164 THE HUMAN BODY. 



In that case, the skin becomes a cooling-apparatus. It 

 works in two ways : 



1. The deep layer of the skin is full of small blood- 

 vessels. The effect of heat is to make these blood-vessels 

 grow larger. The blood, then, flows into them, away from 

 the deeper vessels. That is the reason your face gets red 

 when you are heated. While the blood is in these vessels 

 of the skin, it grows cool much faster than it does when it 

 is deep in the body. It is nearer the cool air. The skin, 

 then, receives more blood when we are heated, and spreads 

 it out in a thin layer near the surface, and so cools it. By 

 cooling the blood, the whole body is cooled. 



s. The two or three million sweat-glands do a most 

 important part of the regulation of the heat of the body. 



In cities, water-carts go about in hot weather sprinkling 

 the streets. This lays the dust, and cools the air. Much 

 of the sprinkled water evaporates as it touches the warm 

 stones ; and, wherever water evaporates, it makes things 

 around it a little cooler. The sweat-glands form a great 

 watering-apparatus for the surface of the skin. The per- 

 spiration evaporates ; and the skin, and the blood in it, are 

 cooled. The hotter it is, the more we perspire ; and, the 

 more we perspire, the more heat is taken away. Men can 

 stay for a time in a temperature of 200 F., and even more 

 if they perspire freely. If perspiration is checked, they 

 can not easily endur even a moderate heat. The reason 

 why we suffer more from heat in what we call a " sticky 

 day" in summer, is that the air is moist, and does not take 

 up the moisture from the skin so fast as drier air would. 

 Our bodies are wet, and evaporation goes on slowly. 



SUGGESTION TO TEACHERS. Show the cooling effect of evaporation by 

 throwing a spray of alcohol or ether on the hand, with an atomizer. 



