THE SKIN. 163 



but that blood immediately passes upward, and is warmed 

 by the warmer stream above, with which it is mixed. A 

 fresh supply of warm blood comes down, and contributes 

 its heat to the feet. They are enabled in this way to 

 maintain nearly the same heat as the rest of the body. 



On the other hand, suppose the brain or the stomach, 

 while they are especially busy, to grow hotter than the 

 rest of the body. Their heated blood is soon mingled with 

 the general stream ; and, while it helps to heat the rest, is 

 itself cooled. The heated parts are cooled by the cooler 

 blood coming from other parts. So, by the constant cir- 

 culation of the blood, the heat of all parts is made nearly 

 equal. 



16, The air is ordinarily cooler than the body, and is 

 constantly taking heat from it. Clothing keeps us from 

 losing heat too fast. Furs and woolens have no warmth 

 in themselves. They only keep off the cold air, and keep 

 in the heat that the body makes. Besides this, in cold 

 weather, we warm the air by fires. 



If we are much out of doors in cold weather, two things 

 help us to keep warm : 



i. We eat more, and so furnish more fuel to our inter- 

 nal fires. We also get more oxygen in each breath, and 

 this is fuel too. 



s. We exercise more, and that keeps these internal fires 

 more active. 



In these ways, more heat is made ; and we can afford to 

 lose more. 



HOW THE BODY IS COOLED. 



17, Sometimes there is too much heat in our bodies, 

 as when we are exercising, or in the sun. It will not go 

 off into the air fast enough, even if we are lightly clothed. 



