CH. XLIIL] HEAT LOSS 639 



of the lungs, the greater will be the loss in warming the expired 

 air and in evaporating the water of respiration. In such animals 

 as the dog, which perspire but little, respiration is a most important 

 means of regulating the temperature; and in these animals a close 

 connection is observed between the production of heat and the 

 respiratory activity. The panting of a dog when overheated is a 

 familiar instance of this. A dog also, under the same circumstances, 

 puts out its tongue, and loses heat from the evaporation that 

 occurs from its surface. The great regulator, however, is un- 

 doubtedly the skin, and this has a double action. In the first 

 place, it regulates the loss of heat by its vaso-motor mechanism ; the 

 more blood passing through the skin, the greater will be the loss of 

 heat by conduction, radiation, and evaporation. Conversely, the loss 

 of heat is diminished by anything that lessens the amount of blood 

 in the skin, such as constriction of the cutaneous vessels, or dilatation 

 of the splanchnic vascular area. In the second place, the special 

 nerves of the sweat-glands are called into action. Familiar instances 

 of the action of these two sets of nerves are the reddening of the 

 skin and sweating that occurs after exercise, on a hot day, or in a 

 hot-air or vapour bath, and the pallor of the skin and absence of 

 sensible perspiration on the application of cold to the body. 



The production of perspiration has a cooling effect, since the 

 latent heat necessary for the evaporation of the sweat is derived 

 chiefly from the body. 



The relative importance of radiation and evaporation depends 

 very much upon the humidity of the atmosphere. Here it is 

 necessary to distinguish between "relative" and "absolute" 

 humidity. The important point is the amount of water which the 

 air can absorb. Now, cold air, even though it is almost dry, is 

 capable of taking up very little aqueous vapour. Warm air may 

 contain a good deal (i.e. the absolute humidity may be greater than 

 that of the cold) and yet may be far from saturated (i.e. its relative 

 humidity may be low). The loss of heat by evaporation is therefore 

 relatively small in cold weather, even though it be dry. The burden 

 of heat regulation then falls upon radiation, and it, to be efficient, 

 demands a warm skin ; hence the glow of heat we experience when 

 we take exercise in still, cold weather. 



In hot climates radiation becomes less important, and the 

 possibility of heat loss from the skin therefore depends upon 

 evaporation. Evaporation in its turn depends upon the " relative " 

 humidity of the air and upon the existence of winds. 



The loss of heat by evaporation is at its maximum in dry hot 

 climates, and is greatly promoted by the wearing of clothes which 

 are relatively porous. In such climates physical "fitness" is pro- 

 moted by the taking of a considerable amount of out-of-door exercise. 



