THE SKIN 285 



that fall out may not be replaced and the head becomes 

 bald. Then the afflicted person generally resorts to hair 

 tonics most of which are utterly useless. The best way 

 to avoid the dreadful fate of having a shiny bald head is by 

 washing the hair occasionally with soap and water, by 

 brushing the hair well and keeping up a good circulation 

 in the scalp. 



We have said that one function of the skin is to regulate 

 the temperature of the body. How does it do this? 

 Most of the lower animals are, as we say, cold blooded. 

 Their temperature goes up and down with the changing 

 temperature of their surroundings. But we are endowed 

 with a remarkable system of heat regulation which is so 

 perfected that between the heat of summer and the ex- 

 treme cold of winter our bodily temperature scarcely 

 fluctuates more than a degree. Of course what keeps 

 up our temperature is the burning of fats, carbohydrates 

 and proteins in our tissues, but our skin regulates 

 the rapidity with which heat is allowed to escape, 

 and in this way keeps our temperature uniform. Our 

 blood vessels are under the control of nerves which regu- 

 late their diameter and thus control the amount of 

 blood that passes through them. When the nerves of 

 the skin are stimulated by cold they generate impulses 

 in the nerves supplying the blood vessels of the skin caus- 

 ing these blood vessels to contract. The blood is driven 

 from the skin and hence does not radiate heat so rapidly 

 to the outside. When the blood is too warm either from 

 surrounding heat or from exercise there is an enlargement 

 of the blood vessels of the skin. More heat is radiated 

 and at the same time the sweat glands secrete more per- 

 spiration which is poured out at the surface where it evapo- 

 rates. Evaporation always produces a lowering of 

 temperature. Wet your fingers and then wave your hand 



