THE DIGESTIVE SECRETIONS 937 



discharge of the acid gastric contents, this precursor is changed into 

 the active secretin, which is then conveyed to the pancreas in the blood- 

 stream'. This organ reacts to it by furnishing an alkaline secretion 

 which in turn neutralizes the acid chyme, thereby setting, a limit to 

 the formation of secretin. The succeeding discharge of chyme gives 

 rise to another stimulation by secretin, and so on until the stomach is 

 empty. The latter point has been proved by Enriquez and Hallion^ 

 in this way. An anastomosis was formed between the arteries and 

 veins of two dogs. Cannulas having been inserted in the pancreatic 

 ducts of both animals, one of them then received an injection of a di- 

 lute acid into the duodenum. Peculiarly enough, a copious secretion 

 of pancreatic juice resulted in both animals. This experiment proves 

 very clearly that secretin is a true chemical messenger and is actively 

 engaged in the formation of normal pancreatic juice. 



Inasmuch as it has been noted that secretin produces at first a 

 very copious flow, which, however, is soon greatly diminished, ^nd that 

 a second dose of this agent does not evoke so pronounced an effect as 

 the first, it may be conjectured that this hormone possesses the func- 

 tion of exciting the pancreatic secretion in the shortest possible time. 

 Accordingly, it may then be assumed that it is the purpose of the nerv- 

 ous mechanism to sustain this secretion as long as required. This 

 view finds support in the fact that the secretion obtained upon stimula- 

 tion of the vagus nerve is characterized by a long latent period, and 

 that the vagus-juice is less watery but contains a larger amount of 

 organic matter than the secre tin-juice. In analogy with salivary 

 and gastric secretion, these differences may be explained by assuming 

 that the nervous and chemical factors just mentioned affect different 

 constituents of the pancreatic acini, one of them, possibly, stimulating 

 the cells of the acini proper, and the other the cells nearer the excretory 

 duct. 



The chemical nature of secretin is still unknown. It is not a fer- 

 ment and may be prepared in the following manner: The mucous 

 membrane of the duodenum is ground up with sand and boiled with 

 0.4 per cent, hydrochloric acid. The filtrate contains the secretin. 

 It is rendered neutral under addition of sodium hydrate. Dale and 

 Laidlaw^ harden the mucosa in HgCl2, boil it, reject the filtrate and 

 extract the residue with a 2 per cent, solution of acetic acid, containing 

 1 per cent, of HgCl2. The filtrate is precipitated by the addition of 

 NaOH to the neutral point. Secretin is stable in acid solutions, but is 

 rapidly oxidized in alkaline, and neutral solutions. Its action is not 

 destroyed by atropin, although this agent paralyzes the secretory 

 mechanism of the vagus. In this connection, brief reference should 

 also be made to the fact that a flow of pancreatic juice may be excited 

 by means of pilocarpin, Witters peptone, and curarin (Popielski). 



1 Compt. rend., Iv, 1903, 233. 



2 Proc. Physiol. Soc, Jour, of Physiol., xUv, 1912. 



