436 PRINCIPLES OF ANIMAL NUTRITION. 
We have already seen reason to believe that this is the case to a 
very limited extent only, even in the fasting animal, an to a still less 
degree in one consuming food. If we are justified in thinking that 
the critical amount of food for herbivorous animals is ordinarily 
less than the maintenance requirement, it follows that the heat 
production on a maintenance ration is in excess of the actual needs 
of the organism for heat by an amount depending upon the avail- 
ability of the metabolizable energy of the food, and that this excess 
of heat is disposed of by “ physical” regulation. That such is the 
case appears to be clearly indicated by the writer’s experiments 
upon timothy hay (p. 424), since there was obviously no such in- 
direct utilization of the heat resulting from the work of digestion 
and assimilation as takes place, according to Rubner’s theory, 
below the critical amount of food. If, now, the temperature to 
which such an animal is exposed falls, it is in accord with all that 
we know regarding the regulative processes in the body to suppose 
that the additional draft on it for heat will be compensated for by 
a fall in the emission constant rather than by an increased produc- 
tion of heat, or, to put it in another way, that some of the heat 
resulting from digestive work will be utilized to maintain the tem- 
perature of the animal instead of being at once dissipated. 
No exact experiments upon the influence of external tempera- 
ture on the maintenance requirement appear to have been made, 
but Kern, Wattenberg & Pfeiffer * have investigated the influence 
of the greater exposure to cold caused by shearing upon the metabo- 
lism of sheep consuming a maintenance ration. A slight decrease 
in the proteid metabolism was found to result, due, as Pfeiffer con- 
jectures, to a more rapid growth of wool after shearing, but the 
corresponding difference in the metabolism of energy is insignificant. 
The removal of a nine-months fleece appears to have caused at first 
an increased excretion of carbon dioxide, but this practically dis- 
appeared within four or five days and is probably to be attributed 
to greater muscular activity on the part of the shorn animals. 
Comparing the results before shearing with those obtained from 
five to sixteen days after, we have the following averages, the 
amount of water-vapor given off being only an approximate esti- 
mate: 
* Jour. f. Landw., 39, 1. 
