116 THE DIGESTIVE FLUIDS. 



of digestion, when hydrochloric acid is found in a free state, lactic 

 acid disappears. Its origin, under normal conditions at least, is 

 referable to the action of certain bacteria, such as the Bacterium 

 lactis, the Bacillus lactis aerogenes, etc., upon starches and sugars, 

 as represented by the equations : 



(1) 2C 5 H,A + H/) = C 12 H 22 O n 



Starch. Lactose. 



(2) C 12 H 22 C n + H 2 = 4C 3 H 6 3 



Lactose. Lactic acid. 



Other organic acids, such as butyric acid and acetic acid, are usu- 

 ally not found in the gastric contents unless large amounts of milk r 

 carbohydrates, or alcohol have been ingested. In such event, how- 

 ever, they may be present, and, like lactic acid, are then referable to 

 the action of certain micro-organisms. They are essentially of patho- 

 logical significance. 



It is thus seen that even during the process of digestion the 

 acidity of the gastric contents is, under normal conditions, scarcely 

 influenced by acids other than hydrochloric acid. It should 

 be noted, however, that following the ingestion of food hydro- 

 chloric acid does not appear in a free state at once, but only 

 after the affinities of the albuminous constituents of the food have 

 been saturated. We consequently find that while in the begin- 

 ning of digestion the acidity of the stomach-contents is largely 

 referable to such combined acid, in the later phases of digestion two 

 factors enter into consideration, viz., free and combined hydrochloric 

 acid. The period at which free acid appears as such varies, of 

 course, with the character of the meal, and directly with the amount 

 of proteids ingested. After the administration of Ewald's test- 

 breakfast free acid is thus found only at the expiration of about 

 thirty-five minutes ; while after the administration of RiegePs test- 

 dinner, which contains much larger amounts of albumin, two hours 

 must elapse before free acid can be demonstrated. 



Acid salts, finally, play only a small part in determining the total 

 acidity of the gastric juice ; and it is thus clear that unless carbo- 

 hydrates, much fat, and alcohol have been ingested, hydrochloric 

 acid, either in a free state or in combination with albumin, or both, 

 is the sole factor which enters into consideration. Under pathological 

 conditions, on the other hand, lactic acid, butyric acid, and acetic 

 acid may also play a part ; but then hydrochloric acid is usually not 

 present, and the acidity of the gastric contents is hence largely 

 referable to fermentative changes which have taken place in the 

 stomach. 



Determination of the Total Acidity of the Gastric Contents. 

 Five or 10 c.c. of the filtered gastric contents are titrated with a 

 decinormal solution of sodium hydrate, using phenolphthalein as an 

 indicator, until the rose color, which appears on the addition of each 

 drop of the sodium hydrate solution, no longer disappears on stirring 

 or is intensified by the addition of a further drop. The number o.f 



