is different from secretin, since acid extracts 

 of the lower end of the ileum, which have no effect on the flow of pan- 

 creatic juice, diminish the blood-pressure. A precursor of secretin, 

 called pro-secretin, exists in the intestinal mucous membrane, and 

 can be extracted from it by physiological salt solution. It does not 

 affect the pancreatic secretion. By boiling or by the action of acid 

 secretin is split off from it. Pro-secretin is most abundant in the 

 duodenum, and diminishes as we pass down the intestine. 



Secretin is very widespread in the animal kingdom. In the 

 monkey, dog, cat, rabbit, man, ox, sheep, pig, squirrel, goose, 

 tortoise, salmon, dog-fish, and skate evidence of its presence has 

 been obtained. The secretin of one animal will excite a flow of pan- 

 creatic juice in an animal of a different kind as well as in one of the 

 same kind. In normal digestion secretin is formed under the 

 influence of the acid chyme, not in the stomach, but after it has 

 passed into the duodenum. The passage of the chyme through the 

 pylorus, as previously mentioned (p. 321), is regulated by the re- 

 action of the duodenal contents, as well as by the consistence of the 

 gastric contents. So long as the liquid in the duodenum is acid, the 

 pylorus remains closed. As soon as the first small portion of acid 

 chyme ejected from the stomach has been neutralized by the in- 

 creased secretion of the pancreatic juice and the outpouring of bile 

 from the gall-bladder in response to the stimulus of the acid, the 

 pylorus opens again. 



According to Pawlow, certain food substances, notably fat, and 



