796 PHYSIOLOGY 



animal, a flow of juice begins in one to one and a half minutes. From 

 this time there is a steady, slow rise of the rate of secretion, which 

 lasts for two to three hours, and then gradually diminishes. The 

 greatest increase in flow is observed at the time when the first por- 

 tions of digested food escape from the stomach into the duodenum. 

 The secretion must therefore be determined in some w^ay by the 

 entry of this food into the duodenum. By experiments on dogs 

 possessing a gastric as well as a pancreatic fistula, it has been shown 

 that the introduction of acid, e.g. 0-4 per cent. HC1, into the stomach 

 evokes, as soon as it passes into the duodenum, a rapid flow of pan- 

 creatic juice. A similar, but smaller, effect is produced by the 

 passage of oil from the stomach into the duodenum. The intro- 

 duction of alkalies is without effect. Weak acids are also effective 

 exciters of secretion if they be introduced directly into the duodenum 

 itself or into a loop of small intestine. The effect gradually diminishes 

 as the loop which is chosen comes nearer to the caecum, and as a rule 

 the injection of dilute acid into the lower foot or eighteen inches 

 of ileum is without effect on the pancreas. The striking resemblance 

 between the secretion thus evoked and that produced in the salivary 

 glands by injection of acid into the mouth suggests that we have 

 here to do with a reflex of the same kind as that which affects the 

 salivary glands. In the search for the channels of this reflex Heiden- 

 hain showed that stimulation of the medulla oblongata occasionally 

 produced a flow of pancreatic juice. He was unable, however, 

 to obtain any secretion on stimulation of the vagus nerve. 

 The pancreas receives fibres from the vagi as well as from the 

 splanchnic nerves (sympathetic system). According to Pawlow 

 the ill success of Heidenhain's experiments was due to the fact that 

 in any operation a gland is played upon by reflex impulses partly 

 of an inhibitory, partly of a secretory nature, in which the inhibitory 

 predominate, and by the further fact that the pancreas is extremely 

 susceptible to alterations in its blood -supply, so that any stimula- 

 tion of the vagus which caused inhibition of the heart would 

 ipso facto prevent the effect of simultaneous excitation of secretory 

 fibres from making its appearance. Pawlow noticed that if in an 

 animal with a permanent fistula the vagus on one side were cut 

 and left for four days in order to allow the cardio-inhibitory fibres 

 to degenerate, repeated stimulation of the peripheral end of the 

 nerve evoked a flow of pancreatic juice. He obtained the same 

 results by stimulating this nerve below the point at which it had 

 given off its cardio-inhibitory fibres, in animals in which the reflex 

 inhibitions from the operation itself were prevented by total section 

 of the medulla. Under certain circumstances he obtained also 

 secretion on stimulation of the splanchnic nerves, and therefore 



