CHAPTER V. 

 ANIMAL HEAT. 



ONE of the characteristic properties of living creatures is that of 

 maintaining, more or less constantly, a standard temperature, not- 

 withstanding the external changes of heat or cold to which they are 

 subjected. If a bar of iron or a vessel of water be heated to a tem- 

 perature above that of the surrounding air, and then left to itself, it 

 will at once begin to lose heat by radiation and conduction ; and this 

 loss will continue until, after a time, its temperature is reduced to that 

 of the atmosphere. It then remains stationary at this point, unless 

 the atmosphere should become warmer or cooler ; in* which case a 

 similar change takes place in the inorganic body, its temperature vary- 

 ing with that of the surrounding medium. 



With man and animals the case is different. If a thermometer be 

 introduced into the rectum, or placed under the tongue, it will indicate 

 in man a temperature of from 37 to 38 C. (about 100 F.),* whether 

 the surrounding atmosphere be warm or cool. This internal bodily 

 temperature is sensibly the samean summer and in winter. Although 

 the external air may be at the freezing point, the internal parts of the 

 body, when examined by the thermometer, will indicate their usual 

 standard of warmth ; and in ordinary summer weather the temperature 

 of the air is, for the most part, many degrees below that of the living 

 body. As the body, however, by exposure to such an atmosphere must 

 be constantly losing heat by radiation and conduction, and yet main- 

 tains a standard temperature, it is plain that a certain amount of heat 

 must be generated in its interior, sufficient to compensate for the ex- 

 ternal loss. The internal heat, so produced, is known by the name of 

 animal heat. 



Thus it is by its own internal heat that the body is warmed. The 

 clothing used by man, and the fur, wool, or feathers by which animals 

 are protected, have no warmth in themselves ; they simply prevent 

 the body from losing heat too rapidly, and thus becoming cooled down 

 below its normal standard. Even the furnaces and fires of a dwelling 

 house only serve to moderate the cooling influence of the air ; for the 

 atmosphere, even in the warmest apartment, never rises to the heat 

 of the living body, which is still the only source of its own vital 

 temperature. 



Difference of Temperature in Different Classes of Jwimals. The 



* To convert degrees of the Centigrade scale into the corresponding value for the 

 Fahrenheit scale, multiply by 1.8 and add 32 to the product. 



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