THE NERVOUS SYSTEM 



22 3 



This unconscious motion is the result of reflex action 

 of the spinal cord. It is called reflex because the impres- 

 sion does not go to the brain, but is reflected, meaning 

 turned back again, from the sensory nerves through the 

 motor nerves. 



300. Importance of Reflex Action. We rarely stop to 

 think how important reflex action is to our health, comfort, 

 and safety. Because we are able to do hundreds of things 



POSTERIOR 



FIG. 140. BLACKBOARD SKETCH. 

 Illustrating the path of a simple nervous reflex action.* 



every day without any effort of the will, we are apt to for- 

 get its importance. In fact, the greater part of nerve power 

 expended in the body goes to produce these numberless 

 reflex actions. 



1 For illustration, let us note what happens when the tips of the fingers 

 are in danger of being burnt. The organ of sensation in the finger tip 

 sends the impression of pain along the sensory fibers (through the ganglion) 

 of the posterior root by means of a long nerve branch to the cells in the 

 posterior horn of the gray matter of the spinal cord. At the cord, the 

 sensation may go to the brain, or instead it may at once pass to the anterior 

 horn of the gray matter of the cord. Thence the impulse is sent forth 

 along the motor fibers to the muscles of the arm. The muscles promptly 

 contract, and the fingers are jerked away from the irritating object even 

 before the brain knows what is taking place. 



