162 UREA. 



The above statements seem to embrace all that can be suggested 

 as to the tissue-antecedents of urea, and it remains now to con- 

 sider the probable mode and seat of their conversion into urea. 

 As regards kreatin it may be that it is split up into urea and sar- 

 kosin, the latter being, like other amido-acids, also converted into 

 urea. When the ainido-acids are compared with urea it is not 

 conceivable, with our present chemical knowledge, how they can 

 give rise to urea in any way other than by being broken down 

 into an ammonia stage and a subsequent synthesis of urea from 

 this product. The synthesis may however involve any one of the 

 three following processes. The ammonia may unite with carbonic 

 acid to form ammonium carbonate, which is then dehydrated into 

 urea (Schmiedeberg). Again, it may unite with carbamic acid to 

 form the carbamate, which again by loss of one molecule of water 

 yields urea (Drechsel). 1 But in the third place the ammonia 

 residues may unite with some cyanic compound to form urea in 

 accordance with the possibilities indicated above (pp. 156, 160) 

 (Salkowski and Hoppe-Seyler). The view that some cyanic resi- 

 dues may be involved in the formation of urea, while at present 

 devoid of any striking positive evidence in its support, is at first 

 sight most attractive, especially when it is borne in mind how 

 great the molecular energy of the cyanogen compounds is, so that 

 during their degradation in the tissues much energy would be set 

 free. Pfliiger, 2 following Liebig, has called attention to this 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 un- 

 stable 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. 



If we accept this view it is perhaps difficult to understand how 

 the cyanic compounds, poisonous as they are known to be, could 

 play a part in the body. But it is apparently the (ON) group 

 which confers on the compounds their poisonous properties ; and 

 if cyanic acid be truly carbamide CO . NH this group is non- 

 existent in it, and it has been recently stated that cyanuric acid 

 (CO . NH) 3 when introduced into the body leads to an increased 

 excretion of urea. 3 



One difficulty in connection with this view is that as yet cyanic 

 acid has never been obtained by the artificial decomposition of pro- 

 teids. But on the other hand the proteids are the chief and only 

 source of the cyanogen compounds, for which the starting-point is 



1 Cf. above sub sarkosin, p. 140, and carbamic acid, p. 151. 



2 Pfluger's Arch. Bd. x. (1875), S. 337. 



3 Coppola, Rendic. d. R. Ace. d. Lined, 1889, pp. 378, 668. Ann. di Chim. e 

 difarmac. (4) T. x. (1889), p. 3. 



