CIRCULATION OF BLOOD AND OF LYMPH 163 



out into the cold air may be a whitening of the skin; but later 

 an expansion of the capillaries causes the skin to become 

 flushed. The expansion and contraction of these vessels ex- 

 plain our feelings of warmth and cold. The blood in the interior 

 of the body is considerably warmer than that on the surface. 

 Since the nerve endings which perceive sensations of heat are 

 located in the skin, and not in the inside of the body, one has 

 no sensation of heat so long as the warm blood is in the in- 

 ternal organs. But if the body is exceptionally warm, from 

 vigorous exercise, for example, the blood vessels in the skin 

 relax for the purpose of allowing the hot blood to flow more 

 rapidly through the skin, that it may be cooled off. This 

 extra rush of warm blood to the skin produces a sensation of 

 heat. In other words, the feeling of warmth which one has 

 on a warm day or after exercising, is simply a sign that the 

 body is cooling off as rapidly as possible. On the other hand, 

 on a cool day, the body wishes to retain its heat, and the 

 blood vessels in the skin are consequently constricted. The 

 skin feels cold because there is so little warm blood flowing 

 through it. Thus the feeling of warmth does not necessarily 

 mean that the body is hotter than usual, but only that the 

 arterioles in the skin are relaxed and that warm blood is 

 flowing rapidly through them. If the skin is flushed, one 

 feels warm even though he is losing heat and so actually be- 

 coming colder. 



Fainting. The common and very unpleasant experience 

 of fainting is due to a smaller supply of blood than usual in 

 the brain. This condition may be brought about by many 

 causes, e.g. by the lack of a sufficient amount of oxygen in the 

 air, by the presence of a disagreeable odor, or by some dis- 

 order in the digestive functions. The last named cause is the 

 most frequent. In such cases the action of the heart may be 

 slower than usual or the vessels in the brain may contract so 

 that it is insufficiently supplied with blood. This causes a 

 stopping of its regular activities, and the person becomes un- 



