ITS STRUCTURE AND FUNCTIONS. 265 



because the loss of heat has to be increased in the hot surround- 

 ings of a Turkish bath. 



Take the reverse picture : a cold winter's day. Most of us 

 look pale and pinched. The blood-vessels of the skin are empty 

 and the skin itself feels cold. The amount of radiant heat given 

 off is diminished, for the blood has been switched back to the 

 blood-vessels of the internal organs. 



Artificially also we regulate our loss by radiation, by exposing 

 or covering the surface of the body. 



Thirdly, we can lose heat by evaporation of moisture from the 

 skin. The sweat-glands are constantly pouring out small quanti- 

 ties of secretion at the orifices of their ducts ; but this is not 

 apparent to us, because as quickly as it is formed it is evaporated 

 or converted into vapour by the heat of the body. If, however, 

 the surrounding conditions of atmosphere, etc., require an increase 

 in the loss of heat, the secretion of sweat is much increased, 

 and the fluid so poured out cannot be converted into vapour 

 sufficiently quickly ; it therefore lies on the skin as drops or beads 

 of sweat. This is popularly called sweating ; but please remember 

 that, physiologically speaking, we are always sweating, and it is 

 only when either the excess of secretion, or the prevention of the 

 ordinary amount of evaporation by a moist condition of the 

 atmosphere on a calm, muggy day hinders its conversion into 

 vapour that the sweat becomes visible, or, as it is called, " sensible 

 sweat." Protected by such a perfect regulating mechanism, it has 

 been possible for experimenters to enter rooms the temperature of 

 which seems fearful, and would be rapidly fatal were it not that 

 the air in them had been absolutely dried, so that loss of heat by 

 evaporation could be fully carried out. 



Thus more than loo years ago two observers were able to 

 remain with impunity in a dry chamber heated to 260^' F., and 

 with ease in one so hot that it became painful for them to touch 

 the metal buttons of their clothes. And, as you all know well, a 

 bath at 120"^ F. is, to say the least, uncomfortable, whereas that 

 temperature in the hot-room of a Turkish bath is nothing. 



Other instances of this regulation of the temperature by loss 

 of heat readily explain themselves, and it is needless for me to 

 say more. 



