THE METABOLISM OF PROTEINS. 109 



The conversion of ammonium carbonate occurs largely in the 

 liver. Our evidence for this is: (1) If solutions containing 

 ammonium carbonate be made to circulate through an excised 

 liver, urea is formed. (2) If this organ be seriously damaged, 

 either experimentally or by disease, less urea and more ammonia 

 appears in the urine. We see therefore that urea is formed in 

 order to prevent the poisonous action of ammonia. But the am- 

 monia may be more usefully employed ; instead of being com- 

 bined with carbonic acid in order that it may be got rid of, it 

 may be employed to neutralize, and thus render harmless, any 

 other acids that make their appearance. Thus, it may be em- 

 ployed to neutralize the acids which sometimes result during the 

 metabolism of fat, as in the disease, diabetes; or the lactic acid 

 that appears in the muscles during strenuous muscular exercise ; 

 or the acids produced on account of inadequate oxygenation. 

 Taking acids by the mouth has a similar effect; thus the am- 

 monia excretion rises after drinking solutions containing weak 

 mineral acids. 



Ammonia is, of course, not the only alkali which is available 

 in the organism for the purpose of neutralizing acids. The 

 fixed alkalies, sodium and potassium are also used. Thus, when 

 we greatly increase the proportion of these, as by taking alkaline 

 drinks, or by eating vegetable foods, the ammonia excretion 

 diminishes. 



Urea is an inert substance, capable of uniting with acids to 

 form unstable salts (urea nitrate and oxalate), and like other 

 amino bodies, being decomposed by nitrous acid so as to yield 

 free nitrogen. This latter reaction is used for the quantitative 

 estimation of urea, the evolved nitrogen being proportional to 

 the amount of urea, thus : 



NH 2 

 CO +2 HN0 2 = 2.C0 2 + 2 N 2 -f 2 H 2 O 



NH 2 



Certain bacteria are capable of causing urea to take up 2 mole- 

 cules of water so as to form ammonium carbonate, a process 



