206 THE HUMAN MECHANISM 



as beginning a degree or two below the ideal or optimum 

 temperature and extending about five degrees below this 

 point. Like the optimum temperature, its exact position 

 varies with atmospheric conditions and with the amount 

 of muscular activity. 



19. The influence of clothing. In the discussion above we 

 have assumed that the clothing has not been changed with 

 the change of external temperature, etc. Clothing, however, 

 may modify greatly the figures given above, for it interferes 

 with the loss of heat from the skin, and the obvious effect 

 of increasing its weight is to lower the optimum temperature 

 and the region of dangerous temperature. By changes of 

 clothing, by muscular activity, and by the use of fans, man 

 has it in his power to supplement the unconscious reflex 

 adjustments which we have thus far been studying by a 

 conscious adaptation to changing conditions of climate or 

 weather. The hygienic use of clothing will be discussed 

 in Chapter XXVI. 



20. Temperature regulation and muscular activity. The 

 reactions of the body to maintain its constant temperature 

 during muscular activity are familiar to everyone, and it 

 is only necessary to sum them up and to point out some 

 practical applications. The arterioles of the skin are dilated 

 (while those of internal organs are constricted) and perspi- 

 ration is secreted. These are the same reactions which are 

 noticed when the body is exposed to external warmth, and 

 their purpose is the same in both cases to facilitate the 

 escape of heat. But in the one case they are made necessary 

 by the fact that climatic conditions interfere with the out- 

 put of heat, in the other by the fact that more heat is being 

 liberated and hence more must be got rid of. 



Seldom indeed is so severe a strain imposed upon the 

 mechanism of heat dissipation as during vigorous muscular 

 exertion, and especially when the external conditions are not 

 favorable for the output of heat. Caution is then urgently 



