CHAPTER XII 



THERMAL PHENOMENA OF THE BODY 

 A. THE 'CONSTANT TEMPERATURE 



1. The normal temperature. No characteristic of the 

 human mechanism is more remarkable than its constant 

 temperature. Whether we are awake or asleep, by night or 

 by day, at work or at rest, at home or abroad, in summer 

 or in winter, in the tropics or in the polar regions, in subter- 

 ranean caves or on lofty mountain peaks, the temperature of 

 healthy human beings is always nearly the same. So steady 

 is this temperature that an increase or decrease of two or 

 three degrees gives just cause for anxiety, and a change of 

 seven or eight degrees is looked upon with alarm. 



In many modern laboratories constant temperatures are 

 obtained by the use of a thermostat, the apparatus of which 

 is visible and easily understood ; but no such special appa- 

 ratus regulates the constant temperature of the human body, 

 and we have rather to seek an explanation in the coordi- 

 nated activities of organs already familiar, such as muscles, 

 skin, blood vessels, and especially the all-controlling nervous 

 system. 



2. Temperature and chemical changes. Every chemical 

 reaction takes place more readily under some external 

 physical conditions than under others, and among these 

 conditions none is more important than temperature. This 

 fact is illustrated in the case of the enzymes. At the freez- 

 ing point saliva exerts no action upon starch paste ; as the 

 temperature rises, the activity of the enzyme increases up 

 to a certain point and then diminishes more or less rapidly 



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