THE CHEMISTRY OF URINE. 231 



is very constant from day to day, and is independent of the diet. 

 It is largely a product of the metabolism of the body tissues. 



Uric Acid. Uric acid is a purine body and its relationship 

 to the other purines, and its mode of formation and significance 

 are fully discussed in the chapter on metabolism (p. 110). It is 

 relatively insoluble in water, and when allowed to crystalize it 

 forms small' rhombic crystals. It can unite with an alkali, such 

 as sodium hydroxide, to form two salts : a neutral or diurate of 

 sodium (C 5 H,N 4 3 Na 2 ) and the biurate or acid urate of sodium 

 (C 5 H 2 N 4 3 HNa). The biurates are neutral in reaction and con- 

 stitute the urates normally found in the blood and urine. They 

 exist in two isomeric forms .(a and the &). The & is more solu- 

 ble than the a form. It may be that the deposition of urate tar- 

 tar on the teeth, and the deposits of urates in the joints of a pa- 

 tient suffering with gout, are due to the change of the b form 

 into the less soluble a type. 



There are a number of other nitrogenous bodies in the urine 

 which are included in the item of unclassified nitrogen in the 

 above analysis. The most important of these is urinary indican, 

 which is derived from the indol produced in the intestines by the 

 action of bacteria on the amino acid trytophane. The yellow 

 color of the urine is produced by a pigment called urochrome, 

 which is believed to be derived from the pigments in the blood. 



THE INORGANIC CONSTITUENTS OP THE URINE. The urinary 

 salts are chiefly the chlorides, sulphates and phosphates of so- 

 dium, potassium, calcium and magnesium. The potassium and 

 sodium salts are found in greatest abundance, since they form 

 the main inorganic constituent of the food, and moreover the 

 greater portion of the salts of the heavier metals, as calcium, iron, 

 bismuth, mercury, etc., is excreted by the intestines. There is 

 very little retention of salts by the body except during the for- 

 nation of bone, so that the amount of the inorganic constituents of 

 urine varies from day to day with the diet. The chlorides are 

 formed for the most part from the inorganic chlorides of the 

 food; the phosphates and the sulphates are derived from the sul- 

 phur and phosphorus of the nucleo-protein molecules. If the 

 urine is neutral or alkaline in reaction, there is apt to be a dc- 



