198 NUTRITIONAL PHYSIOLOGY 



provided and adjustments made to favor its escape or to 

 conserve it, according as the tendency is toward a rise or 

 fall in temperature. 



The first principle is illustrated by those thermostats in 

 which gas flames are automatically caused to rise when the 

 apparatus begins to cool off and are cut down when it 

 begins to be warmed above the intended level. The 

 second way of securing the same result is exemplified by 

 the common incubator used for hatching eggs, in which the 

 heat is furnished by a lamp kept burning at the same height 

 at all times, while a ventilator, opening and closing, dissi- 

 pates or retains the heat as required. In the living 

 body we can recognize the working of both these princi- 

 ples, but it is rather surprising to find how much is ac- 

 complished by the second without the aid of the first. 

 In other words, the constant internal temperature is 

 maintained much of the time by the promotion or the 

 retarding of heat loss without any appeal to the tissues for 

 metabolic support. 



Our practice of adapting our clothing to the season and 

 to in-door and out-door life is an extension of the means 

 which the organism itself employs for the same purpose. 

 Extra clothing hinders the escape of heat from the body 

 and makes possible the maintenance of the normal state 

 with no increase of oxidation in spite of some degree of 

 external cold. A race of naked savages must certainly 

 vary the amount of their metabolism much more positively 

 from summer to winter than we do with the habits of our 

 civilization. Animals may enjoy the protection of heavier 

 coats in cold weather, and they show, moreover, an instinct 

 to assume those positions that reduce to a minimum their 

 surface exposure. 



To explain in detail how changing circumstances are 

 met, let us imagine a man placed successively in atmo- 

 spheres of different temperatures. We will begin with a 

 room at 68 F., a condition which we regard as agreeable 

 and which we aim to produce when artificial heating is in 

 use. If our subject has spent an hour in this room he has 



