SURVEY OF DIGESTION AS A WHOLE 419 



invariable, constituent of bile itself. A portion of it is absorbed by 

 the intestine and then excreted in the urine, the urobilin in which 

 is often much increased in fever (' febrile ' urobilin). No bilirubin 

 or biliverdin occurs in normal faeces, although pathologically they 

 may be present. The dark colour of the faeces with a meat diet is 

 due to haematin and sulphide of iron, the latter being formed by 

 the action of the sulphuretted hydrogen which is constantly present 

 in the large intestine on the organic compounds of iron contained in 

 the food or in the secretions of the alimentary canal. A small 

 amount of altered bile-acids and their products is also found; and 

 in respect to these, and to the altered pigments, bile is an excretion. 

 And although its entrance into the upper instead of the lower end 

 of the intestine, the ascertained importance of its function in diges- 

 tion, and the fact that the greater part of the bile-salts is reabsorbed, 

 show that in the adult it is very far from being solely a waste product, 

 the equally cogent fact, that the intestine of the new-born child 

 is filled with what is practically concentrated bile (meconium), 

 proves that it is just as far from being purely a digestive juice. 

 Skatol and other bodies, formed by putrefactive changes in the 

 proteins of the food, are also present in the faeces, and are responsible 

 for the faecal odour. Masses of bacteria are invariably present, and 

 often make up a very considerable proportion of the total faecal 

 solids. Of the inorganic substances in faeces the numerous crystals 

 of triple phosphate are the most characteristic. When the diet is 

 too large, or contains too much of a particular kind of food, a con- 

 siderable quantity of digestible material may be found in the faeces 

 e.g., muscular fibres and fat. But it should be remembered that 

 under all circumstances the composition of the faeces differs from 

 that of the food. The intestinal contribution is always an important 

 one, although relatively more important with a flesh than with a 

 vegetable diet. The purin bases normally found in human faeces 

 come both from the food directly and from the metabolism of 

 the tissues. They are increased in amount on a diet rich in 

 purin bodies (such as meat extract or thymus), but are also formed 

 on a diet like milk, from which purin bases cannot be obtained. 

 An interesting constituent of faeces on which light has recently been 

 thrown, especially by the researches of Gardner, is the so-called copro- 

 sterin (dihydrocholesterin), which appears to be produced from 

 cholesterin by reduction, probably under the influence of bacteria, 

 and perhaps also from the phytosterins of vegetable food. 



