UREA. 167 



and Mitscherlich,* extirpated the kidneys of animals, and then 

 found no inconsiderable quantity of urea in the blood ; indeed, 

 Marcharidf induced all the symptoms of uraemia in a dog by the 

 mere ligature of the renal nerves, and was able to recognise the 

 presence of urea with the greatest certainty, not only in the blood, 

 but also in the vomited matters. 



The investigations of Marchand have thrown much light upon this 

 subject; this accurate observer could only recover 0'2 of a gramme 

 of urea from 200 grammes of serum to which 1 gramme of urea had 

 been added ; he shows that, even if the urea were only separated from 

 the blood at the end of each successive hour, it could not have 

 accumulated in such quantity as to have been discoverable by the 

 present mode of investigation. The following consideration will 

 give us an idea of the small quantity of urea which, according to 

 Marchand's hypothesis, at the most can accumulate in the blood in 

 one hour. From the experiments of Ed. Weber, which I have in 

 part confirmed, we may assume that there are in an adult man at 

 most 6 or 7 kilogrammes [16 to 19 pounds] of circulating blood; 

 now, if in 24 hours 30 grammes of urea are discharged, at most 

 only 1*25 grammes could accumulate in one hour in the whole 

 mass of the blood, so that only 0*021^ could be contained in it ; 

 this minute quantity can, however, as we have already shown, 

 only be detected in operating on very large masses of blood, and 

 by the aid of the microscope. Hence it is easy to understand 

 why, during my experiments with an animal diet, while the urine 

 was loaded with urea, none of this substance could be discovered 

 in the blood. 



If it be now established, that the urea is not primarily formed 

 in the kidneys, the question still remains to be answered, whether 

 it is produced in the circulating blood or in the individual living 

 organs, (as for instance, the muscles,) and from what materials it is 

 principally formed. In the present state of our knowledge, we may 

 answer, that the urea is formed in the blood, and that it is produced 

 from materials that have become effete, the detritus of tissues, as well 

 as from unserviceable and superfluous nitrogenous substances in the 

 blood. No animal tissue presents such vital activity, is so much 

 used, and so rapidly worn out, as muscular tissue ; it is in this tissue 

 that the metamorphosis of matter proceeds most rapidly and 

 abundantly, and yet, in the large quantities of muscular fluid on 

 which Liebig worked, he could detect no trace of urea, although he 



* Pogg. Ann. Bd.31, S. 303. 



t Journ. f. pr. Ch. Bd. 11, S. 149. 



