THE SKIfl 243 



uppose the temperature of the air falls to 85 F. Profuse 

 sweating ceases; perspiration does not stop, but the sweat is 

 i less abundant, does not collect so profusely in drops, and a 

 | different method of controlling body heat comes into play. The 

 | blood is now warmer than the air, and will be cooled as it 

 | flows through the skin; owing to the heat, the skin vessels re- 

 main relaxed and large amounts of blood from the internal 

 I organs are coming to the surface. The skin is thus filled 

 1 with .warm blood and the person still feels warm, though 

 :! the blood is being cooled by the air. Heat thus passes of 

 i by direct radiation in sufficient quantities to keep body tem- 

 perature at the proper point. 



Suppose now that the temperature falls to about 70 F. 



Sensible perspiration stops entirely; the blood vessels of the 



skin partly close so that less warm blood flows through them; 



[| the body retains its heat in this way, and its general tem- 



I perature remains the same as before. If the external tempera- 



I ture falls still more, a different adjustment is necessary. The 



skin vessels constrict still further, and the warm blood is 



i confined to the internal organs until they are almost over- 



i supplied with blood. Now since so little blood is flowing 



:! through the skin, the person begins for the first time to feel 



i chilly, and it is at this temperature, i.e. between 70 and 60 F., 



lj that one is most liable to take cold. If the temperature falls 



I yet lower, more heat must be produced by increased oxida- 



1 tion. Thus at temperatures above 70 F. regulation of body 



i heat is brought about by increasing or decreasing the loss of 



i heat; below this, chiefly by increasing heat production. 



It is important to realize the significance of a constant 

 body temperature; man can exist only while warmth is as- 

 sured and if its supply were not regulated, he could live only 

 in certain climates. Because of this automatic control of 

 body temperature man can live in summer or winter, in the 

 cold regions of the north, or in the hot climate of the equator. 

 Animals with an abundance of hair do not perspire pro- 



