THE NERVOUS SYSTEM 225 



302. How Reflex Action gives Relief to the Brain. Reflex 

 action relieves the " thinking centers" of the brain of a vast 

 amount of work. If we were forced to use our will power 

 at every step in the process of digestion, the brain would 

 be put to a severe strain. We could not eat, and then 

 quietly go about our business. If we had to plan and will 

 every heart beat, we should soon be ready to give up the 

 struggle for life. 



If we had to exert our will every time we breathed, we 

 should soon get tired of it, and long to die. We could 

 never sleep, for the brain would have to be on the alert 

 to decide if it were time for the next heart beat, the next 

 inspiration, and the proper time for each digestive fluid 

 to flow. 



303. The Spinal Nerves. From the spinal cord thirty- 

 one pairs of spinal nerves proceed to the trunk and the limbs. 

 They pass out on each side of the spinal canal through 

 small openings at the sides of the backbone. 



Each of these spinal nerves has two roots, one going 

 from the front part, and the other from the back part, of 

 the cord. These two roots unite and form one silvery cord 

 as they pass out from the backbone (Fig. 140). 



The root which goes from the front, or anterior, part of 

 the spinal cord consists of motor fibers and controls muscles. 



The root which comes from the back, or posterior, part of 

 the cord consists of sensory fibers and transmits sensations 

 from the various parts of the body to the spinal cord. 



As each nerve trunk leaves the backbone, it subdivides 

 and sends off branches into all parts of the body. Each 

 branch contains fibers from both roots. If any one of these 

 nerves or branches is cut or injured, the power of feeling 

 and movement ceases in all those parts to which it is 



