DIGESTION 393 



stages of intestinal digestion and at different levels of the gut 

 are always slight. There is never a great preponderance either 

 of hydroxyl or of hydrogen ions between the point at which 

 the pancreatic juice and bile are mingled with the gastric chyme 

 and the lower part of the ileum. 



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 inter- 

 mediate 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 reaction. 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 or even 

 to litmus, as pancreatic juice and bile continue to enter the duo- 

 denum. 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 ammonia, produced by the ordinary putrefac- 

 tive 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 dis- 

 charge 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 conditions for the 

 establishment of an alkaline reaction of the intestinal contents are 



