476 HANDBOOK OP PHYSIOLOGY. 



The proportionate quantity of uric acid varies considerably in different 

 animals. In man, and Mammalia generally, especially the Herbivora, 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 unite of soda, of which the so-called chalk-stones 

 of this disease are principally composed. The average amount secreted 

 in twenty-four hours is about one-third of 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. The 

 uric acid exists as urate of soda, produced by the uric acid as soon as it 

 is formed combining with part of the base of the alkaline sodium phos- 

 phate 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 bases 

 sodium and ammonium takes place in the blood, or in the act of secre- 

 tion in the kidney: the latter is more likely; but the quantity of either 

 uric acid or urates in the blood is probably 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, e.g., hydrochloric or nitric. When excreted in excess, however, it 

 is deposited in a crystalline form (fig. 301), mixed with large quanti- 

 ties of ammonium or sodium urate. In such cases it may be procured 

 for microscopic examination by gently warming the portion of urine 

 containing the sediment; this dissolves urate of ammonium and sodium, 

 while the comparatively 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 gran- 

 ules 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. 301). When deposited from urine, the crystals 

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

 the coloring principles of the urine. 



Tests. There are two chief tests for uric acid besides the micro- 



