398 HUMAN PHYSIOLOGY 



It is an exceedingly insoluble substance which tends to 

 crystallise in large irregular crystals, and in the urine these 

 are generally coloured brown by the urinary pigment. 



It occurs as salts of sodium and potassium, and, according to 

 Roberts, the acid salt, NaHlT, is linked to a molecule of the 

 acid to form NaHU-H 2 TT, or, what he calls, a quadriurate ; 

 but the evidence of this is not conclusive. 



Although the salts of uric acid are more soluble than the 

 free acid, only a small quantity can be dissolved in the 

 urine. Apparently inorganic salts, such as phosphate of 

 soda, act as solvents. When the urine cools, especially if it 

 is unusually acid, the urates tend to separate out and fall as 

 a brick-red deposit, which generally shows no crystalline 

 structure under the microscope. 



If the urine has become ammoniacal on standing, the 

 deposit frequently contains characteristic spinous pigmented 

 crystals of urate of ammonia. 



From their insolubility, uric acid and the urates tend to 

 form calculi or concretions in the urinary passages. The 

 presence of uric acid in such concretions is recognised by the 

 murexide test, which depends upon the fact that uric acid 

 heated with nitric acid is oxidised to alloxantin, which 

 strikes a purple colour with ammonia, yielding murexid 

 the ammonium salt of purpuric acid. 



Other members of the series, such as xanthin and 

 hypoxanthin, occur in the urine in small quantities. 



Allantoin, which occurs in the urine of the foetus, and in 

 the urine of dogs after the administration of nucleic acid, is 

 a diureide in which gly coxy lie acid with two carbon atoms is 

 the linking band. 



3. Greatinin. Creatinin is the form in which the creatin 

 of muscle is excreted. 



Creatin is methyl-guanidin-acetic acid (see p. 43). By 

 dehydration creatinin is produced. The amount excreted is 

 always small, and depends upon the amount of muscular 

 tissue broken down in the body. According to the in- 

 vestigations of Folin the amount of creatinin excreted 

 per diem on a flesh-free diet is very constant in each in- 

 dividual, and does not vary with the amount of proteid 

 food taken. 



