344 A MANUAL OF PHYSIOLOGY 



(3) The Influence of Nerves on the Pancreas. Our scanty 

 knowledge may be summed up thus. Stimulation of the 

 medulla oblongata causes secretion, or increases it if already 

 going on. Stimulation of the peripheral end of the vagus* 

 excites secretion after a latent period of 2 to 3 minutes. 

 .Stimulation of the central end of the vagus and of other nerves 

 inhibits the secretion ; the inhibition caused by vomiting is 

 probably due to impulses ascending the vagus. These facts 

 show that a reflex mechanism governing the pancreatic secre- 

 tion exists. The centre is probably in the medulla oblongata ; 

 the efferent fibres run in the vagus. Afferent impulses that 

 increase the activity of the centre can be set up by stimula- 

 tion of the gastric mucous membrane, by ether, e.g. There 



is some evidence that a local 

 centre for the pancreatic secre- 

 tion also exists, possibly in the 

 semilunar ganglion. The natural 

 secretion of pancreatic juice is 

 by no means so intermittent as 

 FIG. 108. RATE OF SECRETION OF that of saliva. In the rabbit the 

 PANCREATIC JUICE pancreatic, like the gastric juice 



S shows the variation in the rate of * 



secretion of the pancreatic juice in a dog ; flo WS Continuously . In a Well- 

 P, the variation in the percentage of solids f , , , ,, , , 



in the juice. It will be seen that the ted QOg it IS probable that the 



rfp*%^tetaC flow seldom st P s altogether, for 



zontal axis are hours since the last meal. a ft e r a meal it has been Seen to 



continue for 20 to 24 hours. It begins abruptly as soon as 

 the food enters the stomach, doubtless through reflex impulses 

 originating in the gastric mucous membrane, rises in two or 

 three hours to a maximum, then falls till the fifth or sixth 

 hour, after which it mounts again about the ninth or tenth 

 hour to a second lower maximum, and then, gradually diminish- 

 ing, ultimately stops. During activity the bloodvessels of the 

 gland are dilated. When the nerves of the pancreas, which 

 pass to it from the solar plexus along the vessels, are divided, 

 ' paralytic ' secretion of thin w r atery juice takes place. There 

 is one remarkable difference between the normal secretion of 



* Slowly recurring stimuli (e.g.> induction shocks at the rate of about 

 one in the second) are used, so as to avoid stimulation of the vaso-con- 

 st/ictor fibres of the pancreas, which run in the vagus, since constriction 

 of the bloodvessels hinders the secretion. (See also footnote on p. 343.) 



