968 



NUTRITION AND HEAT REGULATION. 



with conditions. Thus, high external temperatures will tend to 

 diminish the loss from radiation while increasing that from evapora- 

 tion, owing to the greater production of sweat. The variation 

 in this respect is well illustrated by the following table, compiled 

 by Rubner, from experiments made upon a starving dog:* 



It will be noted that between 25 and 30 C. there was a marked 

 increase in the loss of heat through evaporation. 



In man loss of heat is regulated chiefly by controlling the impor- 

 tant factors of evaporation and radiation. We accomplish this end in 

 part deliberately or voluntarily by the use of appropriate clothing. 

 Clothing of any kind captures a layer of warm and moist air between 

 it and the skin and thus diminishes greatly the loss by evaporation 

 and by radiation. In cold weather the amount and character of the 

 clothing is changed in order to diminish the heat loss. The ideal 

 clothing for this purpose is made of material, such as wool, which, 

 while porous enough to permit adequate ventilation of the air next 

 to the skin, is at the same time a popr conductor of heat and thus 

 diminishes the main factor of loss by radiation. The most impor- 

 tant means of controlling the heat loss, however, is by automatic 

 reflex control through the sweat nerves and the vasomotor nerves. 

 By these means the amount of perspiration evaporated from the 

 skin and the amount of warm blood sent through the skin are 

 controlled. Rubner speaks of this side of the heat regulation 

 as the physical regulation. By its means the body may be safe- 

 guarded from an abnormal rise of temperature. In warm weather 

 the secretion of sweat is greatly increased by reflex stimulation 

 of the sweat nerves. The greater amount of water requires a 

 greater amount of heat to vaporize it, and thus the heat loss 

 is increased. The value of this control is illustrated by a case 

 recorded by Zuntz f of a man who possessed no sweat glands. In 

 summer this individual was incapacitated for work, since even a 

 small degree of muscular activity would cause an increase in his 

 body temperature to 40 or 41 C. 



* Taken from Lusk, "Elements of the Science of Nutrition," Philadelphia, 

 1917. 



t Zuntz, "Deutsch medizinal-Zeitung," 1903, No. 25. 



