68 PHYSIOLOGY. 



found running to the arteries of the limbs. When the 

 muscles of any organ, say the legs, act, they need a greater 

 supply of blood. Now, at the same time that nerve im- 

 pulses are sent to the muscles of the legs to make the 

 muscles shorten, impulses are sent along other fibers of 

 the same nerves to make the arteries dilate, and allow 

 more blood to flow to these muscles. 



Vaso-motor Nerves. The vaso-constrictor and the 

 vaso-dilator nerves taken together are called vaso-motor 

 nerves. 



Centers of Control of Circulation. The centers of 

 control of the blood tubes are in the cerebro-spinal nervous 

 system. There is no evidence that the sympathetic gan- 

 glia are centers of reflex action. 



Blushing. How is it that the face sometimes flushes 

 so suddenly ? Because of some emotion, you say. But 

 how does the emotion bring this about ? We have already 

 learned about the muscles in the wall of the arteries. We 

 are now prepared to understand that in the normal condi- 

 tion nervous impulses are acting on these muscles, keeping 

 them partly shortened, and so keeping the arteries of a 

 moderate size. Under the influence of certain emotions, 

 the caliber of the arteries is suddenly enlarged, and hence 

 the change in color. 



The Regulation of the Size of the Arteries. Through 

 the sympathetic system the blood supply of all the organs 

 of the body is regulated. Any organ needing more blood 

 sends a message (nerve impulse) to some nerve center, and 

 in response nerve impulses are sent to the muscle fibers of 

 the supplying artery, and the amount of blood sent to that 

 organ is regulated. For instance, a piece of ice is laid 



