DIGESTION AND ABSORPTION IN THE INTESTINES. 721 



increased by an increase of the fats in the food or by a deficient 

 secretion of bile. (3) Products of the intestinal secretions. Evi- 

 dence has accumulated in recent years* to show that the feces in 

 man on an average diet are composed in part of the material of the 

 intestinal secretion. The nitrogen of the feces, formerly supposed to 

 represent only undigested food, seems rather to have its origin largely 

 in these secretions, and, therefore, like the nitrogen of the urine, 

 represents so much metabolism in the body. (4) Products of bac- 

 terial decomposition. The most characteristic of these products are 

 indol and skatol. They are crystalline bodies possessing a disagree- 

 able, fecal odor; this is especially true of skatol, to which the odor 

 of the feces is mainly due. (5) Cholesterin, which is found always 

 in small amounts, and is probably derived from the bile. (6) Ex- 

 cretin, a crystallizable, non-nitrogenous substance to which the 

 formula C 78 H 156 S0 2 has been assigned, is found in minute quantities. 



(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. The 

 significance of the calcium and iron salts will be referred to in a subse- 

 quent 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 through the rectum. 

 This gas varies in composition. The following substances have been 

 found at one time or another: CH 4 , C0 2 , H, N, H 2 S. They arise 

 mainly from the bacterial fermentation of the proteids, although 

 some of the N may be derived from air swallowed with the food. 



* Prausnitz, " Zeitschrift f. Biologie," 35, 335, 1897; and Tsuboi, ibid., 

 p. 68. 



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