CHEMICAL BASIS OF THE ANIMAL BODY. 



165 



Uric acid. C 5 H 4 N 4 O 3 - 



The chief constituent of the urine in birds and reptiles ; it occurs 

 only sparingly in this excretion in man ('2 1 grin, in 24 hours) and 

 most mammalia. It is normally present in the spleen, and traces of it 

 have been found in the lungs, muscles of the heart, pancreas, brain 

 and liver. Urinary and renal calculi often consist largely of this 

 substance, or its salts. In gout, accumulations of uric acid salts may 

 occur in various parts of the body, more especially at the joints, forming 

 the so-called gouty concretions. 



It is when pure a colourless, crystalline powder, tasteless, and with- 

 out odour. The crystalline form is very variable, differing according to 

 the concentration of the solution from which the crystals are obtained, 

 the rate at which they are formed, and whether they are separated 

 out spontaneously or by the addition of acids to either solutions of the 

 acid or to urine. Hence it is extremely difficult to illustrate them 

 within reasonable limits, and for figures of the various possible forms 

 some special work must be consulted 1 . The impure acid crystallises 



Rapidly separated. Slowly separated. 



FIG. 17. CBYSTALS OF URIC ACID. (Krukenberg after Kiihne.) 



much more readily than does the purified. The following figure shows 

 additionally some very characteristic forms in which uric acid separates 

 out from urine either spontaneously or after the addition of hydro- 

 chloric acid. 



Uric acid is remarkably insoluble in water (1 in 14,000 or 15,000 

 of cold water, 1600 of boiling.) Ether and alcohol do not dissolve it 

 appreciably. On the other hand, sulphuric acid takes it up in the cold 

 without decomposition, and it is also readily soluble in many salts of 



1 See Ultzmarm and K. B. Hoffmann, Atlas der Harnsedimente, Wien, 1872. 

 Also Funke, A tlas d. physiol. Cliem. Leipzig, 1858. 



