380 A MANUAL OF PHYSIOLOGY 



denum 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 sub- 

 stance 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 pancreatic 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 

 FIG. 149. RATE OF SECRETION OF has been obtained. The secretin 

 PANCREATIC JUICE. o f one an j ma l w iH excite a flow 



s shows the variation in the rate of o f pancreatic juice in an animal 



^rt^tSrit?^ f a different kind as well as in 



centage of solids in the juice, it will one of the same kind. In normal 



be seen that the maxima of s fall digestion secretin is formed under 



at the same time as the maxima of f v i n fl 11pnrp n f f u p ar :j rWmP 



P. The numbers along the horizontal tne influence < tne acid cnyme, 

 axis are hours since the last meal. not in the stomach, but after it 



has passed into the duodenum. 



The passage of the chyme through the pylorus, as previously 

 mentioned (p. 305), is regulated by the reaction 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 re- 

 mains closed. As soon as the first small portion of acid chyme 

 ejected from the stomach has been neutralized by the increased 

 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 

 water stimulate the pancreatic secretion, and with great prompt- 

 ness, even before any acid has been produced in the stomach, 

 and therefore before any can have passed into the duodenum. 

 Possibly this effect is elicited through the long reflex paths 

 already described as running in the vagi or through a local ner- 



