264 THE HUMAN SKIN : 



only within comparatively narrow limits. Thus, in what we call 

 fever, the body-temperature may go up to io5''F. or io6°F. ; but 

 this itself is not altogether without risk, especially if continued for 

 long, and a temperature above this is mostly fatal. Again, loss of 

 heat below a certain point will itself prove fatal ; hence the value 

 of warmth in restoring life after immersion or exposure to severe 

 cold of any kind. 



How this constant temperature is maintained is full of interest. 



It is evident that in a cold climate we must manage somehow 

 to lose less heat, and in a warm climate lose more in order to keep 

 our temperature equable. This regulation of expenditure of heat 

 is chiefly the duty of the skin, governed by the central nervous 

 system, and is performed in three ways : — First, by conduction ; 

 second, by radiation ; third, by evaporation. 



By conduction we mean the loss or gain of heat by contact 

 with colder or warmer bodies. Thus, putting an ice-bag on the 

 head will cool it, and the application of a warm bottle to the feet 

 will warm them. We can in this way vary somewhat our expendi- 

 ture of heat by placing ourselves in contact with hotter or colder 

 bodies, and chiefly by putting on clothing composed of materials 

 which are bad conductors of heat, such as wool, flannel, etc. 



Secondly, by radiation. By this we mean the heat given out 

 in rays by a glowing body, siich as the sun or a fire, which falls on 

 and heats objects placed in its rays, but does not raise the tem- 

 perature of the intervening medium through which it passes. 

 This is thoroughly understood in the practical form by young 

 ladies, who know that radiant heat does not improve the com- 

 plexion, and therefore use parasols, fire-screens, etc. 



Just like any other heat-generating bodies, we give out more or 

 less radiant heat, accordingly as the blood-vessels of the skin are 

 distended with blood or contracted and empty. Stand by a man 

 who has just finished a football match or run a mile race. His 

 skin is red and glowing, the blood-vessels are full, and you cannot 

 fail to perceive how much radiant heat he is giving out. 



Similarly, observe your fellow-men in a Turkish bath. They 

 look like boiled lobsters, for the hot blood has, to borrow an elec- 

 trical term, been switched oft' from the internal parts of the body 

 and is coursing chiefl)- through the blood-vessels of the skin, 



