SECRETION. 449 



excessive, it very frequently happens that the acid is deposited in 

 the form of urinary calculi and gravel. 



To increase the excretion of uric acid, the best means is to 

 increase the secretion of urine by copious draughts of water. 



MUREXIDE TEST. Slowly and gently heat some urine and nitric 

 acid in a porcelain dish to the point of dryness. Decomposition 

 takes place, the color changing to yellow, and N and C0 2 are given 

 off. After allowing the yellow stain to cool, add a drop of dilute 

 ammonia-water to it, when there will be formed with the uric acid 

 a purplish-red color of murexide. On the addition of caustic potash 

 the color becomes a marked blue. 



Hippuric Acid (C 9 H 9 N0 3 ), which, in the herbivora, is the prin- 

 cipal representative of nitrogenized regression, is scarce in human 

 urine. In the latter it appears chiefly after the use of some fruits, 

 such as apples, plums, and grapes. 



Hippuric acid is the product of the coupling of glycocin with 

 benzoic acid. It may also be formed in the kidney itself. It is 

 monobasic, very slightly soluble in cold water and ether, and readily 

 soluble in warm water and alcohol. It crystallizes in vertical rhom- 

 bic prisms, is of a bitterish flavor, and is acid in reaction. When 

 decomposed by heating with acids and alkalies, or when transformed 

 by animal ferments, hippuric acid resolves itself into its components : 

 benzoic acid and glycocin. Ingested benzoic acid and oil of bitter 

 almonds are eliminated with the urine as hippuric acid. 



Some of the hippuric acid, at least, is the product of the activity 

 of the secreting cells of the renal tubules., as is demonstrated by per- 

 fusing. If arterial blood containing benzoic acid and glycocin be 

 forced through the blood-vessels of a freshly excised kidney, hip- 

 puric acid will be found in the perfused blood. 



The food of herbivora seems to be an important factor in the 

 manufacture of hippuric acid. When fed upon grain without the 

 husk, hippuric acid is absent. Crystals of hippuric acid can be 

 readily precipitated from the fresh urine of horses and cows. 



Lactic Acid is a constant component of the urine. Its quantity 

 is increased when it abounds in the blood from deficiency of oxida- 

 tion, or from free derivation from the aliments, or from gastric fer- 

 mentations. 



Oxalic Acid is an inconstant component; it occurs with calcium 

 in the crystalline form of octahedrons. The crystals are insoluble 

 in acetic acid, but are readily dissolved by hydrochloric and nitric 

 acids. The "envelopes-shaped crystals are very characteristic. 



