THE SKIN 241 



ligature. A rubber band tightly wound on the finger gives 

 similar evidence of the same fact. 



Expansion and Contraction of Blood Vessels in the Skin. 

 We have already noticed how readily the small blood vessels 

 are expanded and contracted by the action of muscles in 

 them and that this action is controlled by nerves. In the 

 skin the muscles in the small arteries are especially well 

 developed and quickly relax or contract under the influence of 

 extra heat or cold. If the temperature of the air rises on a 

 warm day, if the body is warm, or if extra heat is being pro- 

 duced, the blood vessels in the skin relax. In short, when- 

 ever the heat being produced in the body is greater than that 

 required to maintain the proper temperature a relaxation of 

 the skin vessels allows more blood to come to the surface, 

 thus increasing the loss of heat. Whenever heat loss from 

 the skin is greater than heat production, so that the body is 

 cooling too rapidly, the skin vessels contract and allow less 

 blood to flow through them. This keeps the warm blood 

 inside, and, of course, prevents the body from cooling too 

 rapidly. The perfect control which the brain thus has over 

 these vessels enables it to regulate the amount of blood 

 flowing through the skin and, consequently, the amount of 

 heat lost through direct radiation from the body. 



Perspiration as a Heat Regulator. There is another means 

 by which loss of heat through the skin is regulated. The 

 sweat glands are constantly pouring their secretion upon the 

 skin, where it is as constantly evaporated. It is a simple 

 law of physics that heat is required to evaporate moisture. 

 Hence the larger the amount of sweat poured upon the skin 

 for evaporation, the greater will be the amount of heat re- 

 quired for evaporating it, and therefore any increase of per- 

 spiration increases the heat lost by this means. Everyone 

 knows that the amount of sweat increases with the tempera- 

 ture. On a hot day one perspires profusely, and on a cold 

 day imperceptibly. All these sweat glands are supplied with 



