EXCRETION 213 



a certain limit, by absence of food. The tissues are constantly 

 throwing off nitrogen-containing molecules, which, if the 

 body is not to waste, must be as constantly renewed. 



Uric Acid. When nitrogenous metabolism has reached the 

 bottom, when albuminous substances have been shaken into 

 the simplest and most stable compound or compounds which 

 the muscles are capable of making (we know not whether the 

 end-products be one or many), they are carried to the liver by 

 the blood. The mammalian liver converts them into urea ; 

 the liver of birds and reptiles changes them into uric acid. 

 Uric acid is not, however, completely absent from the urine of 

 carnivorous animals. In Man the amount excreted is about 

 0-8 gramme per diem, but subject, even in perfect health, to 

 considerable variations (0-2 gramme to 1-4 gramme). There 

 is no reason for thinking that uric acid is made in the liver of 

 mammals. On the contrary, it seems to be either an end- 

 product of the disintegration and oxidation of leucocytes 

 (cf. p. 53), or, like certain other more complex nitrogenous 

 compounds which appear in very small quantities in the urine, 

 the relic of albuminous food which has missed the broad down- 

 path, via muscles and liver, to the kidney. It is a trouble- 

 some burden for lymph and blood, and, unfortunately, the 

 kidney finds difficulty in throwing it out. Uric acid has a 

 pernicious way of accumulating in tissue-spaces, producing 

 all the malevolent symptoms of gout. During an acute attack 

 of gout the quantity of uric acid in the system may be largely 

 increased. It may be so abundant in the blood that, when a 

 sample is allowed to cool, uric acid begins almost immediately 

 to crystallize out. Speaking generally, it is right to ascribe 

 gout to an over-production of uric acid ; but it must be remem- 

 bered that the balance between elimination and production is 

 very delicately adjusted. During an attack of gout the 

 amount excreted in the urine is not increased ; frequently it 

 is less than usual. The clearing up of the attack is accom- 

 panied by abundant excretion of urates, or lithates (Xt'tfo?, 

 stone), as they used to be called, because the " stones " which 

 are found in the bladder consist largely of uric acid. From 

 this it appears that faulty distribution and inadequate excre- 

 tion have more to do with the development of the symptoms 

 of gout than over-production. In a previous chapter (p. 140) 



