HEAT LOST FROM THE SURFACE OF THE BODY 435 



tant loss of heat from the body, probably 90 per cent and upward of the whole 

 amount, is that which takes place by radiation, conduction, and the evapora- 

 tion of moisture from the skin. The actual figures are as follows: For every 

 100 calories of heat produced, 2.6 are lost in heating the food and drink; 2.6 

 in heating the air inspired; 14.7 in evaporation; and 80.1 by radiation and 

 conduction. The means by which the skin is able to act as one of the most 

 important organs for regulating the temperature of the blood, are, i, that 

 it offers a large surface for radiation, conduction, and evaporation; 2, that it 

 contains a large but adjustable amount of blood, and the quantity of blood 

 is greater under those circumstances which demand a loss .of heat from the 

 body, and vice versa ; 3, that it contains the sweat glands, which discharge a 

 quantity of moisture to be evaporated from its surface. 



The circumstance which directly determines the quantity of blood in the 

 skin is that which governs the supply of blood to all the tissues and organs 

 of the body, namely, the power of the vaso-motor nerves to cause a greater 

 or less tension of the muscular element in the walls of the arteries, and, in 

 correspondence with this, a lessening or increase of the caliber of the vessel, 

 accompanied by a less or greater current of blood. A warm or hot atmos- 

 phere so acts on the sensory nerves of the skin as to lead them reflexly to 

 cause a relaxation of the muscular fiber of the blood-vessels; as a result, 

 the skin becomes full-blooded, relatively hot, and moist from sweating; and 

 much heat is lost. With a low temperature, on the other hand, the blood- 

 vessels shrink, and with the consequently diminished blood supply, the skin 

 becomes pale, cold, and dry, an effect produced through the vascular centers 

 in the medulla and spinal cord. 



The activity of the sweat glands of the skin is also regulated reflexly 

 through the sweat centers. The increased blood supply just described is 

 favorable to increased production of sweat by the sweat glands. Thus, 

 by means of the self-regulation the skin becomes the most important of the 

 means by which the temperature of the body is regulated. 



The relative loss of heat by the means given, i.e., radiation, conduction, 

 and evaporation, will depend on two factors: first, the relative temperature 

 of the body to the surrounding air; and, second, the humidity of the air. If the 

 atmospheric temperature is the same as that of the body, of course there will 

 be no loss of heat by radiation and convection; if the air temperature is 

 higher, there will be an actual gain. When the humidity of the air is great, 

 there will be reduced evaporation of perspiration and consequent diminished 

 heat loss by this means. If we assume a moisture-saturated air at the body 

 temperature, then heat loss becomes impossible and the temperature of the 

 body will rise. This is why a hot moist climate is so oppressive, while a 

 hot but dry atmosphere is readily borne by the human body. The increased 

 evaporation of perspiration compensates for the decreased loss by radiation 

 and convection. 



