284 PRINCIPLES OF ANIMAL NUTRITION. 
cordingly containing more potential energy, while the urine of 
sheep and cattle also contains not a little non-nitrogenous matter 
of some sort. Finally, the slow and complicated process of diges- 
tion in the herbivora is accompanied by fermentations and the 
evolution of gaseous hydrocarbons (methane), and perhaps of 
hydrogen, both of which carry off a more or less variable propor- 
tion of the potential energy of the food. By means of experiments 
with approximately pure nutrients it is possible to secure factors 
for the metabolizable energy of the digested nutrients of con- 
centrated feeding-stuffs, but in the case of coarse fodders about 
all that is practicable in this direction is to compute the results 
of experiments upon the total digestible matter. 
There is possible, however, a third method, viz., to compute the 
metabolizable energy upon the total organic matter of the feeding- 
stuff, expressing it either as Calories per gram or pound of organic 
matter or as a percentage of the gross energy. In the latter form 
the result would be analogous to a digestion coefficient and would 
show what proportion of the total energy of the material, as: deter- 
mined by combustion in the calorimeter, was capable of being met- 
abolized in the body. This method of expressing the results has 
certain advantages in directness and simplicity, and especially in 
putting the whole matter on the basis of energy values. In the 
succeeding paragraphs the available data will be considered from 
both the standpoints last named. 
METABOLIZABLE ENERGY OF ORGANIC MATTER. 
For a discussion of the matter from this standpoint we have to 
rely almost entirely upon the Méckern investigations already men- 
tioned. In the case of those earlier experiments in which the ration 
consisted exclusively of a single coarse fodder the computation of 
the metabolizable energy of the latter is, of course, readily made. 
In the experiments in which the food under investigation was added 
to a basal ration the computation is somewhat less simple, it being 
then necessary to compare the gross energy of the added food with 
the increase in the energy of the excreta in the second period as 
compared with the first. The details of both methods will be best 
explained by illustration. 
