THE FOOD AND DIGESTION 353 



sulphuric acid. The fate of the cholalic acid is not known, 

 but it is supposed to be excreted in the faeces. 2. The 

 pigments undergo a change and lose their power of giving 

 Gmelin's reaction. They appear in the faeces as what may 

 be called stercobilin. It is probably formed by reduction of 

 bilirubin in the intestines as the result of the action of 

 micro-organisms. 3. The cholesterin is passed out in the 

 faeces. 



Faeces. 



The materials not absorbed from the intestine, whether 

 these are derived from the food or from the alimentary 

 canal, are thrown off from the rectum as the faeces. In 

 fasting animals these are passed at long intervals, and 

 consist of mucin, shed epithelium, the various products 

 of the bile constituents, and inorganic salts. In feed- 

 ing animals the amount and character of the faeces de- 

 pends largely upon the amount and character of the food, 

 and upon the bacteria which are growing in the large in- 

 testine. The unabsorbed material, as it passes down the 

 large intestine, becomes inspissated from the absorption of 

 water, but, if much undigested matter is present, water 

 may also be added, and the consistence of the faeces may 

 thus be varied. In the average condition they contain 

 about 70 or 80 per cent, of water. The colour is normally 

 brown, from the haematin of the flesh eaten, while the 

 sulphide of iron formed by the splitting of the haematin 

 compounds in the intestine may make them darker in 

 colour. On a milk diet they are light yellow in colour, 

 and if a large excess of fatty food is taken, or if fat is not 

 absorbed, as in jaundice, they become clay coloured. 



The derivatives of the bile pigments play but a small part 

 in colouring the fasces. In infants, before bacteria are 

 introduced and begin to exert their reducing action, the 

 faeces may be green from the presence of unaltered biliverdin. 

 The reaction of the faeces varies. Usually the outside of the 

 mass is alkaline from the alkaline secretion of the intestine, 

 while the inside is acid from the free fatty acids and other 

 acids formed by the action of such acid-forming bacteria as 



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