6& HUMAN BIOLOGY 



vasomotor nerves which control the little muscle fibers 

 in the walls of the blood vessels. The relaxing and 

 enlarging of the blood vessels decreases the resistance to 

 the blood flow, and the heart beats faster under its lighter 

 load. The narcotic effect of alcohol is much more power- - 

 ful than its irritating or stimulating effect. The effect 

 of alcohol in causing fatty degeneration of the muscles'- 

 often weakens the heart and other blood vessels. 



Climate and Brain Work. In going to sleep the vessels in the skin 

 dilate and blood is drawn from the brain to the skin. It is difficult to 

 go to sleep when cold, for cold sends the blood to the brain and keeps 

 the mind active. On the same principle, mental work is difficult in very 

 warm weather because of the enlarged capillaries in the skin and the 

 withdrawal of blood from the brain to the skin. This increases the 

 perspiration and keeps the temperature of the body down to normal, but 

 it deprives the brain of blood needed for good mental work. Mental 

 workers in warm weather and in warm climates should seek every con- 

 dition favoring coolness . Benjamin Franklin was accustomed to strip 

 himself almost entirely of clothing when he was writing and wanted his 

 brain to work at its best. The wearing of barefoot sandals and the thin- 

 nest cotton clothing, light in color, helps to prevent mental inertia in hot 

 weather. In the Gulf states in summer and in our tropical islands the 

 best mental work can be done by rising at dawn and working before 

 the hot part of the day begins. Some of the greatest thinkers in the 

 world have lived in warm climates (Greece and India), but they wore 

 very few clothes and ate moderately of the simplest food (see p. 44). 



Congestion is a swelling of the blood vessels of some part, with the 

 accumulation of blood therein. Congestion is active when a rapid flow 

 of blood distends the capillaries. Example, flushing of face when 

 running. Congestion is passive when there is a narrowing of the out- 

 let of the capillaries, the blood moves slowly and partly stagnates in the 

 swollen vessels. Example, when the nose feels stopped up during a 

 cold. If a syringe is worked so fast that the rubber tube swells, this is 

 like active congestion ; if the end of the tube is pinched together so 

 that moderate pumping causes it to swell, this is like passive con- 

 gestion. 



Inflammation is congestion where the vessels of any part are strained 

 and injured. White corpuscles collect there to repair the vessels and 

 devour the blood that escapes and stagnates there. They also destroy 

 germs that have usually found lodgment and begun to multiply. The 



