362 HUMAN PHYSIOLOGY 



70 per cent, of all the heat is lost by conduction and 

 radiation. 



By the evaporation of sweat, heat is rendered latent, and 

 is taken from the body, which is thus cooled just as the hand 

 may be cooled by allowing ether to evaporate upon it. If 

 the amount of sweat vaporised is known, it is possible to 

 calculate the amount of heat removed from the body hi 

 this way. The loss is comparatively small only about 

 14 per cent, of the whole. The extent depends upon the 

 rapidity with which evaporation goes on, and this is governed 

 by the amount of sweat secreted, and by the dryness and 

 temperature of the atmosphere. Thus a warm dry climate 

 is better borne than one which is warm and moist, since in 

 the former the loss of heat by evaporation is so much freer. 

 Of the various factors increasing sweat secretion, heat is 

 probably the most important. 



Since the temperature of the skin is governed by the state 

 of the cutaneous vessels, and the amount of sweat produced 

 by the state of the sweat glands, and since both of these 

 are under the control of the nervous system, the elimina- 

 tion of heat from the skin is presided over by a nervous 

 mechanism. 



B. Respiratory Passages. By conduction and radiation, 

 and by evaporation from the respiratory passages, about 10 

 per cent, of the heat is got rid of in man. In the dog and 

 some other animals, the proportion of heat eliminated in 

 this way is considerably greater. 



C. Urine and Faeces. Since these are warmer than the 

 surrounding air, a certain amount of heat is lost through 

 them. The amount is small something less than 2 per 

 cent, of the whole. 



Temperature. In all higher animals, the loss of heat and 

 the production of heat are so nicely balanced that the tem- 

 perature of the body remains fairly constant under all con- 

 ditions. If an extra amount of heat is produced, say in 

 muscular exercise, it is at once eliminated by the skin, and, 

 if the body is exposed to a low temperature, loss of heat is 

 rapidly checked by contraction of the cutaneous vessels and 

 diminished activity of the sweat glands. 



