340 The Outline of Science 



engine. It enables the organs to work to do the work we 

 describe here and it produces heat. And in connection with 

 this heat we employ, all our lives, wonderful mechanisms which 

 even modern science has only partially mastered. 



The blood must be kept at a temperature of (in a normal 

 human body) about 98.4 F. When the air sinks very low in 

 temperature, we shiver, or stamp our feet, or rub our hands. The 

 shiver is an automatic warning to take exercise, to increase the 

 combustion in the muscles. When, on the other hand, the out- 

 side temperature rises too high, we get the stopcocks of our 

 arteries, which are tightened on a cold day, now opened wide, 

 to let the blood's heat escape by the skin. If this does not 

 suffice, automatic messages go from the nerve-centres to the 

 millions of sweat-glands in the skin, and we "sweat." To raise 

 the temperature of the watery fluid so much heat has had to be 

 extracted from the blood. If the air is dry as well as warm, 

 this mechanism is generally sufficient, but if we are in a "moist 

 heat" everybody knows how much worse it is than dry heat 

 the evaporation through the skin is checked, and the temperature 

 of the blood rises until it may be too much for our brain. Even 

 cold moist weather is trying. Our vitality is lowered in meet- 

 ing it, and the cold-microbes get their chances to invade the 

 body. 



A wonderful mechanism surely! But there seem to be 

 unintended effects at times of these ingenious devices. Take the 

 "crimson flood" on a girl's cheek at some ugly word, or some 

 word of praise, or some consciousness of guilt. The stopcocks 

 to the capillaries in her cheeks are opened wide, but we can 

 hardly suppose that some nervous reaction was evolved for that 

 purpose. Sudden paleness is more intelligible. The cheek 

 blanches in the face of danger, because the stream of blood has 

 been directed to the brain and muscles that may have to meet 

 the situation, and such temporarily useless organs as the cheek 

 have the supply cut off. 



