204 THE HUMAN MECHANISM 



remained fairly constant in amount between 90 F. and 65 F. 

 now increase to compensate the inevitable loss, and continue to 

 increase as the atmospheric temperature continues to fall. The 

 body is now striving against the effects of a rapid and inevitable 

 loss of heat by producing more heat, and continues to do so until 

 somewhere near the freezing point (32 F.) it can no longer pro- 

 duce enough heat to balance the loss ; the temperature of the 

 body then falls and the man ultimately freezes to death. 1 



Briefly, then, at an external temperature somewhere be- 

 tween 65 and 70 heat production exactly equals heat trans- 

 fer, and it is not necessary that the body make any special 

 effort to get rid of heat or to compensate for heat loss. 

 The blood is properly distributed between the skin and internal 

 organs, and there is no excess in either. This we may call the 

 ideal or optimum temperature, for the given conditions. 

 Above this point measures must be taken to provide for an 

 adequate heat output by sending a larger quantity of blood 

 to the skin and by the secretion of perspiration ; below this 

 point measures of the opposite kind must be taken to check 

 heat loss or even to increase heat production. 



17. Changes of the optimum temperature with high humid- 

 ity, with wind, and with muscular activity. High humidity, 

 by facilitating the transfer of heat from the body, raises the 

 optimum temperature a few degrees; a room is comfortable 

 at 65 when the air is dry ; it is too cool when the air is 

 moist. Wind may raise the optimum temperature still more, 

 and for the same reason; it may be safe to sit in a breeze 

 at 75 when it is decidedly unsafe to do so at 65 or 70. 



Muscular activity on the other hand, because of the produc- 

 tion of larger quantities of heat, lowers the optimum tem- 

 perature, for at the lower temperature the agencies of heat 

 transfer can get rid of the excess of heat without a large 

 blood flow to the skin and without inducing perspiration. 



1 In all this it must be remembered that the body is still lightly clad and 

 at rest. 



