288 APPLIED PHYSIOLOGY 



be acid, the bilirubin may be converted into biliverdin, 

 and they then become green. The usual amount of 

 water in the faeces is about 75 per cent., but if there 

 be any delay in the large intestine, the stools may 

 become much drier, as is often seen in constipation. 

 The bulkiness of the stools on a vegetable diet is largely 

 due to the presence of an excess of moisture. 



As will be seen from what has been said, the influence 

 of the food upon the physical and chemical character 

 of the stools is so great that it is necessary, when one 

 wishes to investigate the digestive processes in the intes- 

 tine clinically, to put the patient upon a standard or 

 ' test diet,' the character of the faeces that result from 

 which in health is known. 



The intestine is also responsible for the excretion of the 

 greater part of the calcium which is absorbed from the 

 food, and crystals of calcium phosphate are often found 

 even in normal faeces, and make up a large part of the 

 ' intestinal sand ' sometimes excreted in cases of disease. 



In some pathological conditions the intestine seems to 

 be unable to excrete calcium, which then appears in the 

 urine in abnormal quantity, producing one variety of 

 so-called ' phosphaturia ' (see p. 281). 



Iron is also mainly excreted by the intestine, and so 

 probably is copper. The recovery of these metals from 

 the stools is therefore no proof that they have not been 

 absorbed and passed through the body, although the 

 artificial colouring of vegetables with sulphate of copper 

 has sometimes been excused on the ground that most 

 of the metal can be recovered from the stools, and has 

 therefore not been absorbed. 



