74 PHYSIOLOGY 



the body is more free ; this counteracts the tendency 

 for the internal temperature to rise in the warm sur- 

 rounding. The skin becomes warmer but the internal 

 temperature remains unaltered, or often is a little lowered 

 from over compensation. 



Moreover, when we are exposed to warmth the skin 

 becomes suffused with moisture. In the little valleys 

 crossing the ridges sculptured on the skin lie the mouths 

 of tiny glands. Their deep ends lie coiled among the tiny 

 blood-vessels in the deep layer of the skin. These 

 glands constantly secrete water which they draw from 

 the blood. Usually they secrete this so slowly that it 

 evaporates as quickly as it is formed. The evaporation 

 contributes to cool the surface of the skin. Whether 

 water exists in the solid, liquid, or gaseous state of 

 course depends only on the amount of heat in it. The 

 water, as it evaporates from the skin, abstracts heat from 

 it, large in quantity since the specific heat of water is so 

 high. The skin becomes moist in a hot room because 

 its glands become more active, so active that the 

 supply of water is greater than the evaporation. But 

 the evaporation is increased and increases the efficiency 

 of the skin as a cooling zone for the circulating blood. 



How is this mechanism worked ? What is it causes 

 the vessels to dilate, the glands to secrete more, when 

 warm air plays upon the skin ? Does the heat act 

 directly on the vessels and the glands ? It is found by 

 experiment that certain nerves control the vessels and 

 glands and cause them to dilate and secrete. These 

 nerves run from nerve-centres in the depth of the body. 

 These centres are too remote from the surface to be 

 acted on by the warmth of the air directly. But the 

 skin has also many nerves which run from it to the 

 nerve-centres. If a lighted match is held near my skin 



