METABOLISM. 45 
these animals contains substances of the aromatic series capable 
of yielding benzoic acid or its equivalent in the body, but in none 
of the feeding-stuffs known to be efficient in causing an excretion 
of hippuric acid have such compounds been discovered in quantity 
even remotely sufficient to account for the hippuric acid produced. 
On the other hand, the hypothesis that the benzoyl radicle of 
the hippuric acid is derived to any large extent from the proteids 
of the food appears to be decisively negatived by several facts: 
First, the quantity of proteids in the ordinary rations of herbivora 
is relatively small, and even if it all underwent putrefaction the 
amount of aromatic products which could be formed, on any reason- 
able estimate, would account for only a small fraction of the hip- 
puric acid actually found.* Second, in several instances it has 
been observed that variations in the extent of the putrefactive 
processes in the intestines, as measured by the amount of con- 
jugated sulphuric acid in the urine (compare p. 46), bore no rela- 
tion to the variations in the production of hippuric acid. Third, 
the addition of pure proteids or of foods very rich in proteids to a 
ration does not increase the production of hippuric acid, and in at 
least one case | was found to diminish it and even stop it alto- 
gether. 
Apparently we must regard the non-nitrogerious ingredients of 
feeding-stuffs as the chief source of hippuric acid formation, but be- 
yond this our knowledge is rather vague. It is well established 
that the coarse fodders are the chief producers of hippuric acid, 
while the concentrated feeding-stuffs give rise to little or none, and 
may even reduce the amount previously produced on coarse fodder, 
as may also starch. Among the coarse fodders, the graminez give 
rise to a markedly greater production of hippuric acid than the 
leguminose. This effect of the coarse fodders naturally led to the 
suspicion that the crude fiber contained in them in large amounts 
might be the source of the hippuric acid, and in fact numerous 
experiments seem to show that some relation exists between the 
two, although the results of various investigators are far from con- 
cordant. 
Finally, the investigations of Goetze & Pfeiffer, { and of 
* Compare Salkowski, Zeit. physiol. Chem., 9, 234. 
+ Henneberg and Pfeiffer, Jour. f. Landw., 38, 239. 
t Landw. Vers. Stat., 47, 59. 
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