4 i 4 DIGESTION 



Reaction of Intestinal Contents. A consideration of the properties of 

 the indicators mentioned enables us to interpret in some measure these 

 results, which at first sight appear so confusing. Methyl orange, the 

 most stable of the series, is not affected by weak organic acids, but reacts 

 acid to inorganic, and the stronger organic acids like lactic, acetic and 

 butyric acids, and alkaline to salts of the weaker acids, such as sodium 

 carbonate and bicarbonate. Phenolphthalein is very sensitive to acids, 

 even to weak organic acids such as the fatty acids derived from the fat 

 of meat, and to carbonic acid. Litmus is intermediate between methyl 

 orange and phenolphthalein. The chyme, as it passes through the 

 pylorus, contains free hydrochloric acid. It mingles immediately with 

 the alkaline contents of the duodenum. If these contain a sufficient 

 quantity of bases to combine with the whole of the acids which would 

 affect methyl orange, that indicator will show a neutral or alkaline re- 

 action. Phenolphthalein may at the same time react acid on account of 

 the presence of weaker acids, including carbonic acid, either originally 

 dissolved in the intestinal fluid or liberated by the action of the acids 

 of the chyme on the carbonates. If there is not enough alkali to combine 

 with the whole of the stronger acids, the reaction will be at first acid to all 

 the indicators, but may soon become alkaline to methyl orange ov even 

 to litmus, as pancreatic juice and bile continue to enter the duodenum. 

 As the food progresses along the intestine a certain amount of lactic 

 acid is produced by the action of micro-organisms on the carbo- 

 hydrates. The alkalies of the intestinal secretions are being continually 

 used up, both to neutralize this acid, and to form soaps with the fatty 

 acids set free from the fats by the steapsin and the fat-splitting bacteria. 

 The point may easily be reached, and as a rule is reached, at which 

 enough of the weak acids or of acid salts is present to give an acid 

 reaction with phenolphthalein or litmus, while the reaction is still 

 alkaline to methyl orange. By the time the food has arrived at the 

 lower end of the small intestine the greater part of the fat -splitting 

 may be supposed to be over, and the greater part of the fatty acids 

 absorbed. The acids that remain may be easily neutralized by the 

 alkaline succus entericus, reinforced by the alkalies, especially am- 

 monia, produced by the ordinary putrefactive bacteria from proteins; 

 and the reaction, previously alkaline to methyl orange only, may thus 

 become alkaline to litmus as well. Dissolved carbonic acid will still 

 account for the acid reaction to phenolphthalein. Towards the end of 

 intestinal digestion the discharge of pancreatic juice, bile and succus 

 entericus having almost or entirely ceased, the acid-forming bacteria 

 appear again to get the upper hand ; and since the reaction is acid to 

 methyl orange as well as to the other indicators, we must assume that 

 strong organic acids, like lactic acid, are present. Very early in the 

 meal the inflow of alkaline pancreatic juice, and perhaps of succus 

 entericus, into the intestine begins; and for a considerable time this is 

 not counteracted by the escape of any large quantity of acid chyme 

 through the pylorus. We must accordingly suppose that the con- 

 ditions for the establishment of an alkaline reaction of the intestinal 

 contents are unfavourable at the end of intestinal digestion, and 

 favourable at the beginning of gastric digestion. 



This question of reaction has significance in two ways: in the first 

 place the reaction affects the activity of this or that ferment on 

 the food substances; and, secondly, it determines whether a given 

 ferment shall be destroyed or not by another ferment or by the 

 alkalinity or acidity of the medium. Thus pepsin can be destroyed 



