PHYSIOLOGY OF THE DIGESTIVE GLANDS 461 



blood pressure. It is believed that this effect is due to the action of histainme 

 (/?-imidazolylethylamine). A very strong preparation of secretin can also 

 be prepared by the method of Dale and Laidlaw, which depends on pre- 

 cipitation by mercuric chloride. 9 



Secretin does not exist preformed in the epithelial cells, as is shown by 

 the fact that an extract, made with neutral saline solution, does not as a 

 rule, have any secretory action when injected intravenously. Sometimes 

 a slight secretion may be produced, but this is probably to be explained 

 by the fact that some secretin remains behind in the cells as a result of a 

 preceding phase of activity. If, on the other hand, the above neutral or 

 slightly alkaline opalescent solution of the mucous membrane is boiled 

 with acid, secretin may become developed in it. The interpretation put 

 upon these results is that a substance, called prosecretin, exists in the 

 epithelial cells, and that this becomes converted into 



of acid on the cells. The secretin thus produced is then taken up by the 



blood, none of ft passing into the intestinal canal, because the free borders 

 of the cells are impervious to secretin. That this is actually the case has 

 been shown by finding that the introduction of neutralized secretin solu- 

 tion into the duodenum, or other parts of the small intestine, does not 

 cause a secretion of pancreatic juice. Similar treatment of the gastric 

 mucosa, after pepsin has been destroyed by heat, also yields secretin 

 solutions (Luckhart). This fact indicates that the foregoing hypothesis 

 concerning the role of secretin in bringing about secretion of pancreatic 

 juice under normal conditions in the intact animal is as yet uncertain. 



We know practically nothing concerning the chemical nature of secretin. 

 Being soluble in about 90 per cent alcohol and in fairly weak acids, it can 

 not belong to any of the better known groups of proteins. As it is 

 readily diffusible through parchment membrane, it can not be of very 

 complex structure, and as it withstands heat, it can not be an enzyme. 

 It rapidly deteriorates in strength in the presence of alkalies. 



Any acid when_applied to th_ mucous membrane is capable of producing 

 secretin, and so are certain other substances, such as mustard oil. 

 Secretin is very susceptible to destruction by such digestive enzymes 

 as those present in the pancreatic, gastric, and intestinal juices. 

 That secretin is present in the blood when acid is in contact with the 

 duodenal mucosa has been shown by the fact that injection into a normal 

 dog of blood from one in which secretin formation is going on (as a 

 result of acid in the duodenum), excites pancreatic secretion. 



The pancreatic juice produced by the injection of secretin, like that 

 which is produced under normal conditions, does 'not contain any active 

 trypsin, but instead contains its precursor, trypsinogen. This becomes 

 converted into trypsin in the intestine, being activated by contact with 



