164 UREA. 



urine only of those animals which excrete their nitrogen chiefly in the form of 

 urea 1 . 



The various ways by which it has been suggested that urea may 

 arise in the body all imply that whatever be the form in which the 

 nitrogen initially leaves the tissues, the substance or substances in which 

 it makes its exit undergo their final (synthetic?) conversion in some other 

 organ of the body. In the case of leucin there is distinct evidence that 

 the conversion is effected in the liver, and there is increasing evidence 

 that this organ is largely concerned in the presumably synthetic 

 changes which lead to the formation of urea in mammals and of uric 

 acid in birds. Thus Schroder has shown that the conversion of 

 ammonium carbonate into urea occurs in the liver 2 , and a similar 

 relationship to the formation of uric acid in birds has additionally been 

 proved 3 . Further there are many observations which show, when the 

 liver is diseased, a marked diminution in the excretion of urea with a 

 frequently increased output of ammonia 4 . After extirpation of the 

 liver in birds the urine contains not only more ammonia but a large 

 amount of sarcolactic acid 5 . It would be however premature to regard 

 this fact as showing that in birds uric acid is partly formed by the 

 converting activity of the liver brought to bear upon ammonia and 

 lactic acid. When urea is given to birds it reappears externally as 

 uric acid 6 , but this change is not effected after extirpation of the 

 liver. 



Substituted Ureas. The hydrogen atoms of urea can be replaced by alcohol- and 

 acid-radicles. The results are substituted ureas in the first case, or ureides as they 

 are called in the second, when the hydrogen is replaced by the radicle of an acid. 

 Many of them are called acids, since the hydrogen from the amido group, if not all 

 replaced as above, can be replaced by a metal. Thus the substitution of oxalyl 



NH.CO 

 (oxalic acid) gives parabanic acid, CO. I ; of tartronyl (tartronic acid), 



X NH.CO 

 NH.COv 

 dialuric acid, CO X CHOH ; of mesoxalyl (mesoxalic acid), alloxan 



X NH.CO X 

 .NH.CO, 

 CO J^CO. These substances are interesting as being also obtained by 



X NH.CO / 



the artificial oxidation of uric acid. The close chemical relationship of urea to 

 uric acid will be explained below. 



1 Bruylants, Bull, de I'acad. de med. de Belgique, (4) T. n. (1888), p. 18 et seq. 



2 Arch. f. exp. Path. u. Pharm. Bd. xv. (1882), S. 364 ; Bd. xix. (1885), S. 373. 

 Cf. W. Salomon, Virchow's Arch. Bd. xcvn. (1884), S. 149. 



3 Minkowski, Arch. f. exp. Path. u. Pharm. Bd. xxi. (1886), S. 40. 



4 Koster, Lo Sperimentale, T. XLIV. (1879), p. 153. Hallervorden, Arch. f. exp. 

 Path. u. Pharm. Bd. xn. (1880), S. 237. Stadelmann, Deutsch. Arch. f. klin. Med. 

 Bd. xxxm. (1883), S. 526. 



5 Minkowski, loc. cit. See also Marcuse, Pfliiger's Arch. Bd. xxxix. (1886), S. 

 425. 



6 Meyer u. Jaffe, Ber. d. d. chem. Gesell. Bd. x. (1877), S. 1930. 



