CIRCULATION 83 



the blood forms the stimulus for a corresponding varia- 

 tion in the amount of blood brought to the surface and 

 consequently in the amount of heat given off. If the 

 blood is too warm, the surface vessels dilate, more blood 

 comes to the surface, and the skin becomes red. This 

 is very noticeable after violent exercise or exposure 

 to heat. The sweat glands are also stimulated to activ- 

 ity, and perspiration occurs, which in evaporating takes 

 heat from the body. If there is not enough heat in the 

 blood the surface vessels contract. Blood is with- 

 drawn from the surface and the skin appears white or 

 purplish. 



Curiously enough these blood vessels are also very 

 easily affected by the emotions. The face grows red 

 or white under stress of shame, anger or fear. 



Interference with the Vaso-Motor Mechanism. — If 

 there is any interference with the mechanism by which 

 the giving off of heat is regulated, heat may be produced 

 in the body by chemical action faster than it is given 

 off by perspiration and the surface blood vessels. 

 Fever results. We should therefore have some regard 

 for the extreme sensitiveness of these blood vessels. If 

 a draught strikes the body when the surface blood vessels 

 are dilated they are forced to contract, the blood is with- 

 drawn from the skin and the giving off of heat is inter- 

 fered with at a critical moment. A cold, which is 

 merely another name for fever, is apt to follow. Some 

 of the important blood vessels are very near the surface 

 in the wrists and in the ankles. In cold weather, then, 

 the wrists and ankles should be covered. When the sur- 

 face vessels are enlarged and full of blood, one feels 



