EXCRETION 281 



stant. It is retained in the body whenever water is 

 being retained (e.g., in dropsy), and also, although for 

 unknown reasons, in some febrile conditions. The 

 phosphates of the urine are derived mainly from the 

 food, but also to some extent from the decomposition 

 of phosphorus compounds e.g., nuclein, in metabolism. 

 The condition termed * phosphaturia ' is really in most 

 cases not an indication of an excessive excretion of 

 phosphorus, but of a diminished acidity of the urine, 

 which causes the earthy phosphates to be thrown out. 

 In some instances it appears to be the result of an 

 excessive excretion of calcium by the urine, so that the 

 normal mono-di-calcium phosphates are replaced by the 

 much less soluble tri-calcium salt. It would appear, too, 

 that the amount of phosphates in the urine is determined 

 to some extent by the amount of calcium in the food. 

 When this is high, as in herbivora, phosphorus is excreted 

 by the bowel as calcium phosphate. 



Oxalic acid is present in the urine to a small extent, 

 chiefly combined with calcium. It is derived partly 

 from vegetable foods, of which some, such as rhubarb 

 and spinach, are specially rich in it, and is partly pro- 

 duced in the body in some unknown way as the result 

 of metabolic processes. It cannot, therefore, be made 

 to disappear entirely from the urine even if the diet be 

 free from it. The presence of magnesium salts in the 

 urine helps to keep it in solution, and poverty of these 

 is one cause of * oxaluria.' Of the origin of normal 

 urine pigment (urochrome) we know nothing, although 

 it is probably derived somehow from haemoglobin, and it 

 is therefore of little clinical interest. Urobilin is present 



