VRINE AND ITS CONSTITUENTS. 715 



carried over, and the excess of acid is titrated with ~ sodium hydroxide 

 with alizarin as an indicator. 



Creatinine is normally present in urine to the amount of 1 or 2 

 grammes in twenty-four hours. It is believed to be derived from the 

 creatine of muscle, and mainly from the body muscle, not the food. 

 Its significance is still much disputed, as an accurate method of esti- 

 mation has been only comparatively recently devised (Folin, 1905). 

 Creatine is not normally present in urine. 



Creatinine is best recognized in the urine by removing the phos- 

 phates and coloring-matter by milk of lime, concentrating the filtrate 

 by evaporation, and applying the tests mentioned before. As crea- 

 tinine is a reducing agent, its presence in urine will influence the tests 

 for sugar based on its deoxidizing power. 



Make tests 2 and 3 of Experiment 77, to show the presence of creatinine in 

 urine. 



NH CO 



Uric acid, H 2 C 5 H 2 N 4 O 3 . 2.6.8. Oxypurine, CO C-NH^ 



,co. 



NH C NH X 



Uric acid is found in small quantities in human urine, chiefly in com- 

 bination with sodium, potassium, and ammonium, but also with cal- 

 cium and magnesium. In larger proportions, uric acid is found in 

 the excrement of birds, mollusks, insects, and chiefly of serpents, the 

 solid urine of the latter consisting almost entirely of uric acid and 

 urates. It is also found in Peruvian guano. The proportion of uric 

 acid to urea in human urine is normally between 1 : 50 and 1 : 70. 

 The normal amount for twenty-four hours is about 0.7 gramme. 



Pure uric acid is a white, crystalline, tasteless, and odorless sub- 

 stance, almost insoluble in water, requiring 1900 parts of boiling and 

 15,000 parts of cold water for its solution; it is also insoluble, or 

 nearly so, in alcohol and ether. The great insolubility of uric acid 

 causes its separation in the solid state, both in the bladder and in the 

 tissues. 



It is believed that uric acid is derived by oxidation from the purine 

 bodies of the nucleins, and is increased when there is an increase in 

 nuclein metabolism. That coming from the tissue nucleins is termed 

 " endogenous " uric acid; that from the food nucleins is termed 

 " exogenous." While uric acid is formed synthetically in the liver 

 of birds, such a synthesis has not been proved for man. It seems 

 probable that most of the waste nitrogen in birds, as in man, is con- 

 verted into urea ; but is further changed to uric acid for excretion. 



