DIGESTION 397 



invariably present, and often make up a very considerable pro- 

 portion 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 considerable 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 xanthin or 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 xanthin bases cannot be 

 obtained. 



