DIGESTION 351 



That some pepsin is secreted by the pyloric end of the stomach 

 is not difficult to prove. The secretion collected from the isolated 

 pyloric portion is, indeed, like the secretion of the Brunner's 

 glands in the duodenum, quite unable to digest protein until 

 dilute hydrochloric acid is added. But this is because in both cases 

 the juice as it flows from the glands is slightly alkaline, and, as we 

 have already seen, pepsin only acts in the presence of an acid. 

 The milk-curdling action of these two juices also unfolds itself 

 only when the secretions are first acidulated, and later on again 

 neutralized ; in other words, the ferments must be activated by 

 the addition of an acid. In normal digestion the pepsin of the 

 (in itself) alkaline secretion of the pyloric end of the stomach 

 becomes a constituent of the acid gastric juice ; and it may, 

 perhaps, be considered a morphological accident, so to speak, 

 that the oxyntic cells of the fundus should mingle their acid 

 products with the (presumedly) alkaline secretion of the chief 

 cells in the lumen of each gland-tube, instead of being massed 

 as a separate organ with a special duct. 



We are not without other examples of digestive juices fitted or 

 destined to act in a medium with an opposite reaction to their 

 own. The ' saliva ' of the cephalopod Octopus macropus, strongly 

 acid though it is, contains a proteolytic ferment which in vitro 

 acts, like trypsin, better in an alkaline than in an acid solution. 

 And trypsin, whose precursor is a constituent of the alkaline 

 pancreatic juice and the enterokinase which activates it, a con- 

 stituent of the alkaline succus entericus, performs a part at least 

 of its work in an acid medium. 



Attempts made to demonstrate an acid reaction in the border cells 

 have hitherto failed, perhaps because the acid is poured into the 

 ducts as fast as it is formed. But it may be mentioned, although 

 only as a matter of historical interest, that some observers have 

 denied that the acid is secreted in the depths of any cell from the 

 chlorides of the blood, and have asserted that it is formed at the 

 surface of contact of the stomach-wall with the gastric contents from 

 the sodium chloride of the food by an exchange of sodium ions 

 (p. 400) for hydrogen ions from the blood or lymph. It was pointed 

 out in favour of this view that when, instead of sodium chloride, 

 sodium bromide is given in the food, the hydrochloric acid in the 

 stomach is to a large extent replaced by hydrobromic acid. And 

 it was argued that this cannot be due to the decomposition of the 

 bromide by hydrochloric acid, since it occurs in animals deprived 

 for a considerable time of salts, and in ' salt-hunger ' the stomach 

 contains no acid (Koeppe). It may be, however, that even in ' salt- 

 hunger ' the presence of sodium bromide in the stomach stimulates 

 the secretion of hydrochloric acid, which then decomposes the 

 bromide, with the formation of hydrobromic acid. The sodium 

 chloride formed in the double decomposition might be re-absorbed, 

 and the stock of chlorides in the blood remain undiminished. It 

 is in any case a decisive objection to this now defunct theory that 

 a copious secretion of gastric juice, containing hydrochloric acid in 





