CHAPTER XXI 



THE MAINTENANCE OF THE BODY 

 TEMPERATURE 



ALL animals are producers of heat, and must, therefore, 

 maintain temperatures above those of their surroundings 

 unless the evaporation of water more than compensates 

 for the heat of metabolism. We speak, however, of warm- 

 blooded and cold-blooded animals as though some were 

 far more liberal than others in their calorific output. 

 This is actually the case. But a better distinction be- 

 tween the two classes may be found in the fact that some 

 animals allow themselves to be warmed and cooled readily, 

 while others keep near to a constant standard of internal 

 temperature. We call a frog a cold-blooded animal, but 

 what we really mean is that it accepts for its own the tem- 

 perature of its environment. On a very hot day the frog 

 may be as warm as a man; in winter the frog is chilled 

 through and through, while the tissues of a human being 

 everywhere save at the surface are kept at the same tem- 

 perature as in summer. 



Animals which have a fixed standard to which they ad- 

 here under all ordinary conditions are spoken of as homo- 

 thermous. This trait is shared by birds and mammals. 

 Our interest, of course, centers in man's remarkable capac- 

 ity to keep his deeper tissues at the same level in spite of 

 radical external changes and equally sweeping changes in 

 his own metabolism. The means by which this result is 

 attained differ somewhat with different animals of the 

 homothermous order. We shall confine our attention 

 almost entirely to the human problem. 



The common clinical thermometer bears the mark 

 "Normal" opposite the point on its scale corresponding to 



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