322 PRINCIPLES OF ANIMAL NUTRITION. 
ucts are a normal constituent of the urine of cattle both on hay and 
on mixed rations. Their effect on the computation becomes more 
obvious in the case of the straws, simply because of the relatively 
small amount of protein in the latter feeding-stuffs. In these cases 
we get impossible results when we assume that all the potential 
energy of the urine is derived from the proteids metabolized, but it 
is clear that the results on the hays must be affected by the same 
error, and there is little question that the low and variable results 
noted in the table are to be explained in part in this way. We 
know no essential difference between the real proteids of the differ- 
ent coarse fodders, nor between those of coarse fodders and grain, 
nor any reason why they should not be metabolized in substantially 
the same way in the body and possess approximately the same 
metabolizable energy. It would seem more reasonable, then, to 
assume that the proteids of coarse fodders are metabolized sub- 
stantially like those of concentrated fodders, and to take provision- 
ally the results obtained for the protein of wheat gluten as repre- 
senting approximately the metabolizable energy of the digested 
protein of the total ration, while we regard the remaining energy 
of the urine as derived largely from the non-nitrogenous nutrients 
of the food. 
Hippuric Acid.—The statement last made, however, requires 
some modification. Not a little of the potential energy of the urine 
of cattle is contained in the hippuric acid which these animals. 
excrete so abundantly. This being a nitrogenous product, it is 
natural to look upon it as derived from the proteids of the food, 
but it must not be forgotten that this is only partially true. Its. 
glycocol portion originates in the proteids, but its pheny] radicle 
appears to be derived in these animals largely, if not wholly, from 
the non-nitrogenous ingredients of the food (compare p. 45). If 
the metabolism of one gram of protein is arrested at the glycocol 
stage by the presence in the organism of benzoic acid, there has 
already been liberated from it about 3 Cals. of energy, while about 2.7 
Cals. remainin the glycocol. The resulting hippuric acid, however, 
contains about 11.6 Cals. of potential energy, or more than the 
original protein. In this case, then, the larger share of the energy 
of the excretory product (8.9 Cals. out of 11.6 Cals.), although con- 
tained in a nitrogenous substance, is derived ultimately from the 
