APP.] CHEMICAL BASIS OF THE ANIMAL BODY. 749 



' that urea may be obtained readily either from ammonium cyanate or from 

 ammonic carbamate, and may with the greatest ease be converted into 

 ammonic carbonate 1 . Now urea is a much more stable body than 

 amnionic cyanate, and in the transformation of the latter into the 

 former, energy is set free; and it is worthy of notice that though the 

 presence of sulphocyanides in the saliva probably indicates the existence 

 of cyanic residues in the body, the nitrogenous products of the decom- 

 position of proteids belong chiefly to the class of amides, cyanogen 

 compounds being rare among them. Pfliiger 2 has called attention to the 

 great molecular energy of the cyanogen compounds, and has suggested 

 that the functional metabolism of protoplasm by which energy is set 

 free, may be compared to the conversion of the energetic unstable 

 cyanogen compounds into the less energetic and more stable amides. In 

 other words, ammonium cyanate is a type of living, and urea of dead 

 nitrogen, and the conversion of the former into the latter is an image of 

 the essential change which takes place when a living proteid dies. 



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

 acid radicles. The results are compound ureas or ureides when the hydrogen is 

 replaced by an acid radicle. Many of them are called acids, since the hydrogen 

 from the amide group, if not all replaced as above, can be replaced by a metal. 

 Thus the substitution of oxalyl (oxalic acid) gives parabanic acid, 



CO 



/ 



N 2 H 2 



a 



Ic 2 o 2 



of tartronyl (tartronic acid), dialuric acid, CO, NH 2 , N.C 3 H 2 3 ; of rnesoxalyl 

 (mesoxalic acid), alloxan, CO, NE^, N . C 3 3 . These bodies are interesting as being 

 also obtained by the artificial oxidation of uric acid. (See below). 



Uric acid. C 5 H 4 N 4 3 . 



The chief constituent of the urine in birds and reptiles ; it occurs 

 only sparingly in this excretion in man and most mammalia. It is 

 normally present in the spleen, and traces of it have been found in 

 the lungs, muscles of the heart, pancreas, brain and liver. Urinary 

 and renal calculi often consist largely of this body, or its salts. In gout, 

 accumulations of uric acid salts may occur in various parts of the body, 

 forming the so-called gouty concretions. 



It is when pure a colourless, crystalline powder, tasteless, and with- 

 out odour. The crystalline form is very variable, but usually tends to- 



1 The following literature is interesting in connection with the question of the 

 cyanic or amide origin of urea. Drechsel: Ber. d. k. s. Gesell. d. Wiss. Leipzig: 

 Sitz. 25. Juli. 1875; Arch. f. PhysioL, 1880, S. 550. v. Knieriem: Zt.f. Biol. t Bd. 

 x. (1874), S. 263. Munk: Zt. f. physiol. Chem., Bd. 11. (1878), S. 29. . E. Salkow- 

 ski : Centralb. f. d. Med. Wiss.\ 1875. No. 58 ; Ber. d. dtutsch. Chem. Gesell., 1875, 

 S. 116; Zeitsch. f. physiol. Chem., Bd. I. (1877), Sn. 1. u. 374; Bd. rv. (1880), Sn. 

 54. u. 103. Schmiedeberg: Arch. f. exp. PathoL, Bd. vni. (1877), S. 1. 



2 Pfliiger's Archiv, Bd. x. (1875) S. 337. 



