786 PHYSIOLOGY OF DIGESTION AND SECRETION. 



meal of bread alone than after a meal of meat alone. It seems 

 possible that the latter point, the time at which the maximum flow 

 is reached, may depend upon the difference in rate at which these 

 foods are ejected from the stomach. Cannon (p. 719) has shown that 

 the carbohydrate foods leave the stomach sooner than the proteins or 

 fats. It is stated, however, that the composition of the secretion 

 varies also with the character of the food, and indeed shows an 

 adaptation to the character of the food. The secretion caused by 

 protein food is especially rich in trypsin, that caused by fatty food 

 in lipase, etc. The mechanism by which this adaptation is secured 

 is not understood. Glaessner* has measured the rate of flow in 

 man, and his curve for a mixed diet is represented also (in 

 red) in Fig. 296. These curves indicate in general that the secretion 

 of pancreatic juice begins very soon after food enters the stomach, 

 and increases rapidly to a maximum, which is reached somewhere 

 between the second and fourth hour. According to Glaessner's 

 case, there is a continuous small secretion of the juice during fast- 

 ing. The observations on dogs, on the contrary, indicate an 

 entire cessation of the flow when the stomach is empty. 



Normal Mechanism of the Pancreatic Secretion — Secretin. 

 — Much light was thrown upon the mechanism of pancreatic secretion 

 by the discovery (Dolinsky, 1895) that acids brought into contact 

 with the mucous membrane of the duodenum set up promptly a 

 secretion of pancreatic juice. Since this discovery it has been be- 

 lieved that the acid gastric juice is the means that serves to inaugurate 

 the flow from the pancreas. As soon as any of the acid contents of 

 the stomach pass through the pylorus this action begins. Just as the 

 chewing and swallowing of the food initiate the gastric secretion, so 

 the acid of the latter starts the pancreatic secretion. Assuming 

 that the pancreatic gland possesses secretory fibers it was thought at 

 first that the acid acts reflexly through these fibers— that is, the acid 

 in the duodenum acting upon sensory endings causes a reflex stimu- 

 lation of the efferent secretory fibers. It has been shown, however, 

 that the same effect takes place after section of the vagus and 

 splanchnic nerves (Popielski) , and Bay liss and Starling f have called 

 attention to another more probable explanation. These authors find 

 that if the mucous membrane of the duodenum (or jejunum) is 

 scraped off and treated with acid (0.4 per cent. HCl) the extract 

 thus made when injected into the blood sets up an active secretion of 

 pancreatic juice. They have shown that tliis effect is due to a special 

 substance, secretin, which is formed by the action of the acid upon 

 some substance (prosecretin) present in the mucous membrane, 



* Glaessner, Loc. cit. 



t Bayliss and Starling, "Journal of Physiology," 28, 235, 1902. 



