BL U SUING. 87 



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 mod- 

 erate size. When the nerve is cut, these nervous impulses 

 are interrupted, and the muscle fibers, being no longer stimu- 

 lated, relax, and allow the artery to widen, and so permit 

 more blood to flow through it. Thus more blood goes to the 

 parts supplied, and they become redder, and, for the time at 

 least, warmer. 



When the end of the nerve is stimulated, the muscle fibers 

 are made to shorten and diminish the caliber of the artery, 

 and to reduce the blood supply ; and this lessened blood sup- 

 ply is shown by the pallor and coldness of the part. 



Under the influence of varying emotions, the caliber of the 

 arteries is changed, largely through the nerves of the sympa- 

 thetic system. 



In the same way, 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 upon the skin of the hand. The part becomes 

 pale, as the arteries have become narrowed. If this action 

 be continued, there may set in a decided reaction, and the 

 part become more red than usual when the reaction has made 

 the artery wider than it was before the constriction. 



As there is only a certain amount of blood in the body, it 

 is evident that if one organ receives an extra supply, some 

 other organ or organs must, for the time, receive less. For 

 instance, one begins to walk vigorously. The large muscles 

 of the lower limbs and trunk become active, and they need 

 more blood. They therefore send messages to some nerve 



