NERVOUS SYSTEM UPON SECRETION 283 



The subject of chemical stimulation will be treated in a sub- 

 sequent section, and we shall here deal with the subject of 

 pancreatic innervation which properly leads up to it. 



The nerve- supply of the pancreas is similar in plan to that of the 

 stomach, being provided by cerebrospinal fibres coming from the 

 vagi, and sympathetic fibres derived from the solar plexus. 



The methods for studying the effect of these nerves upon the 

 secretion of pancreatic juice devised by Pawlow and his co-workers 

 closely resemble those employed in the case of the gastric secretion, 

 with one important exception namely, that no means was devised 

 similar to the oesophageal fistula for preventing escape of material 

 from the stomach into the duodenum, as a result of, or accom- 

 panying, the stimulation of the nerves. This difference is of 

 importance, because it was shown by the workers of the St. Peters- 

 burg school themselves that the application of dilute acid solutions 

 or of acid chyme to the duodenal wall gives rise to a copious and 

 long- continued flow of pancreatic juice. This flow was ascribed 

 by these experimenters to a stimulation by the acid of afferent 

 nerve endings of a local reflex nervous mechanism in the duodenal 

 wall; but it was later shown by Bayliss and Starling, as we shall 

 see in detail in the next section, that the flow of pancreatic juice 

 so obtained was not due to a direct action of the acid upon afferent 

 nerve endings in the duodenum, nor indeed to nervous mechanism 

 at all, but to a chemical action of the acid upon a substance formed 

 in the duodenal mucous membrane cells which they named pro- 

 secretin. This prosecretin is formed during rest in the duodenal 

 cells, and when acid arrives from the stomach is converted into 

 an active substance called secretin, which enters the blood-stream, 

 is carried to the pancreatic cells, and excites these cells to secre- 

 tion by acting as a direct chemical stimulus. 



For the present, what concerns us here as a preliminary to 

 the description of the experiments of the Pawlow school upon 

 the effect of stimulation of the vagus and sympathetic nerves 

 on the process of pancreatic secretion, is the experimental fact 

 that passage of acid chyme from the stomach to the duodenum 

 is capable of calling forth a secretion of pancreatic juice. It is 

 accordingly just as important, in experimenting upon the effects of 

 nerve stimulation upon secretion by the pancreas, to make certain 

 that no chyme passes from the stomach to the duodenum, as it 

 is in similar experiments upon gastric secretion to make certain 



