HIPPURIC ACID. 



363 



urates in the blood is probably too small to allow of this ques- 

 tion being solved. 



The source of uric acid is probably in the disintegrated ele- 

 ments of albuminous tissues. The relation which uric acid 

 and urea bear to each other is, however, still obscure. The 

 fact that they often exist together in the same urine, makes it 

 seem probable that they have different origins or different 

 offices to perform ; but the entire replacement of either by the 

 other, as of urea by uric acid in the urine of birds, serpents, 

 and many insects, and of uric acid by urea, in the urine of the 

 feline tribe of Mammalia, shows that each alone may dis- 

 charge all the important functions of the two. 



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 ex- 

 creted in excess, it is deposited in a crystalline form (Fig. 127), 

 mixed with large quantities of urate of ammonia or soda (Fig. 

 130). In such cases it may be procured for microscopic exam- 

 ination, by gently warming the portion of urine containing the 

 sediment ; this dissolves urate of ammonia and soda, 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 granules of urate of ammonia or soda. When 

 deposited in crystals, it is most frequently in rhombic or dia- 

 mond-shaped laminae, but other forms are not uncommon (Fig. 

 127). When deposited from urine, the crystals are generally 

 more or less deeply colored, by being combined with the color- 

 ing principles of the urine. 



Hippuric Add has long been known to exist in the urine of 

 herbivorous animals in combi- 

 nation with soda. Liebig has 

 shown that it also exists nat- 

 urally in the urine of man, 

 in quantity equal to the uric 

 acid, and Weismann's obser- 

 vations agree with this. It is 

 a nitrogenous compound with 

 the formula CgHgNOg. It is 

 closely allied to ben zoic acid ; 

 and this substance when in- 

 troduced into the system, is 

 excreted by the kidneys as 

 hippuric acid (Ure). * Its 

 source is not satisfactorily de- Crystals of hippuric acid. 



