6 ELEMENTS OF PHYSIOLOGY 



from the blood so freely furnished them. This supply 

 is controlled by the nerves which regulate the size of the 

 arteries leading to the skin. The sweat glands take up 

 water and various other substances from the blood and 

 pour them out upon the surface of the true skin. The 

 water evaporates, but the salt and other solids in the per- 

 spiration are deposited on the skin ; just as salt will not 

 evaporate from the ocean, and as solids form in the bottom 

 of a kettle of boiling water. Usually the amount of per- 

 spiration from each gland is so small that it evaporates, 

 or is dried up by the air, as soon as it reaches the surface, and 

 hence does not become visible. On this account it is called 

 insensible perspiration ; it becomes sensible perspiration 

 when it is formed rapidly in warm weather or during vigor- 

 ous exercise. It does not evaporate so quickly in a moist 

 atmosphere ; and those who live near the seacoast or in 

 rainy regions show more perspiration than those who live 

 in dry regions, although the former may not perspire so 

 much. The amount of perspiration averages about one 

 and one half pints per day. In the study of physics, we 

 are taught that the evaporation of even a small amount 

 of water, as from a teakettle, uses up a large amount of 

 heat. Hence, perspiration is very effective in cooling the 

 body. 



Why is humid weather in summer so oppressive and 

 disagreeable ? Is the skin more active in throwing off 

 impurities in winter or in summer ? The mucous membrane 

 is a kind of inner skin, lining the nose, throat, and all 

 inner passages to which air has access. In case of need, 

 when the skin fails to act and ceases to throw off the usual 

 amount of impurities, the mucous membrane may become 

 swollen with blood and throw off impurities in the form 

 of phlegm and mucus. Such a condition is called a cold. 



7. The Number of Sweat Glands. Count the sweat 

 glands shown in Figure 5. This row is one half inch long. 



