\\i AMMM. HKAT 175 



tliis extra heat, and so tends to keep the temperature of all 

 parts the same. The skin is a little colder than the internal 

 organs, and the blood passing through the skin gives up some 

 of its heat, which is lost from the body, and the blood thus 

 cooled is wanned again as it is subsequently distributed through 

 the various organs. 



Regulation of Heat. If the temperature of a man doing 

 ,u ti\r work is taken with a thermometer it will be found to be 

 the same as when he is at rest. The man is producing a 

 great deal more heat by the activity of his muscles, but he 

 does not become any hotter. He must therefore be losing 

 more heat to make up for the increased production. When 

 more heat than usual is formed, more blood is sent to the skin 

 and so more heat is lost. The increased amount of blood sent 

 to the skin favours an increased perspiration, and this leads to 

 the increased loss of heat. A man feels warmer when he 

 is working because the skin is warmer, on account of the 

 greater quantity of blood sent to it, since we form our 

 judgment of the warmth of the body from the temperature 

 of the skin. We feel warm when the skin is warm, and 

 cold when the skin is cold. The temperature of the body, 

 then, is regulated by means of the skin, so that if the pro- 

 duction of heat varies the loss varies hand in hand with it. On 

 a cold day the loss of heat from tkc body would be much 

 greater and would go on much faster than on a hot day if the 

 blood-supply to the skin was always the same, the radiation to 

 the cold things around being much greater. Hut, as we have 

 seen, cold causes the blood-vessels of the skin to constrict, and 

 thus diminishes the blood-supply and at the same time checks 

 the perspiration, so that the loss of heat is diminished. On a 

 hot day the loss of heat from the body would be but little if it 

 were not for the increased perspiration, for the radiation to 

 the warm things around may be but little, or there may be 

 none. A man can enter a hot chamber or oven where the 

 air is very much hotter than he is without his body being 

 raised in temperature; this is due to the profuse perspira- 

 tion set up. Thus when the temperature of the air around 

 varies, the loss of heat is still kept very nearly the sanu- 

 by the regulation of the supply of blood to the skin and 

 of the perspiration. In cold weather, however, a trifle 



