SURVEY OF DIGESTION AS A WHOLE 425 



solids. Of the inorganic substances in faeces the numerous crystals 

 ot 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 fasces 

 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. 



