STRUCTURE AND FUNCTION OF THE KIDNEYS. 367 



and a little more so in hot water, quite insoluble in alcohol and ether. 

 It dissolves freely in solution of the alkaline carbonates and other salts. 



The proportionate quantity of uric acid varies considerably in differ- 

 ent animals. In man, and Mammalia generally, especially the Herbi- 

 vora, it is comparatively small. In the whole tribe of birds, and of 

 serpents, on the other hand, the quantity is very large, greatly exceeding 

 that of the urea. In the urine of granivorous birds, indeed, urea is 

 rarely if ever found, its place being entirely supplied by uric acid. 



Variations in Quantity. The quantity of uric acid, like that of 

 urea, in human urine, is increased by the use of animal food, and de- 

 creased by the use of food free from nitrogen, or by an exclusively vege- 

 table diet. In most febrile diseases, and in plethora, it is formed in 

 unnaturally large quantities; and in gout it is deposited in and around 

 joints, in the form of urate of soda, of which the so-called chalk-stones 

 of this disease are principally composed. The average amount secreted 

 in twenty-four hours is 8.5 grains (rather more than half a gramme). 



Condition in the Urine. The condition in which uric acid exists in 

 solution in the urine has formed the subject of some discussion, because 

 of its difficult solubility in water. The uric acid exists as urate of soda, 

 produced by the uric acid as soon as it it is formed combining with part 

 of the base of the alkaline sodium phosphate of the blood. Hippuric 

 acid, which exists in human urine also, acts upon the alkaline phosphate 

 in the same way, and increases still more the quantity of acid phosphate, 

 on the presence of which it is probable that a part of the natural acidity 

 of the urine depends. It is scarcely possible to say whether the union 

 of uric acid with the base sodium, and probably ammonium, takes place 

 in the blood, or in the act of secretion in the kidney: the latter is more 

 likely; but the quantity of either uric acid or urates in the blood is proba- 

 bly too small to allow of this question being solved. 



Owing to its existence in combination in healthy urine, uric acid for 

 examination must generally be precipitated from its bases by a stronger 

 acid. Frequently, however, when excreted in excess, it is deposited in 

 a crystalline form (Fig. 254), mixed with large quantities of ammonium 

 or sodium urate. In such cases it may be procured for microscopic ex- 

 amination by gently warming the portion of urine containing the sedi- 

 ment; this dissolves urate of ammonium and sodium, while the compara- 

 tively insoluble crystals of uric acid subside to the bottom. 



The most common form in which uric acid is deposited in urine, is 

 that of a brownish or yellowish powdery substance, consisting of granules 

 of ammonium or sodium urate. When deposited in crystals, it is most 

 frequently in rhombic or diamond-shaped laminae, but other forms are 

 not uncommon (Fig. 254). When deposited from urine, the crystals are 

 generally more or less deeply colored, from being combined with the 

 coloring principles of the urine. 



