UKEA. 199 



dence of the direct transformation of these principles into 

 urea before they have become part of the organized struc- 

 tures, except in a comparison of the proportions of nitrogen 

 ingested and discharged ; and this proves nothing with re- 

 gard to the nature of the intermediate processes. At the 

 present time, the most rational supposition is, that the nitro- 

 genized elements of food nourish the corresponding constitu- 

 ents of the body, which are constantly undergoing conversion 

 into excrementitious matters. Observations which have ap- 

 peared to demonstrate the formation of urea directly from 

 albuminoid substances have not been confirmed. 1 



There are certain arguments, based upon comparisons 

 of the atomic constitution of urea with the elements of uric 

 acid, creatine, and creatinine, in favor of the view that urea 

 is the product of a higher degree of oxidation of the other ex- 

 crementitious matters above-mentioned. It has been found, 

 also, that urea may be formed artificially from uric acid, 

 creatine, creatinine, xanthine, hypoxanthine, and some other 

 bodies of similar nature. 8 That certain bodies are mutually 

 convertible by the addition or subtraction of a few elements 

 of water, there can be no doubt. Examples of these simple 

 transformations are, the change of starch (C 12 H 10 O 10 ), dex- 

 trine, etc., into glucose (C 13 H 14 O 14 ) ; the change of creatine 

 (C 8 H 9 X 3 O 4 ) into creatinine (C 8 H 7 ^" 3 O 3 ), etc. ; but the atomic 

 changes necessary for the conversion into urea of the princi- 

 ples from which this substance has been assumed to be pro- 

 duced are much more complicated. There is no positive 

 proof that the proportion of these various principles in the 

 muscles, blood, and urine, bears an inverse ratio to the pro- 

 portion of urea. Again, the argument that the excrements 

 of reptiles contain an excess of uric acid because the activity 

 of oxidation is less than in the mammalia is met by the fact 

 that in birds, where the amount of oxygen consumed is 



1 MILNE-EDWARDS, Lemons sur la physiologic, Paris, 1862, tome vii., pp. 

 400, 401. 



8 XEUBAUER AND VOGEL, op. tit., p. 9 



