HUMAN STRUCTURE AND LIFE-ACTIVITIES 523 



The muscular system is the chief source of increased heat 

 production, while the other organs probably generate the heat 

 which is more or less constant, as during sleep or complete 

 rest. When these organs are unable to supply the necessary 

 heat, shivering, which is involuntary activity of muscles, 

 may begin and thus increase the internal supply of heat. 



Loss of heat is partly by respiration. This is very impor- 

 tant for dogs, which " pant " when overheated because their 

 skin perspires little except on the pads of the feet. They 

 also lose heat rapidly from the evaporation that occurs on 

 the surface of the protruded tongue. Chickens are often 

 seen breathing rapidly with the mouth open ; and thus birds 

 lose heat from their lungs and air-sacs. In man, however, 

 the skin is the one great heat-regulator. 



The loss of heat from the human skin is controlled by 

 nerves, some of which regulate ( 414) the flow of blood to 

 the skin and sweat-glands, and some stimulate these glands 

 into activity. Rapid exercise causes the sweat-glands to 

 become active. The effect of the sweat is illustrated by the 

 familiar cooling of bottles of water wrapped with wet towels 

 exposed to warm air ; of water allowed to evaporate from the 

 hands or face on a summer's day; or of the porous water- 

 jugs and canvas bags which the inhabitants of some hot 

 countries use for their drinking water because the small 

 amount of oozing water is evaporated and cools the water 

 in the jug. We also know that following a bath on a hot 

 day the body cools rapidly, and the explanation is that the 

 heat of the skin was used to evaporate the water. Now, the 

 sweat-glands are simply mechanisms for covering the skin 

 with water ready for evaporation at all times when the skin 

 is warmed by blood circulating rapidly. 



In addition to the heat lost by evaporation of water on 

 the surface, there is much loss by radiation, especially in cold 

 weather when the skin gets warm after exercise which is not 

 often active enough to cause moistening the skin with sweat. 



