122 CONTENTS OF THE INTESTINAL CANAL. 



always apparent in the duodenum and jejunum ; in the ileum it 

 begins to diminish, so that for a great extent before we reach the 

 ceecum, it has often entirely disappeared. As a general rule, the 

 contents of the large intestine are alkaline; it very often, how- 

 ever, happens (as has been previously mentioned) that the inner 

 portions of the contents are still strongly acid, while the outer parts, 

 moistened or permeated with the alkaline intestinal juice, are 

 neutral or alkaline. This acid reaction is usually dependent on 

 the presence of lactic acid, but occasionally on that of butyric, 

 acetic or other acids. The sources of the lactic acid are, however, 

 very various, being dependent both on the nature of the food 

 that has been taken, and on the part of the intestine from which the 

 mass has been obtained. In the duodenum, where, notwithstand- 

 ing the access of bile and pancreatic juice, a strong acid reaction is 

 observed, the free acid depends chiefly on the acid of the gastric 

 juice, whatever kind of food may have been taken ; after the use 

 of flesh, sour milk, or acidified food, the acid of the food naturally 

 takes part in the reaction of the contents. In the normal state, it 

 cannot depend on a lactic fermentation, or on any other acid fer- 

 mentation, since any such fermentation is prevented by the normal 

 gastric juice. On the other hand, it is generally only found in the 

 lower part of the small intestine, and in the large intestine after 

 the use of amylaceous substances ; hence, we must conclude that 

 here the reaction is not dependent on the digestive juices, but on 

 the metamorphosed starch. That the free acid which occurs there, is 

 lactic acid, may be readily proved by analysis (see vol. 1, p. 95). 

 But in the normal condition both starch and sugar are converted 

 in the ileum and the rectum into lactic acid. Moreover, as 

 Frerichs has shown, the lactic acid sometimes becomes trans- 

 formed into butyric acid in these parts, when all other relations 

 seem perfectly normal. Among the free acids occurring in the 

 small intestine, but exerting less influence on the reaction of its 

 contents, we may mention cholic, glycocholic, and choloidic 

 acids. Frerichs has very thoroughly traced the changes which the 

 biliary constituents undergo in the intestinal canal, and has 

 proved that in the large intestine for the most part we find only 

 dyslysin, but sometimes also a little cholic or choloidic acid. 



As a general rule we can, by means of Pettenkofer's test, trace 

 the presence of the resinous constituents of the bile as far as the 

 lower extremity of the ileum. (See vol. 1, p. 125.) 



Among the less soluble substances which we may extract from 

 the contents of the intestine, we very often meet with grape-sugar 



