190 THE HUMAN MECHANISM 



until a point is finally reached at which its peculiar chemi- 

 cal properties are destroyed. 



3. Temperature and vital activities. When we come to 

 the activities of living cells activities which, it will be 

 recalled, depend on chemical changes precisely the same 

 thing holds true and in so striking a manner as to create a 

 widespread but erroneous impression that this dependence 

 upon temperature is peculiarly characteristic of living things. 

 The one-celled animal, amoeba, moves about more actively 

 and digests more food at 20 C. than at 10 C. ; bacteria 

 grow more rapidly at the room temperature than near the 

 freezing point; the pitch of the note made by a cricket rises 

 with the temperature, indicating that the movements of the 

 wing covers which produce the sound are being made more 

 rapidly ; and in the winter sleep of hibernating animals we 

 have a beautiful example of the decline of vital activities 

 with the fall of external temperature. 



Nor are the living cells of the human body exceptions to 

 this rule. The rate of the heart beat varies directly with the 

 temperature of the blood, and the character of the breathing 

 movements is influenced by the same cause; a cooled muscle 

 contracts more slowly, a cooled gland secretes less abun- 

 dantly. If the temperature of the body itself falls, every 

 vital activity is depressed, and death itself may result from 

 undue cooling. 



4. The constant temperature of the body. This depression 

 of nervous, muscular, and glandular activity results, how- 

 ever, only from a fall of the temperature of the body, not of 

 that of the surrounding air or other medium. These two 

 things are by no means the same, as may be readily seen 

 from the fact that a thermometer placed in the mouth indi- 

 cates almost the same temperature of the body on warm and 

 on cold days ; even while we are shivering with cold the 

 thermometer gives about the same reading as when we are 

 enjoying the warmest summer weather. The temperature of 



