386 PHYSIOLOGY CHAP. 



supposition urea can be derived from cyanic acid as follows 

 (Salkowski) : 



2(HOCN) + H 2 O = CO<' +C0 2 



Cyanic acid. Urea. 



It may also be assumed (Hoppe-Seyler) that the cyanic acid 

 on combining with ammonia forms ammonium cyanate (NH 4 OdN T ), 

 which is isomeric with urea (CO'(N'H 2 ) 2 ) and from which, as we 

 saw, Wohler obtained urea by synthesis. 



To this theory Hofmeister objected that cyanic acid had not 

 yet been discovered in the body, and particularly in the liver. It 

 was also noted that if salts of ammonium, or ammonia, were 

 introduced into an organism poisoned with cyanic acid, the 

 syndrome was not altered, showing that the two substances did 

 not combine to form urea, as assumed by Hoppe-Seyler. 



Hofmeister experimentally discovered an important fact, viz. : 

 that urea can be obtained from a large series of organic com- 

 pounds, by oxidation with permanganate of potash, in the presence 

 of ammonia. Among such are ovalbumin, glutin, asparagine, 

 leucine, glycocoll, hydrocyanic acid, and lactic, malic and tartaric 

 acids. This shows the extreme importance of the oxidising 

 processes to the formation of urea outside the body. But it has 

 not at present been decided how far similar processes may occur 

 within the living body. 



III. In the urine of birds and reptiles nearly all the nitrogen 

 introduced is eliminated in the form, not of urea, but of uric acid. 

 Uric acid is often absent from the urine of carnivora, while that of 

 herbivora shows no trace of it. In human urine, however, it is 

 always present in a small quantity which normally fluctuates 

 between 0'25 grm. and 140 grm. On an exclusively vegetable 

 diet it rarely exceeds 0'3 grm. in the 24 hours : on an exclusively 

 flesh diet it may reach 2 grms. or more. Its relation with the 

 total nitrogen of the urine is from 1 to 3 per cent ; its relation with 

 the urea may vary from 1*15 to 1*82. 



Uric acid is not normally present in human blood, but it 

 has been noted in minute quantities in the glandular organs, 

 particularly in the liver, spleen, and lungs (Gorup-Besanez and 

 others). Evidently, therefore, it is not a product of the kidneys, 

 the more so as in pathological conditions (arthritis, leucaemia, 

 pneumonitis, pulmonary tuberculosis) it also occurs in the blood. 

 It is normally present in the blood of birds (Salomon, Meissner), 

 and becomes abundant there after ligation of the ureters 

 (Colasanti). 



The empirical formula of uric acid is C 5 H 4 N 4 3 . One atom 

 of hydrogen is easily replaced by bases. On bringing uric acid 



