NET AVAILABLE ENERGY—MAINTENANCE. 423 
its average metabolizable energy. In other words, it is computed 
that under the conditions of these experiments, with a ration more 
than sufficient for maintenance, the net availability of the energy 
of the crude fiber was practically zero. The authors report no 
experiments upon rations below the maintenance requirement, but 
appear to regard the metabolizable energy of the crude fiber as being 
indirectly available, under such conditions, substantially in the 
manner assumed by Rubner and already explained. 
As has been noted, Zuntz & Hagemann’s conclusions as to the 
value of crude fiber for work production are in apparent harmony 
with those of Wolff, which will be discussed in the next chapter, but 
on the other hand they contrast sharply with the results of Kellner 
(see Chapter XIII, §1), who observed a high percentage utilization 
of the energy of one form of crude fiber in the ration of fatten- 
ing cattle. On previous pages some reasons were presented for 
questioning the quantitative accuracy of Zuntz & Hagemann’s 
computations, but even aside from these their conclusions as re- 
gards the value of crude fiber are difficult to reconcile with obvious 
facts. Thus they compute (loc. cit., p. 280) that the expenditure 
of energy in the mastication and digestion of average straw is 
greater than its metabolizable energy, so that for the horse this 
material has a negative value. When forming part of a mainte- 
nance ration we mey probably assume that below the critical 
amount of food (p. 408) the heat generated during the digestion of 
the straw would be of use to maintain the body temperature, but 
this could not possibly suspend the expenditure of energy in the 
various forms of internal work, such as respiration and circulation. 
Since, however, by hypothesis, the straw can contribute no energy 
directly for these purposes, it follows that the consumption of this 
material alone cannot reduce the loss of tissue below the amount 
requisite to supply energy for the internal work, while on an 
exclusive straw ration above the critical amount of food the more 
straw the animal consumed the sooner it would starve. 
Orcanic Acips.—The results of a considerable number of ex- 
periments in which salts of organic acids were ‘njected into the 
blood have already been presented in Chapter V (p. 157). The 
general result was that lactic and butyric acids caused little or 
no increase in the heat production of the animal—in other words 
