blubber in the form of clothing, shelter, and the use of fire, 

 enabling him to live without freezing. 



In the winter the internal heating plant of man is stepped 

 up. Recall the high muscle tone in winter in contrast to the 

 inertia and diminished muscle tone in summer. Recall the 

 thyroid-gland enlargement in winter and its demobilization 

 in summer. Recall the high incidence of deaths from cardio- 

 vascular disease in winter and the fewer number in summer. 

 Recall the aggravation and higher incidence of diabetes 

 in winter than in summer and the aggravation and the 

 higher incidence of death from exophthalmic goiter in 

 the autumn and winter compared with that of spring and 

 summer. 



Thus far in our analysis we have attributed only to the 

 thinking brain and the thyroid gland the task of pro- 

 viding sufficient external supplementary heat to man in 

 winter to prevent death from freezing and sufficient loss of 

 heat in summer to prevent death from heatstroke. 



The mere maintenance of the warm-blooded state taxes 

 man beyond all other energy requirements, because 75 

 per cent of the food intake of man is required to maintain 

 his warm-blooded state. But heat and cold are two of the 

 factors that make up variability in climate to which man 

 must make adaptation. 



Another factor to which man must make adaptation is 

 humidity. When there is an increase in the humidity great 

 suffering from depression and heat ensues. Although being 

 naked increases man's heat loss by 35 per cent, heat loss is 

 in direct ratio to the amount of moisture that the air will 

 absorb. Therefore, the absorption of heat from the skin of 

 man is diminished in proportion to the amount of water 

 vapor in the air. 



In a steam room a maximum of depression is experienced, 

 because the skin loses little heat, and the rate of heat 

 produced by the large brain, the large thyroid gland, the 



240 



