ANIMAL HEAT. 253 



CHAPTER XIII. 



ANIMAL HEAT. 



OXE of the most important phenomena presented by animals and 

 vegetables is the property which they possess of maintaining, more 

 or less constantly, a standard temperature, notwithstanding the 

 external vicissitudes of heat and cold to which they may be sub- 

 jected. If a bar of iron, or a jar of water, be heated up to 100 or 

 200 F., and then exposed to the air at 50 or 60, it will imme- 

 diately begin to lose heat by radiation and conduction ; and this 

 loss of heat will steadily continue, until, after a certain time, the 

 temperature of the heated body has become reduced to that of the 

 surrounding atmosphere. It then remains stationary at this point, 

 unless the temperature of the atmosphere should happen to rise or 

 fall : 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 living animals the case is different. If a thermometer be 

 introduced into the stomach of a dog, or placed under the tongue 

 of the human subject, it will indicate a temperature of 100 F., very 

 nearly, whatever may be the condition of the surrounding atmo- 

 sphere at the time. This internal temperature is the same in sum- 

 mer and in winter. If the individual upon whom the experiment 

 has been tried be afterward exposed to a cold of zero, or even of 20 

 or 30 below zero, the thermometer introduced into the interior of 

 the body will still stand at 100 F. As the body, during the whole 

 period of its exposure, must have been losing heat by radiation and 

 conduction, like any inorganic mass, and has, notwithstanding, main- 

 tained a constant temperature, it is plain that a certain amount of 

 heat has been generated in the interior of the body by means of the 

 vital processes, sufficient to compensate for the external loss. The 

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

 heat. x 



There are two classes of animals in which the production of vital 



