REGULATION OF THE HEAT LOSS. 829 



The relative values of these different means of heat loss are 

 estimated as follows by Vierordt: 



1. By urine and feces 1.8 per cent, or 47,500 calories. 



2. By expired air: Warming of air 3.5 " " 84,500 



Vaporization of water from lungs 7.2 " " 182,120 



3. By evaporation from skin 14.5 " " 364,120 



4. By radiation and conduction from skin .73.0 " " 1,791,820 



Total daily loss = 2,500,000 



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

 portant factors 3 and 4. We accomplish this end in part deliber- 

 ately 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 poor 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. 

 In warm weather the secretion of sweat is greatly increased by re- 

 flex 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 * 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. 



The control through the vasomotor nerves is doubtless even 

 more important. The blood-vessels bring the warm blood to the 

 skin, where it loses its heat by conduction and especially by radia- 

 tion to the cooler air. When the surrounding air is much below the 

 temperature of the body the vasoconstrictor center is stimulated, 

 the blood-vessels in the skin are constricted, the supply of warm 

 blood to the skin is diminished, and therefore the amount of heat lost 

 is less. The reflex in this case may be attributed primarily to the 

 action of the cool air on the cold nerves of the skin. The impulses 

 carried by these fibers to the nerve centers stimulate the vasocon- 

 strictor center or that part of it controlling the vasomotor fibers 

 to the skin. On warm days, on the contrary, the blood-vessels 

 in the skin are dilated sometimes to an extreme extent, the 

 supply of warm blood is therefore increased, and more heat 

 is lost if the air is lower in temperature than the blood. The 



* Zuntz, " Deutsche medizinal-Zeitung, " 1903, No. 25. 



