212 THE HEAT OF THE BODY 



of the skin to relax. As a result, much more blood is 

 allowed to pass to the surface of the body, where it can 

 be quickly cooled by contact with the air. If the blood 

 is not sufficiently cooled in this way, the sweat glands 

 are also stimulated to activity and secrete a copious 

 amount of perspiration, which they pour upon the sur- 

 face. The heat of the blood then disappears much more 

 rapidly because it is doing the work of evaporating the 

 moisture. In health, the temperature of the body, even 

 in severe exercise, never rises much above the normal 

 point of 98.4 F. 



Loss of heat. The heat-controlling mechanism has 

 to provide not only against a possible increase of body 

 temperature but also against a too rapid loss of heat 

 from exposure to cold. When there is but little activity 

 on the part of muscles and glands and the air is cold, 



the heat centre causes the 

 arterioles of the skin to 

 contract, so that compara- 

 tively little blood passes 

 through them to be chilled 

 at the surface. This with- 



FIG. 114. Diagram to show distribution drawal of blood from the 

 of blood in the body when heat is . . 



being saved (A , and is being thrown body S Surf ace IS in a HieaS- 

 off (B). The black areas represent 



the parts containing blood. The ure equivalent to putting On 

 innermost white area represents the . . 



layer of subcutaneous fat ; the outer, an QVerCOat, Since it shuts 

 the epidermis. 



the heat inside the fatty 



layer of the skin. In case this is not sufficient to maintain 

 the normal temperature, the arterioles in the feet and 

 hands, and even those in the legs, arms, ears and nose, also 

 contract to shut the blood within the trunk and thus pre- 

 serve life, even at the sacrifice of these parts by freezing. 1 



1 Those who habitually suffer from cold hands and feet simply 

 demonstrate the efficiency of this control and prove that they have 



