348 PRINCIPLES OF ANIMAL NUTRITION. 
body of a man were unable to give off the heat which it pro- 
duces, a single day would suffice to raise it to a pasteurizing 
temperature, while in the course of a year, at the same rate, a 
temperature of over 17,000° C. would be reached. 
Furthermore, since the external conditions of temperature are 
subject to frequent and sudden changes, it is obvious that the 
balance between heat production and emission must be capable of 
prompt adjustment to varying circumstances. , 
Tuermic Rance.—The ability of the animal body to adapt 
itself to changes of temperature has, however, often been ex- 
aggerated. Asa matter of fact this adaptation is possible only 
within a comparatively narrow range, and unless we hold fast to 
this fundamental idea we are in danger of reaching fallacious 
and absurd conclusions. Man has considerably extended the range 
of climate within which he can exist by means of clothing, shelter, 
artificial heat, and even to a slight extent artificial refrigeration, 
and this fact often leads unconsciously to an overestimate of the 
possible thermic range. These means of artificial protection re- 
sult essentially in modifying the temperature to which the body is 
actually exposed, and the same is true in a less degree of the differ- 
ences in the summer and winter coats of animals. The fact still 
remains that the actual thermic range of any species is and must be 
strictly limited. All life implies a certain amount of metabclism, 
and consequently of heat production. With rising temperature a 
point must sooner or later be reached at which the animal is unable 
to impart this heat to its surroundings as fast as it is produced, and 
in which the rise in temperature necessarily resulting will prove 
fatal. With falling temperature a point will be reached at which 
the greatest possible amount of metabolism in the body will be 
unable to equal the rate at which heat is lost to the surroundings 
and the animal will perish from cold. Both the maximum and 
minimum points and the extent of the thermic range will vary for 
different species and varieties of animals, but at best the range is | 
relatively small. 
Means oF ReGuiaTion.—Within the thermic range of a given 
animal the adjustment to its thermal environment may be effected 
in one or both of two ways, viz., by a regulation of the rate of emis- 
sion of heat or by a variation in the heat production. 
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