386 ESSENTIALS OF PHYSIOLOGY. 



characteristic whetstone or dumb-bell shape. It is almost completely 

 insoluble in water, but dissolves in weak alkalies. It slightly reduces 

 Fehling's solution (p. 387), and will also reduce an alkaline solution 

 of silver nitrate (Schiff's test). A solution of uric acid, when evaporated 

 to dryness with nitric acid at a low temperature, yields a purple 

 colour on the subsequent addition of an alkali (murexide test). 



The uric acid is derived partly from the nuclein in food and partly 

 from the breaking down of the tissue nucleins. 



In 'addition to uric acid, small amounts of the purine bases are 

 found in urine. 



Hippuric acid is synthesised in the kidney from benzoic acid and 

 glycine, the synthesis being brought about by an enzyme. It is the 

 only urinary constituent which is formed in the kidney itself. 



Sulphates. The sulphates are of two kinds, namely, (1) inorganic, 

 and (2) ethereal. The latter are compounds of sulphuric acid with 

 phenol, indoxyl, or skatoxyl, and potassium. Indol and skatol are 

 formed from tryptophane by bacterial action in the digestive tract ; 

 and after absorption into the blood they are converted by oxidation 

 into indoxyl and skatoxyl, combined with sulphuric acid, and excreted 

 by the kidneys. Phenol, also, is a product of protein decomposition. 



As a rule the ethereal sulphates form about one-tenth of the total 

 sulphates, but when bacterial changes in the digestive tract become 

 excessive (e.g. in constipation), the proportion of ethereal sulphates is 

 increased. 



Some sulphur is also excreted in organic combination and is known 

 as "neutral" sulphur. 



The sulphur of the urinary sulphates is formed almost entirely by 

 the oxidation of the sulphur contained in protein, and the total 

 amount of sulphates, varies therefore with the quantity of protein food 

 ingested. 



Urine also contains sodium chloride and phosphates, the latter 

 being of two kinds, namely, (1) alkaline phosphates, of sodium and 

 potassium, and (2) earthy phosphates, of calcium and magnesium. 



URINARY DEPOSITS. 



On standing, normal urine deposits a cloud of mucus (nucleo- 

 protein) derived from the urinary passages. When the urine is 

 concentrated, biurates of sodium and potassium are often deposited as 

 an amorphous sediment, coloured pink by uroerythrin, and dissolving 

 when warmed. Earthy phosphates are deposited from neutral or 

 alkaline urine ; they dissolve on the addition of acetic acid. Crystalline 



