MAINTENANCE OF THE BODY TEMPERATURE 367 



pected with a moderate external temperature. If the 

 chamber is made warmer the share represented by 

 evaporation will be progressively increased until, with 

 an outside temperature equal to that of the body, all 

 the heat produced must be made latent by evaporation. 

 To get rid of the whole 2500 Calories in twenty-four 

 hours, a man in a chamber at 99F. would be compelled 

 to secrete through the respiratory tract and the sweat 

 glands nearly 5 liters (more than a gallon) of water. 

 He could not add measurable heat to his environment 

 that is, he could not make it warmer when it was at 

 his own temperature in the first place. 



A man subjected to this severe trial would not give 

 off extra water to any extent by increasing his breath- 

 ing, but almost wholly by increasing his secretion of 

 sweat. Animals like the dog which pant when they 

 are in a warm atmosphere rid themselves of surplus 

 heat almost entirely by increasing evaporation from 

 the mouth and throat and the extended tongue. We 

 are not to assume that their behavior indicates any such 

 distress as it would in the human subject. 



As the skin is warmed the receptors which lie just 

 under its surface are stimulated. The reflexes which 

 result are both vasomotor and secretory. The vessels 

 of the skin are dilated and a larger share of the total 

 blood-stream than usual finds its way through them. 

 This favors the cooling of the blood unless the external 

 temperature is equal to or above the internal. With 

 the ordinary balance reversed the cutaneous dilation 

 would cease to be a protective reaction if it did not 

 help to sustain the sweat glands in their augmented 

 activity. 



Humidity. The possibility of evaporating water de- 

 pends on the extent to which the air is already satu- 

 rated in other words, on the humidity. If the air in 

 contact with the skin holds all the water which it can, 

 we cannot expect it to take more save under one im- 

 portant condition: it may be warmed and gain in its 



