DIGESTION AND ABSORPTION IN THE STOMACH. 773 



free and combined acid.* The application of these methods has 

 shown that after a protein (meat) diet so-called free acid may not 

 appear in the gastric contents for an hour or more. For physio- 

 logical purposes it is preferable to abandon the use of the terms free 

 and combined acid, and instead to express the degree of acidity in 

 terms of the actual hydrogen-ion concentration. To determine 

 this factor physiochemical methods of some complexity are re- 

 quired, but it has been shown that in place of these methods a 

 series of indicators may be employed which exhibit a range of 

 color changes for concentrations of hydrogen ions that lie within 

 the limits to be expected from specimens of gastric juice. For 

 example, the indicators, methyl-violet, Congo-red, tropaeolin, and 

 methyl-orange, may be used to detect variations between 0.03 and 

 0.00001 n. Making use of these methods, Michaelis and David- 

 sohnj find that after a test-meal the hydrogen-ion concentration of 

 the gastric contents is equal to 1.7 X 10~ 2 (0.017), which would 



60 

 correspond to about a solution of acid, a solution containing, 



therefore, less than 0.1 per cent, of hydrochloric acid. Since this 

 gastric juice at the time of secretion contains HC1 in the concentra- 

 tion of 0.5 per cent, it is evident that during digestion the acidity 

 is reduced to a much lower level by dilution, and by neutralization 

 by means of the alkaline salts and the proteins of the foods and the 

 secretions. 



The Origin of the HC1. That the acid of the gastric juice is a 

 mineral acid and is present in considerable strength is a remarkable 

 fact that has excited much interest. Attempts have been made to 

 ascertain the histological elements concerned in its secretion and 

 the nature of the chemical reaction or reactions by which it is pro- 

 duced. With regard to the first point, it is generally believed that 

 the parietal cells of the gastric tubules constitute the acid-secreting 

 cells. This belief is founded upon the general fact that in the 

 regions in which these cells are chiefly present that is, the middle 

 region of the stomach the secretion is distinctly acid, and where 

 they are absent or scanty in number the secretion is alkaline or less 

 acid. In the pyloric region, for instance, these cells are lacking 

 entirely and the secretion is alkaline. Moreover microchemical 

 reactions seem to show clearly that the parietal cells are particu- 

 larly rich in chlorids, and this fact serves to connect them with 

 the production of the acid. It seems perfectly evident that the 

 HC1 must be formed in the long run from the chlorids of the 

 blood. The chief chlorid is NaCl, and by some means this com- 



* Simon, "A Manual of Clinical Diagnosis." 



f Michaelis and Davidsohn, "Zeitschrift f. exp. Pathol.," 8, 398, 1911: also 

 Boldyreff, "Quarterly Journal of Exp. Physiol.," 8, 1, 1914. 



