FUNCTIONS OF THE NERVOUS SYSTEM 301 



sensory fibers of the nerve through the posterior root to 

 the spinal cord. There it may pass up to the brain 

 (Fig. 52) in one of the white columns of the cord, and, 

 affecting certain cells in the cortex, result in conscious- 

 ness and sensation, followed by voluntary motion. Or 

 it may reach directly certain .cells in the posterior horn 

 of gray matter in the cord, and be thence transmitted by 

 communicating fibers across to other cells in the anterior 

 horn, from which a motor impulse is sent forth to certain 

 muscles, and they are called into action. By this shorter 

 path motion takes place without necessarily affecting con- 

 sciousness, and without the interference of the brain ; 

 that is, the nerve cells in the cord which receive the 

 stimulus reflect the impulse to other cells of the cord, 

 which then issue motor orders, without waiting for in- 

 structions from the overruling brain. It is as if the cap- 

 tain of a company of soldiers in a great army, having 

 received from incoming scouts news of an attack by hostile 

 forces, immediately orders his men, without waiting for 

 orders from his superior officers, to turn upon the enemy 

 and repulse them. 



The medulla oblongata is also a great reflex center, 

 very many afferent-efferent circuits being completed there 

 without affecting the higher cerebral centers. 



439. Examples of Reflex Action. If a sudden flash of 

 light strikes the eyes, the lids are immediately closed. 

 Any part of the body touching a hot object is at once 

 drawn back. A person in sleep may raise his hand to 

 brush away a fly from his face, or he may start at a sud- 

 den noise whether awake or asleep. All these acts may 

 take place without conscious orders from the brain. Many 

 of the vital processes are wholly or mainly reflex acts. 

 Of most of them we are, in health, wholly unconscious. 

 MACY'S PHYS. 19 



