EXCRETION OF MATTER FROM THE BODY 397 



nitrogen is normally excreted as ammonium salts. But 

 under certain conditions the proportion is greatly increased. 

 Anything which causes an increased breaking down of 

 proteid, and an increased formation of acids, leads to an 

 increased excretion of ammonia the ammonia being formed 

 from the proteids to neutralise the acids. 



2. Diureides. The members of this series of bodies 

 consist of two unmodified or modified urea molecules, linked 

 together by an acid nucleus. The most important of the 

 series have as the linking molecule acrylic acid, and they 

 constitute the purin bodies. 







II 

 j H N C N H i Urea. 



C=C C = Acrylic acid nucleus. 



; H N N H ! 



i \/ I 



C Urea. 



II 

 



In birds and reptiles they replace urea as the substances 

 in which nitrogen is chiefly eliminated. In these animals 

 they are formed in the liver from lactate of ammonia, 

 derived from proteids, 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 con- 

 stituents 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 Acid is the most important member of the series. 

 Its constitution is shown above. 



