I2O THE PHYSIOLOGY OF DIGESTION AND ABSORPTION 



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 decomposi- 

 tion, 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 alka- 

 line 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 ac- 

 tivity, and it would seem that proteid putrefaction would en- 

 sue. But it is supposed that in health these bacteria set up 

 fermentative changes in the carbohydrates, with the produc- 

 tion of acids which inhibit proteid putrefaction, and account 

 for the acid reaction at the ileo-cecal valve. When the mass 

 has entered the colon the acidity is soon overcome and putre- 

 faction 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 pro- 

 teids are luecin, tyrosin, indol, skatol, phenol, lactic and bu- 

 tyric 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 con- 

 tained in the intestinal secretions, and the alimentary canal is 

 more important in excretion than was formerly supposed. 

 These substances are waste matters from tissue metabolism. 

 Besides these materials, feces normally contain indigestible 

 and undigested matters, inactive salts, stercorin, mucus, epi- 

 thelium from the intestinal wall, coloring matter and sub- 

 stances resulting from bacterial activity. Stercorin is the 

 converted form of cholesterin, a constituent of the bile. The 

 coloring matter is from the pigment (bilirubin) of the same 

 fluid. Of the bacterial products the most important are in- 



