506 FAECES. [BOOK n. 



mucus in variable amount, sometimes albumin, cholesterin, butyric 

 and other fatty acids, lime and magnesia soaps, colouring matters, 

 and inorganic salts, especially earthy phosphates, crystals of 

 ammonia-magnesia phosphates being very conspicuous. The reac- 

 tion of the faeces is generally but not always acid. They also contain 

 a ferment similar in its action to pepsin, and an amylolytic ferment 

 similar to that of saliva or pancreatic juice. The bile salts are 

 represented by a small quantity of cholalic acid, or some product 

 of that body, and sometimes a very small quantity of taurin. The 

 glycin and most or all of the taurin have been absorbed from the 

 intestine, and the cholalic acid has been partly absorbed and partly 

 decomposed. The fact that the faeces become 'clay-coloured' 

 when the bile is cut off from the intestine shews that the bile- 

 pigment is at least the mother of the faecal pigment; and a 

 special pigment, which has been isolated and called stercobilin, is 

 said to be identical with the substance called urobilin, which may 

 be formed from bilirubin. As other special constituents of the 

 faeces may be mentioned excretin, a somewhat complex nitrogenous 

 body, whose exact chemical nature is at present uncertain, and 

 skatol (C 9 H 9 N), a nitrogenous body which like indol is derived 

 from the decomposition of proteids by means of micro-organisms, 

 and which is the chief cause of the faecal odour, since only a small 

 quantity of indol remains in the faeces. These odoriferous bodies 

 are derived directly from the food ; at the same time it is quite 

 possible that other specific odoriferous substances may be secreted 

 directly from the intestinal wall, especially from that of the large 

 intestine. 



