44 PRINCIPLES OF ANIMAL NUTRITION. 
be regarded as established, and appears difficult to reconcile with its 
relation to the proteid metabolism of birds. Others regard it as an 
intermediate product in the production of urea, a small portion of 
which escapes further oxidation by being excreted by the kidneys. 
Hippuric Actp.—This substance is a normal ingredient of the 
urine of mammals, but in that of man and the carnivora is found in 
but very small amounts. In the urine of herbivora, on the other 
hand, it occurs abundantly. 
Light was thrown upon its origin by the well-known discovery 
by Wohler, in 1824, that it is also found in large amount in the urine 
of man or of carnivora after the administration of benzoic acid. 
Chemically, hippuric acid is benzamido-acetic acid, or benzoyl 
glycocol. When the food contains benzoic acid the latter unites 
with glycocol resulting from the metabolism of the proteids and 
forms hippuric acid, while otherwise the glycocol would be further 
oxidized to simpler nitrogenous products. The synthesis of hip- 
puric acid has been shown to occur only in the kidneys in the dog, 
but in the case of the rabbit and frog they appear to share this 
capacity with other organs. 
In this action of benzoic acid we have the most familiar demon- 
stration of the formation of metabolic products intermediate be- 
tween the proteids and the comparatively simple nitrogenous sub- 
stances found in the urine. Glycocol has never been detected in the 
‘body, obviously because as fast as it is formed it is again decom- 
posed. Benzoic acid reveals its presence by seizing upon it and 
converting it into a compound which is incapable of further oxida- 
tion, and is therefore excreted. Other less familiar examples of 
the same fact might be cited did space permit. 
The normal presence of small quantities of hippuric acid in the 
urine, even when no benzoic acid is contained in the food, arises 
from the fact that the putrefaction of the proteids in the intestines 
yields aromatic compounds, containing the benzoyl radicle, which 
are resorbed and combine with glycocol to form hippuric acid. 
The origin of the large quantities of hippuric acid ordinarily ex- 
creted by herbivora, however, or rather of its benzoyl radicle, 
is still more or less of a puzzle, notwithstanding the consider- 
‘able amount of investigation which has been devoted to its 
study. The most natural supposition would be that the food of 
