VENTILATION, HEAT, AND CLOTHING 95 



enough to prevent any odor in the room, for air that has 

 no odor is generally pure and safe for use. 



174. Warmth of the body. Heat is produced by oxi- 

 dation. This warms the whole body and also gives it 

 power to think, and move, and work. Oxidation takes 

 place in every cell of the body, but most of the fat is 

 oxidized in the cells of the lungs, and most of the sugar 

 in the cells of the liver. Muscle cells also produce a 

 great deal of heat when they work. The body has the 

 power of making its fires burn high or low as its work 

 needs, but its own warmth always remains the same. A 

 thermometer shows its temperature to be 98.5 degrees F., 

 whether we feel warm or cold. 



175. The feeling of heat and cold. We sometimes feel 

 very warm and again very cold, but our bodies always 

 have the same degree of heat. We feel warmth mostly 

 in the skin. So if the skin is warm, we feel warm all 

 over, but if it is cold, we feel chilly all over. In fevers a 

 sick person often feels very cold, for his skin may be cold 

 while his body may really be in a hot fever. 



176. How the body varies the heat. When we work 

 hard, we need a great deal more heat than when we are 

 still. In winter we also need much heat to warm the body, 

 while in summer we need but little. So we must vary 

 the amount of heat. We can do this by varying our food. 

 Fat makes a great deal of heat. So in winter we like fat 

 meat. In summer we do not like fat so well, but prefer 

 fruit. This has little fat, but a great deal of sugar, which 

 produces less heat. In winter we eat more food than in 

 summer. When we move about, we are warmer than 

 when we keep still. So in winter we feel like working, for 

 we need the heat of exercise to keep ourselves warm. 



177. How the skin gives off heat. The body has a 



