CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 397 



formed from bilirubin. As other special constituents of the 

 faeces may be mentioned excretin, a somewhat complex nitrog- 

 enous body, whose exact chemical nature is at present uncer- 

 tain, 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 fseces. 

 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. 



