354 A MANUAL OF PHYSIOLOGY 



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 of the 

 chyme the reaction will be at first acid to all the indicators, but may 

 soon become alkaline to methyl orange or even to litmus, as pan- 

 creatic juice and bile continue to enter the duodenum. As the food^ 

 progresses along the intestine a certain amount of lactic acid is pro- 

 duced by the action of micro-organisms on the carbo-hydrates, and 

 perhaps also on the proteids. 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 putrefactive bacteria 

 from proteids ; 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. To- 

 wards 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, 

 me must assume that strong organic acids, like lactic acid, are 

 present. On the entrance of food into the stomach 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 

 unfavourable at the end of intestinal digestion, and favourable at the 

 beginning of gastric digestion. 



Trypsin, like pepsin, performs its work in part in an acid 

 medium; and although the cause of the acidity and the 

 character of the medium are far from being the same as in 

 the gastric juice, it is obviously an advantage that the chief" 

 proteolytic ferment should be able to act upon the proteids 

 in all parts of the intestine and at every stage of intestinal, 

 digestion whether the reaction is acid or alkaline. The 

 proteids of the chyme are all carried by the trypsin to the 

 stage of peptone. Much of the peptone is absorbed as it is 

 formed, but some, even in perfectly normal digestion, is 

 further split up into leucin, tyrosin, and similar bodies. 



