UREA. 
55 
from the blood of the placenta. M. Paul Hervier has obtained 
urea from human blood, when operating only upon 250 or 
500 grammes of it. The blood he used had been taken from 
patients suffering under rheumatism, pneumonia, or erysi- 
pelas ; he concludes, however, that urea is normally present 
in that fluid. According to Prout and Dumas, the blood 
contains an excess of this substance after the removal of the 
kidneys. At the approach of death, when the intestinal 
secretions diminish, Bernard and Barreswill will have also 
observed a decided increase of the urea existing in the blood. 
The amount of the urea of urine has been found to vary 
according to a number of circumstances ; but, unfortunately, 
the methods for the extraction of this principle have led to 
no satisfactory result. We are much indebted to Liebig, who 
has very recently pointed out a new process for ascertaining 
the amount of urea existing in the urine, which cannot but 
lead to very accurate results. The urine of young children 
hardly contains a trace of urea. According to Lehmann, 
normal urine will yield from 31*45 to 32*90 per 1000 of this 
substance. The amount of this principle secreted in the 
twenty-four hours is modified by a variety of circumstances, 
including especially the nature and quantity of the solid and 
fluid ingesta. 
Urea is always found dissolved in the urine, from its being 
exceedingly soluble in water. Several theories have been 
given as to the source of urea in the system; it is known, 
however, not to be formed by the kidneys, for the excision of 
those organs causes an increase of urea in the blood. Ac- 
cording to chemists, this substance is the result of an 
oxidation of tissues. Their carbon and hydrogen are con- 
verted into carbonic acid and w r ater, to be eliminated through 
the lungs, whilst the nitrogen, combining wdth hydrogen, 
will assume the form of ammonia. This gas, in the presence 
of carbonic acid, is supposed to lose one equivalent of water, 
and the final combination will be urea. Other theorists sup- 
pose that cyanate of ammonia is formed in the blood (from 
the oxidation of its nitrogenised elements), which is sub- 
sequently converted into its isomeric compound, urea. 
Urea is secreted from the blood by the kidneys, and thus 
expelled from the body. An excess of that principle in the 
blood is not necessarily followed by an increased amount of 
it in the urine ; and if its proportion in the urine be dimin- 
ished, as happens in some diseases, we are not certain of 
finding its quantity in the blood larger than usual: for M. Cl. 
Bernard has observed, that urea is often secreted by the 
stomach and intestines, where it loses its nature, and assumes 
