300 CHANGES OF FOOD IN THE FAECES. [BOOK n. 



containing sulphur, obtained by Marcet), colouring matters, and 

 salts, especially those of magnesia. Cholalic acid (and dyslysin) 

 are found in very small quantities only, thus indicating that the 

 bile-salts have been in part at least destroyed (they may have 

 been in part reabsorbed, see p. 280), the less stable taurocholic 

 acid (of the dog) disappearing more readily than the glycocholic 

 acid (of the cow). 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 1 . We have already seen that during artificial 

 pancreatic digestion, a distinctly faecal odour due to the presence of 

 indol is generated ; and the fact that the presence of bacteria, or 

 other similar organisms, is essential to the production of this body, 

 does not preclude the possibility of it (or of the allied body skatol, 

 having an evil faecal odour, formed after prolonged putrefaction of 

 the pancreas and present in human excrement) being the chief 

 cause of the natural odour of faeces, for undoubtedly bacteria may 

 exist throughout the whole length of the intestinal canal. At the 

 same time it is quite possible, that specific odoriferous substances 

 may be secreted directly from the intestinal wall, especially from 

 that of the large intestine. 



1 See Appendix. 



V 



