506 A MANUAL OF PHYSIOLOGY 



in the tissues in general, including the liver. It has been shown 

 that when air is blown through a mixture of splenic pulp and 

 blood, uric acid is formed from purin bodies already present in 

 the spleen. When the quantity of these is increased by the 

 decomposition of nucleins induced by slight putrefaction, the 

 yield of uric acid is also increased. Uric acid is also formed by 

 the perfectly fresh surviving spleen, liver, and thymus in the 

 presence of oxygen, and the quantity is increased when purin 

 bodies are artificially added. 



As to the source of the uric acid, it is well established that 

 in the bird it arises both from the end-products of protein meta- 

 bolism and from nuclein compounds and their derivatives in the 

 food and tissues. In the mammal, the taking of food rich in 

 nucleated cells, and therefore in nucleo-proteins and nucleins 

 (thymus gland, pig's pancreas, and herring roe), or of food rich 

 in purin bases (Liebig's meat extract), increases the quantity 

 of uric acid in the urine. The increase is mainly due to the 

 production of uric acid from the nuclein substances. But this 

 is not the only source of the uric acid, since extracts of the 

 thymus gland containing only traces of nucleins or nucleic acid 

 cause, when injected, a characteristic increase in the uric acid 

 excretion, just as the entire gland does when taken by the 

 mouth. And during the period of increased nitrogen excretion 

 occasioned by a meal containing protein the increase in the 

 uric acid occurs particularly in the hours immediately following 

 the ingestion of the food, and does not last so long as the increase 

 in the urea. Now, the nucleins of the food are comparatively 

 little affected during the earlier stages of digestion (Hopkins and 

 Hope). We may conclude, therefore, that in the mammal, as 

 well as in the bird, a portion of the uric acid, although certainly 

 a far smaller portion in the mammal, is derived from bodies other 

 than the nuclein substances of the food that is to say, from the 

 nuclein substances of the tissues contained particularly in the 

 cell-nuclei, and from the ordinary proteins of both food and 

 tissues. The portion derived from the proteins is that small 

 fraction which has already been spoken of as synthetically formed. 



Our knowledge of the metabolism of the nucleo-proteins and 

 nucleins has been greatly augmented in recent years. When 

 nucleo-protein is digested by gastric juice a certain amount of 

 protein is easily split off, and hydrolysed to peptone and the 

 other ordinary products of proteolysis. An insoluble residue of 

 nuclein remains. This is acted upon with difficulty by gastric 

 juice, although eventually an active juice will split it up also. 

 By heating with dilute acids it is more easily hydrolysed, yielding 

 a further quantity of protein along with nucleic acid. This 

 second fraction of protein, which is split off with so much more 



