

EXCRETION OF MATTER FROM THE BODY 417 



they are formed in the liver from lactate of ammonia derived 

 from proteins, but in mammals they appear to be very largely 

 derived from the decomposition of nucleic acid. Even when 

 all supplies from without of nucleins and purin bodies are 

 cut off, a certain amount of these purin bodies is daily 

 eliminated. These have been called the "endogenous" purins, 

 while those derived from the constituents of the food are 

 termed the "exogenous" purins. A certain amount of the 

 purins formed are changed to urea before being excreted, and, 

 therefore, when disturbances of the chemical processes in 

 the liver occur, the purins appear to be increased at the 

 expense of the urea. 



Uric Aeid is the most important member of the series in 

 man, but in the dog, horse, and ox its amount is very small. 

 Its constitution is shown on p. 416. 



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. 



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 

 rnurexide 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 murexide the ammonium 

 salt of purpuric acid. 



Other members of the series, such as xanthin and hypo- 

 xanthin, occur in the urine in small quantities. 



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

 urine of cows, and possibly of other herbivora, and in the 

 urine of dogs both before and after the administration of 

 nucleic acid, is a diureide in which glyoxylic acid with two 

 carbon atoms is the linking band. 



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

 of muscle is excreted. 



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

 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 investi- 

 gations of Folin the amount of creatinin excreted per diem 



27 



