120 THE PHYSIOLOGY OF DIGESTION AND ABSORPTION 



form. The consistence of the fecal matter increases in its pas- 

 sage through the colon, owing to the absorption of its more fluid 

 portions. The bile pigment is responsible for the character- 

 istic color. The odor is mainly due to bacterial decomposition, 

 but partly to the secretion of the mucous membrane. 



Bacteria in Intestinal Digestion. The entrance of the bile 

 and pancreatic juice into the duodenum changes to alkaline 

 the previously acid reaction of the chyme. But it is found that, 

 when an ordinary mixed diet is given, the mass leaving the ileo- 

 cecal valve has an acid (proteid) reaction, and that the proteids 

 have not undergone putrefaction. The alkaline medium of the 

 upper intestine favors bacterial activity, and it would seem that 

 proteid putrefaction would ensue. But it is supposed that in 

 health these bacteria set up fermentative changes in the carbohy- 

 drates, with the production of acids which inhibit proteid putre- 

 faction, and account for the acid reaction at the ileo-cecal valve. 

 When the mass has entered the colon the acidity is soon overcome 

 and putrefaction is the Usual consequence. It can be seen how 

 readily this delicately adjusted balance may be disturbed by 

 errors in the proper kind and proportion of food, etc. Some of 

 the products of bacterial activity upon carbohydrates and proteids 

 are leucin, tyrosin, indol, skatol, phenol, lactic and butyric acid. 

 The object of the production of these substances is unknown. 



Composition of Feces. It seems at present that the main 

 bulk of fecal matter is made up of substances which are contained 

 in the intestinal secretions, and the alimentary canal is more im- 

 portant in excretion than was formerly supposed. These sub- 

 stances are waste matters from tissue metabolism. Besides these 

 materials, the feces normally contain indigestible and undigested 

 matters, inactive salts, stercorin, mucus, epithelium from the 

 intestinal wall, coloring matter and substances resulting from 

 bacterial activity. Stercorin is the converted form of cholesterin, 

 a constituent of the bile. The coloring matter is from the 

 pigment (bilimbiri) of the same fluid. Of the bacterial products 



