304 THE NERVOUS SYSTEM 



training he will have acquired voluntary control over 

 his muscles, the scream will be restrained, and his muscles 

 will remain quiet, or will remove him from the dangerous 

 spot according to the direction of his brain. Many a sol- 

 dier has thrown down his gun and beat an inglorious 

 retreat from the scene of his first battle, and yet has after- 

 ward become the honored veteran of many a hard-fought 

 field. A child may roar with pain and clutch with all his 

 strength at the dentist's hand when his first tooth is drawn, 

 while men have been known to endure the amputation of 

 a limb without flinching. Important as it is to educate 

 the reflex nervous centers to the supervision of swift and 

 accurate useful movements, it is of even more importance 

 that the brain should still be able to exercise a superior 

 restraining power when needful in respect to even those 

 acts which may be ordinarily left to the reflex centers. 



443. Functions of the Sympathetic System. Experiments 

 have shown the functions of the sympathetic system to be 

 threefold. 



(1) This system has control of the contractile coats of 

 the blood vessels. Certain fibers, called "vasoconstrictors," 

 carry impulses by which the tone of the walls of arteries 

 and veins is maintained. If the sympathetic nerve is 

 divided in the neck, there is a general dilation of the 

 blood vessels on the same side, and a fall of blood pres- 

 sure in the arteries. Other fibers, called " vasodilators," 

 seem to possess the power of inhibiting the action of the 

 44 constrictors," and so causing dilation of the vessels. 

 A large part of the action of the vital organs is controlled 

 by these vasomotor nerve fibers which regulate the amount 

 of blood distributed to those organs. 



(2) Other fibers of the sympathetic nerves seem to stimu- 

 late directly the activity of secreting cells in the glands. 



