ORIGIN OF ACID SECRETIONS 327 



be reduced in diameter by the swelling of the droplets. As soon as the 

 droplets come to touch one another, however, any further increase in 

 their diameter will push their peripheries further apart and increase 

 the diameter of the interstitial pores. 



This will be clear from the accompanying diagram depicting the 

 three conditions indicated (Fig. 18). It can readily be seen, therefore, 

 how a lipoid-solvent may, in small doses, decrease the permeability of 

 cells for water-soluble substances and, in larger doses, increase it. 



THE ORIGIN OF ACID SECRETIONS. 



It has always been, until within very recent years, a fact exceedingly 

 puzzling to physiologists that certain secretory glands, particularly 

 the glands of the gastric mucosa and the "salivary" glands of car- 

 nivorous molluscs, elaborate a strongly acid secretion from an alkaline 

 medium, namely blood or other tissue-fluids. The alkalinity of the 

 medium was, of course, greatly overestimated by the earlier observers. 

 On the other hand, however, the results of the most exact measure- 

 ments show that the blood and tissue-fluids are on the alkaline side of 

 neutrality, while the acidity of gastric juice, of which the components 

 must in the long run have been derived from the blood, is comparable 

 with that of a hundredth-molecular solution of hydrochloric acid. 



The first hypothesis which was advanced in explanation of this 

 phenomenon is usually but erroneously attributed to the German 

 biological chemist, Maly, who published it in 1874. It actually 

 originated with an American, E. N. Horsford whose account of this 

 hypothesis is contained in an article contributed to the Proceedings of 

 the Royal Society of London in 1869. He observed that if a mixture 

 of neutral or even weakly alkaline salts, such as the Phosphates, be 

 enclosed within a parchment-membrane and allowed to diffuse through 

 it into distilled water, the water outside the membrane becomes acid 

 in reaction, while that within the membrane becomes correspondingly 

 more alkaline. This phenomenon is due to the fact that the diffusion- 

 velocity of acids is more rapid than that of the alkaline salts which 

 are formed within the .dialyzer. From the alkaline blood, containing 

 chlorides and phosphates, therefore, the acid hydrochloric juice was 

 supposed to arise in an analogous manner. The difficulty which con- 

 fronts this hypothesis is, however, that it proves too much, since by 

 parity of reasoning all the secretions of the tissues should be acid in 

 reaction, whereas, as a matter of fact, the majority of the secretions 

 resemble the blood in reaction or else, as in the case of the pancreatic 

 juice, are actually more alkaline than the blood. Moreover the effects 

 observed in the dialysis of salt mixtures are too small in magnitude to 

 account for the relatively high acidity of gastric juice. An alternative 

 hypothesis advanced by Koeppe is even more difficult of acceptance. 

 This investigator supposes that the gastric mucosa is permeable to 

 sodium ions but not for chlorine ions. As sodium ions in the food 



