METABOLISM. 43 
least a considerable part if not all of the urea is formed in the liver, 
and that its immediate antecedent is ammonium carbonate, to 
which it is closely related chemically. This theory of Schmiede- 
berg’s is supported by the facts: 
ist. That ammonium salts, and also the amid radicle NH, in 
the amido acids ofthe fatty series, when administered in the food 
are converted into urea. 
2d. That ammonium carbonate or formiate injected into the 
portal vein is converted in the liver into urea which appears in the 
blood of the hepatic vein. 
3d. That the administration of inorganic acids to the dog and to 
man results in the excretion of ammonium salts in the urine, it 
being supposed that the acid displaces the weaker carbonic acid 
and that the resulting ammonium salt is incapable of conversion 
into urea in the liver. 
4th. Severe disease of the liver has been observed to result in 
a decreased production of urea and an excretion of ammonium salts 
in the urine. 
Later investigations by Minkowski * and others have followed 
the process of the formation of urea one step further back and ren- 
dered it highly probable that the ammonium salts out of which urea 
is formed reach the liver in the form of ammonium lactate. It has 
been shown that sarcolactic acid is one of the products of the meta- 
bolism of the muscles. It would appear that this acid unites with 
the ammonium radicle derived from the proteids to form ammonium 
lactate, and that the latter on reaching the liver is first oxidized to 
the carbonate, which is then converted into urea. If, by disease 
or surgical interference, this action of the liver is prevented, ammo- 
nium lactate appears in the urine, and the same effect may even be 
produced by excessive stimulation of the proteid metabolism, so 
that the production of ammonium lactate exceeds the capacity of 
the liver to convert it. 
Uric Acip.—Uric acid is contained in small amounts in the 
urine of mammals. With birds it constitutes the chief nitrogenous 
product of the proteid metabolism. Of its antecedents in the 
organism scarcely anything is known. One theory regards it as a 
specific product of the metabolism of the nucleins, but this cannot 
*Cf. Neumeister, Physiologische Chemie, pp. 313-318. 
Digitized by Microsoft® 
