NET AVAILABLE ENERGY—MAINTENANCE. 431 
actual gain obtained (expressed in terms of energy), at least within 
certain limits, is proportional to the amount of metabolizable energy 
supplied in excess of maintenance. This would mean that above 
the maintenance ration the energy required for digestion and 
assimilation plus that consumed in the chemical changes incident 
to the formation of new tissue (compare p. 396) is proportional 
to the amount of food. If this be true it seems more reasonable 
to conclude that each of these forms of work separately is propor- 
tional to the amount of food than to assume a compensation between 
the two, and granting this, we should have every reason to suppose 
that the same proportionality would hold good for the work of 
digestion and assimilation below the maintenance requirement. 
Character of Food.—The investigations of Zuntz & Hagemann 
(pp. 385-393) have shown that, in the case of the horse at least 
and doubtless with other animals also, the work of digestion and 
assimilation varies with the kind of food, a result which is entirely 
in accordance with what we should expect. For reasons stated in 
describing their experiments, their results are to be regarded as 
qualitative rather than quantitative, but they suffice to demon- 
strate the very marked difference as regards availability which 
exists between the relatively pure nutrients employed in the exper- 
iments of Pettenkofer & Voit, Magnus-Levy, Rubner, and others 
and the feeding-stuffs consumed by our herbivorous domestic 
animals, and to show the fallacy involved in applying the results 
of the former experiments directly to the latter case. The same 
conclusion is also indicated by the few results upon timothy hay 
on p. 424. 
Unfortunately no other direct determinations of the availability 
of the food of herbivorous animals in amounts below the mainte- 
nance ration are on record, so that we are unable to compare either 
different feeding-stuffs or different species of animals in this respect. 
The extensive investigations of the Méckern Experiment Station 
mentioned in the previous paragraph show how large a proportion 
of the metabolizable energy of the food of fattening animals becomes 
economically waste energy, thus fully confirming the conclusions 
drawn from Zuntz & Hagemann’s experiments upon the horse, but 
they afford no means of distinguishing between the work of diges- 
