34 SECRETION 



thetic nerve supply. Those in the tenth nerve are revealed 

 if this nerve is stimulated while the pancreas is active, for the 

 flow then ceases. Stimulation of the central end of the vagus 

 and of other nerves reflexly inhibits the pancreatic flow. So 

 do painful impressions. Inhibition during vomiting is due to 

 the vagus. It has been maintained that a psychical secretion 

 exists, but this is doubtful, since the results brought forward 

 to prove this can be interpreted as due to the passage of acid 

 chyme into the duodenum. 



Mechanism of Pancreatic Secretion. Pancreatic juice begins 

 to flow in two to three minutes after 0.4 per cent, hydrochloric 

 acid is put into the duodenum. The effect becomes less pro- 

 nounced in passing along the intestine and is absent in the 

 ileum. This experiment gives positive results from an isolated 

 loop of the intestine, all nerves to which have been cut, the 

 solar plexus extirpated, and after the administration of atro- 

 pine. The effect, therefore, takes place through the blood, 

 but it is not obtainable by directly injecting the acid into the 

 animal's blood. An extract of the mucous membrane of the 

 duodenum, made with 0.4 per cent, hydrochloric acid, neu- 

 tralized, and then injected, is effective. The conclusion which 

 is to be drawn, therefore, is that the hydrochloric acid acting 

 on the mucous membrane gives rise to a new body which after 

 absorption into the blood is carried to the pancreatic cells 

 and acts as the effective stimulus. This substance is called 

 secretin. It is a diffusible body of low molecular weight, soluble 

 in alcohol or in alcohol plus ether. It withstands boiling and 

 digestion, and is, therefore, not a protein. It is also an effec- 

 tive stimulus to bile secretion, but to no other glands. An 

 extract of the mucosa with 0.7 per cent, sodium chloride gives 

 prosecretin, which does not excite the pancreas, but which can 

 be changed to secretin by boiling or by adding hydrochloric 

 acid. Secretin is widespread and interchangeable between 

 different animals. Since, normally, hydrochloric acid is supplied 

 by the chyme the normal mechanism of pancreatic secretion 

 is through the formation of secretin. 



The pancreatic secretion is said to vary in character with 

 the nature of the food. On a bread diet it is poor; on a flesh 

 diet, richer; and on a milk diet, richest in fat-splitting ferment. 

 Bread gives a secretion rich in amylolytic ferment. When 



