URINE 561 



3. Purin Bodies. — A very small proportion of the nitrogen is 

 found in these bodies. They consist of two unmodified or 

 modified urea molecules, linked together by a nucleus of an 

 acid radicle. In birds and reptiles the most important have 

 as the linking part an oxy-acid with three carbons in 

 series — Uric Acid— Tri-oxy-purin — an exceedingly insoluble 

 substance which tends to crystallise in large polymorphic 

 crystals. 



O 



II 

 H— X— C 



I I 

 O = C C— N— H 



I II >c = o 



H— N— C— N— H 



Uric Acid. 



In these animals uric acid largely replaces urea as the sub- 

 stances in which nitrogen is eliminated, and they are formed 

 in the liver from the various products of the decomposition 

 of protein molecules. But in mammals the purins appear 

 to be very largely derived from the decomposition of nucleic 

 acid (p. 556). Even when all supplies of nucleins and purin 

 bodies from without are cut off, a certain amount of these 

 purins is daily eliminated. These have been called the " endo- 

 genous " purins, Avhile those derived from the constituents of 

 the food are termed the " exogenous " purins. A small 

 amount is undoubtedly formed from the purins of muscle 

 (p. 210). 



In most mammals the chief purin is Allantoin. In this 

 two urea molecules are linked by the radicle of glyoxylic 

 acid. 



H 



H— N— C— N— H 



= C 



C = 



H— N C— N— H 

 H 



36 



