1248 



PHYSIOLOGY 



colour is produced. This reaction is made use of for the quantitative estimation 

 of creatinine in the urine. 



Like the other nitrogenous constituents, creatinine can be regarded 

 as partly exogenous, partly endogenous in origin. The exogenous 

 part is derived from the creatine contained in meat. When meat is 

 excluded from the diet the output of creatinine becomes remarkably 

 constant and is little affected by the total amount of proteins taken, 

 the amount excreted during starvation being practically identical with 



FIG. 516. Creatinine. (FuNKE.) 



FIG. 517. Uric acid. (FuNKE.) 



that excreted on a full protein-free diet. It is said to be increased 

 during febrile conditions and as a result of violent muscular exercise. 

 URIC ACID. Uric acid is 2-6-8-trioxypurin. 



HN CO 



00 C NH, 



)CO 



HN NH- 



N-C(OH) 



or (HO)C C NIL 



N C N 



C(OH) 



Uric acid forms small rhombic crystals. The crystalline form varies 

 considerably in the presence of impurities. The different forms of uric 

 acid crystal which may occur in the urine are shown in the accompany- 

 ing figure (Fig. 517). It is extremely insoluble in pure water, one part 

 of uric acid requiring 39,000 parts of water at 18 C. for its solution. 

 It is easily soluble in concentrated sulphuric acid and alkalies. 



It may be prepared from human urine or from guano, which consists almost 

 entirely of urates. In order to prepare it from guano, this is dissolved with 

 the aid of heat in dilute sodium carbonate, filtered, and the filtrate treated with 

 a few drops of concentrated hydrochloric acid and boiled. On allowing to cool, 

 the uric acid crystallises out. From urine uric acid may be obtained by adding 

 one-fiftieth of its volume of concentrated hydrochloric acid and allowing to 



