768 PHYSIOLOGY OF DIGESTION AND SECRETION. 



states that the acidity is reduced mainly by a regurgitation of the 

 alkahne duodenal contents which occurs at periods during diges- 

 tion. He considers that this regurgitation is a self -regulating 

 mechanism for maintaining the low acidity of the gastric con- 

 tents. The normal occurrence of regurgitation during gastric 

 digestion is confirmed by several observers. Chnicians make a 

 distinction between free and combined acid in the gastric secre- 

 tion. 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. Un- 

 der the second term is included the acid that is combined in some 

 way with the protein material. In this form the acid is less dis- 

 sociated 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 combined acid.* 

 For physiological 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 concentra- 

 tion, since the activity of the pepsin is controlled by this factor. 

 The hydrogen-ion concentration may be determined 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 colorimetric method. Using 

 the former method, Mentenf reports for the normal juice as 

 secreted a concentration in hydrogen ions varying round 1 X 10~^' 

 or a hydrogen exponent of pH = 1 (see p. 415), which is equal to 

 a tenth-normal solution. Others give a lower value to the acidity 

 of the normal juice, pH 1.3. Experiments show that an acidity 

 as high as this is not favorable to the digestive action of pepsin, 

 and, as a matter of fact, it is found, as was stated above, that in 

 ordinary digestion the contents of the stomach exhibit always a 

 lower acidity. It is fair to assume that this reduction in the 

 acidity is not a matter of chance, but is an adaptation properly 

 regulated to ensure the rnost favorable conditions. According to 

 Michaelis and DavidsohnJ the acidity of the gastric contents after 

 a test-meal is equal to a concentration in H-ions of 0.017 n or 

 pH 1.67. The optimum acidity for peptic digestion is placed at 

 about pH 1.6. Concentrations of acid above or below this point 

 would, therefore, present less favorable conditions for digestion. 

 When there is no free HCl in the gastric contents, the acid all 

 being combined with protein, the acidity is low, the H-ion con- 

 centration not rising above pH 3, that is, a thousandth normal 



* Simon, "A Manual of Clinical Diagnosis." 

 t Menten, "Journal of Biological Chemistry," 22, 341, 1915. 

 i Michaelis and David?ohn, "Zeitschrift f. exp. Pathol.," 8, 398, 1911; 

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



