186 ON THE CONVERSION OF 
The urine of herbivorous animals does not contain any uric acid, but another acid, 
called hippuric acid, from ‘os, a horse, in whose urine it was first discovered. It only 
contains seven per cent. of nitrogen, while uric acid contains thirty-three per cent. By 
oxidation, it yields, among other products, benzoic acid. 
The urine of the horse sometimes contains benzoic acid instead of hippuric. Ben- 
zoic acid is generally obtained from gum-benzoin by subjecting it to a gentle heat; 
when it sublimes in crystalline needles. It will be observed that it contains no nitrogen. 
It belongs to a series of compounds, which in a historical point of view are of interest, as 
containing the first well established radical in organic chemistry; that is to say, an ima- 
ginary body, which, though never isolated by itself, may be carried through a series of 
combinations with most of the simple elementary bodies. 
The question now naturally arises, how is this transformation of benzoic acid into 
hippuric effected? We have seen that the normal quantity of uric acid is too small to 
suffice, if its formation depended on this acid, and, besides, their formation seems indepen- 
dent of each other. That the transformation should be effected in the stomach and in- 
testines during the digestion is less probable: it seems more probable to suppose that it 
takes place during the subsequent circulation of the blood, and that its formation is con- 
nected with that of the urea. We know that at least the principal source of urea is the 
transformation of the tissues in the body, or what Prout calls the destructive assimilation, 
since its formation continues, when animals are fed exclusively on non-nitrogenized food, 
and when they are starved; and that it is not secreted by the kidneys, but actually 
formed in the body, and merely excreted by them, is proved by removing them, when urea 
will be found in the blood. The same occurs by suppression of urine. I have myself 
lately had an opportunity to confirm this observation, in examining, at the request of Drs. 
Pepper and Sargent, the blood of a patient who for some time previous to her death had 
a complete suppression of the urine. But the question then is,—is it merely a simple 
chemical reaction between the benzoic acid and the products of the transformations of 
the tissues, or does it actually affect the chemical changes, by which the latter are effected, 
so as to cause new chemical products to be their results. Liebig takes this latter view 
in his Animal Chemistry. We quote his own words: 
“Benzoic acid, when administered internally, appears in the form of hippuric acid. 
Should the observation be confirmed, it will acquire great physiological significance, since 
it would plainly prove, that the act of the transformation of the tissues in the animal 
body under the influence of certain matters taken in the food, assumes a new form with 
respect to the products, which are its results; for hippuric acid contains the elements of 
one at. of urea, one of lactic acid, and two of benzoic acid.” 
Liebig then at once proceeds to make an application of this fact to the action of medi- 
cinal agents. We are not disposed to dispute his theories on this point, nor deny that 
such may be the case in active remedies, but we believe that he is too prompt in drawing 
the conclusion, that because benzoic acid is converted into hippuric by entering into the 
system, it should, therefore, necessarily affect the usual chemical changes in the body. 
We are disposed to consider it as a mere chemical reaction between the benzoic acid and 
the substances of the blood, either those which are used for the formation of the tissues, 
or, more probably, those which result from their dissolution. But if this be correct, it fol- 
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