DIGESTION AND ABSORPTION IN THE INTESTINES. 807 



the odor of the feces is mainly due. (5) Cholesterin, or a deriva- 

 tive, which is found always in small amounts, and is probably 

 derived from the bile. (6) Some of the purin bases, especially 

 guanin and adenin. (7) Mucus and epithelial cells thrown off 

 from the intestinal wall. (8) Pigment. In addition to the color 

 due to the undigested food or to the metallic compounds contained 

 in it, there is normally present in the feces a pigment, urobilin or 

 stercobilin, derived from the pigments (bilirubin) of the bile. 

 Urobilin is formed from the bilirubin by reduction in the large 

 intestine. (9) Inorganic salts — salts of sodium, potassium, 

 calcium, magnesium, and iron, but chiefly the last three together 

 with phosphoric acid. The significance of the calcium and iron 

 salts will be referred to in a subsequent chapter, when speaking 

 of their nutritive importance. (10) Micro-organisms. Great 

 quantities of bacteria of different kinds are found in the feces. 



In addition to the feces, there is found often in the large 

 intestine a quantity of gas that may also be eliminated thrc^ugh 

 the rectum. This gas varies in composition. The following 

 substances have been found at one time or another: CH4, CO2, 

 H, N, H2S. They arise mainly from the bacterial fermentation 

 of the proteins, although some of the N may be derived from air 

 swallowed with the food. 



