CHAPTER XLlX 

 PHYSIOLOGY OF THE DIGESTIVE GLANDS (Cont'd) 



THE HORMONE CONTROL 



This is exhibited best in the case of the pancreas. The crucial experi- 

 ment demonstrating that this gland is not primarily dependent upon 

 nervous impulses for the control of its activity was performed by Bay- 

 liss and Starling. 2 Starting with the well-known fact that the application 

 of weak acid to the duodenal mucous membrane excites secretion of pan- 

 creatic juice, these workers carefully severed all the nerve connections of 

 a portion of the duodenum, and found on again applying acid to the mucous 

 membrane that the secretion persisted. To explain this result they postu- 

 lated that the acid must cause some substance to be liberated into the 

 blood stream, which carries it to the pancreas, the cells of which it then 

 excites to activity. To test this hypothesis they scraped off the mucous 

 membrane of the duodenum and ground it in a mortar with weak hydro- 

 chloric acid (0.6 per cent), and, after boiling the solution so as to remove 

 the protein and nearly neutralizing it, they obtained a fluid which, when 

 injected intravenously, immediately caused a copious secretion of pan- 

 creatic juice. 



Accompanying the secretion, however, a marked fall in arterial blood 

 pressure was observed, making it possible that the secretion might have 

 been due to a vasodilatation occurring in the pancreatic blood vessels. To 

 eliminate this possibility they prepared an extract that was free of the 

 depressor substances by extracting intestinal epithelium without any of the 

 submucous tissue. The resulting extract had merely the secretory effect 

 and produced no fall in blood pressure. This secretagoguary substance 

 they named secretin. 



Further evidence that the action of secretin is independent of the 

 depressor substances has been obtained by taking advantage of the fact 

 that the depressor substance is more soluble in alcohol than the secretin. 

 If an acid decoction of duodenal mucous membrane is poured into abso- 

 lute alcohol, a precipitate is formed. If this precipitate is redissolved 

 in water and reprecipitated several times by absolute alcohol, then after 

 drying a white powder is obtained, which is easily soluble in water. The 

 resulting solution injected intravenously has a powerful secretory action, 

 but produces no effect on blood pressure. The concentrated alcoholic 



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