INTERNAL WORK. 347 
and we recognize from this that we have to do here with a constant 
metabolism which is indissolubly connected with life itself. The 
animal in the fasting state adjusts itselj to the minimum metabolism.” 
In other words, the metabolism and consequent heat production 
of the fasting, quiescent animal speedily reaches a minimum which 
represents the aggregate demands of the vital activities of the 
organism for energy; that is, which represents the internal work of 
the body in the sense in which the words are here used, plus the 
metabolism required for any direct production of heat which may 
be necessary to maintain the normal temperature of the animal. 
The relative importance of the internal work in the narrower 
sense and of the direct heat production as regards their demands for 
a supply of energy will appear more clearly when we consider, in 
the following paragraphs, the effects of varying conditions, and 
particularly of the thermal environment, upon the heat produc- 
tion of the fasting animal. 
Influence of Thermal Environment on Heat Production.*—An 
animal, particularly in the temperate zones, is subject to consider- 
able variations of external conditions, particularly of temperature, 
which, in the first place, tend to affect the rate at which it emits 
heat, and secondarily, within certain limits to modify the amount 
of heat produced in the body. 
Bopy TrmprrarurE.—As regards their body temperature, 
animals have been divided into two great classes: the cold-blooded 
(poikilothermic), whose temperature as a rule differs but slightly 
from that of their surroundings, and the warm-blooded (homoio- 
thermic), whose temperature remains approximately constant dur- 
ing health whatever be that of their surroundings. Since all our 
domestic animals, as well as man himself, belong to the second 
group, it alone will be considered in the following paragraphs. 
Since the animal is constantly producing heat in the various 
ways already indicated, it is obvious that in order to maintain a 
constant body temperature it must be able to give off this heat at 
the same average rate at which it is produced. Nanke illustrates 
this necessity in a striking manner by computing that if the 
* The discussion of this subject follows to a considerable extent that of 
Ranke in the introduction to his ‘Einwirkung des Tropenklimas auf die 
Ernihrung des Menschen,” Berlin, 1900. 
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