460 



A TEXTBOOK OF PHYSIOLOGY 



A strong solution of uric acid in alkali reduces Fehling's solution 

 on heating, but not Nylander's solution (see p. 61). It will also 

 reduce silver nitrate in the cold. If a drop of an alkaline uric acid 

 solution be placed on a filter-paper, and a drop of silver nitrate 

 solution be added, a blackened area of reduced silver results (Schiff's 

 test). 



Uric acid is estimated quantitatively by precipitating it as am- 

 monium urate, as described above, washing the precipitate carefully 

 to remove all traces of chlorides, setting free the uric acid by the 

 addition of sulphuric acid, and titrating with a standard solution of 

 potassium permanganate until the rose colour, which at first dis- 

 appears, just persists. 



Uric acid is both exogenous and endogenous in origin (see p. 444). 

 It is increased by ingestion of bodies rich in nuclei (sweetbreads, etc.). 

 In disease, it is increased when tissue destruction is going on, as in 



FIG. 223. URIC ACID CRYSTALS. (Savill.) 



leukaemia and acute fevers. Much has been made of uric acid as a 

 cause of gout. There is no evidence to show that its formation is 

 increased in gout. It has no toxic properties, and there is nothing 

 to justify its ill reputation, or the advertisements of the quack medicine 

 vendors. The precursors of uric acid and the site of formation have 

 already been discussed (p. 444). 



Purin Bodies. Hypoxanthin, adenin, and xanthin, are present in 

 normal urine in small amounts. These are supposed to be derived 

 from the decomposition of nucleic acid. Methylxanthin results from 

 the taking of coffee and tea which contain caffeine (tri-methyl- 

 xanthin), and of cocoa which contains theobromine (di -methyl - 

 xanthin). Guanin is present in the urine of several of the inverte- 

 brata, 



AUantoin is the chief end-product of the metabolism of nuclein in 

 some animals, and occurs in human urine in traces and in allantoic 



