CHAPTER XIV. 



ANIMAL HEAT. 



ONE of the characteristic properties of living creatures is that of 

 maintaining, more or less constantly, a standard temperature, notwith- 

 standing the external changes of heat or cold to which they are sub- 

 jected. If a bar of iron or a vessel of water be heated to a temperature 

 above that of the external air, and then left to itself, it will at once 

 begin to lose heat by radiation and conduction ; and this loss of heat 

 will continue until, after a certain time, the temperature of the heated 

 body has been reduced to that of the surrounding 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 remaining constant or varying with 

 that of the surrounding medium. 



With man and many animals the case is strikingly different. If a 

 thermometer be introduced into the stomach or rectum of a dog, or 

 placed under the tongue of the human subject, it will indicate a tem- 

 perature of from 37 to 38 (about 100 F.), 1 whether the surrounding 

 atmosphere at the time be warm or cool. This internal temperature of 

 the body is sensibly the same in summer and in winter. Although the 

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

 in a condition of health, will indicate their usual standard of warmth 

 when examined by the thermometer; and even 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, like any inorganic mass, and yet maintains a standard tem- 

 perature, it is plain that a certain amount of heat must be generated in 

 its interior, sufficient to compensate for the external loss. The internal 

 -heat, so produced, is known by the name of vital or 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 the 

 bodies of animals are protected, have, of course, no warmth in them- 

 selves ; they simply prevent the body from losing heat too rapidly and 

 thus becoming cooled down below its normal standard. Even the fur- 

 naces and fires of a dwelling house only serve in a similar way to 

 moderate the cooling influence of the air ; for the atmosphere, even in 







1 To convert any given number of degrees of the Centigrade scale into the 

 corresponding value for the Fahrenheit scale, multiply by 1.8 and add 32 to the 

 product. 



(300) 



