DIGESTION AND ABSORPTION IN THE STOMACH. 779 



is reduced mainly by a regurgitation of the alkaline duodenal con- 

 tents which occurs at periods during digestion. He considers that 

 this regurgitation is a self-regulating mechanism for maintaining 

 the low acidity of the gastric contents. The normal occurrence of 

 regurgitation during gastric digestion is confirmed by several ob- 

 servers. Clinicians make a distinction between free and com- 

 bined acid in the gastric secretion. By the first term is meant 

 that the acid exists in solution as in so much water, and is, therefore, 

 largely dissociated with the production of a corresponding amount 

 of hydrogen ions. Under the second term is included the acid 

 that is combined in some way with the protein material. In 

 this form the acid is less dissociated and the acidity, that is to say, 

 the concentration of hydrogen ions, is much less. Methods have 

 been devised for estimating the total acidity and the free and com- 

 bined 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 physiological pur- 

 poses 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. This factor may be de- 

 termined by the use of hydrogen gas electrodes, the electrometric 

 method, or by the use of various indicators which give a change 

 of color at different concentrations of hydrogen ions, the colori- 

 metric method. Using the former method, Mentenf reports for 

 the normal juice as secreted a concentration in hydrogen ions 

 varying round 1 X 10" 1 , or a hydrogen exponent of pH = 1 (see 

 p. 417), Using the colorimetric method, Michaelis and Davidsohn t 

 report the acidity of the gastric contents after a test-meal as equal 

 to 1.7 X 1(T 2 (0.017). 



The Origin of the HCL 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- 



* Simon, "A Manual of Clinical Diagnosis." 

 f Menten, "Journal of Biological Chemistry/' 22, 341, 1915. 

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

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



