510 APPLIED BIOLOGY 



438. Reflex Action. How the nervous system exerts 

 its coodinating power on other organs may be clearer after 

 a brief account of some simple cases of control. 



If by accident one touches a finger to a hot stove, a sudden 

 contraction of the muscles will cause the hand to be jerked 

 away before the brain becomes conscious of the burn. The 

 explanation of such an action is that the hot stove stimulated 

 sensory nerve-endings in the finger, the stimulus was trans- 

 mitted to nerve-cells in the spinal cord between the shoulders, 

 and at once turned back as a motor impulse, which, trans- 

 mitted along other nerve-fibers back to the finger, caused 

 the muscles to contract. Since the impulse originating 

 in the stimulated sensory nerve-endings appears to be re- 

 flected back to the muscles by the spinal cord, the process is 

 called reflex action. 



Headless frogs and other animals will make the same 

 reflex movements if their toes are stimulated. This proves 

 that reflex action is quite independent of consciousness 

 (knowing, feeling), for all observations on injured men and 

 animals indicate that the brain is the organ of consciousness. 

 For example, a man with the spinal cord seriously injured, 

 say in the middle of the back, would feel no pain in the legs 

 and could not move them voluntarily ; that is, by conscious 

 action from the brain. Such facts, of which many have been 

 recorded in medical books, prove that there must be uninjured 

 nerve-fibers extending to the brain in order to have conscious 

 control of organs. Hence, a headless animal or one with the 

 spinal cord cut off just back of the head could not feel or 

 be conscious of changes occurring in the body. 



In the case of instantly closing an eyelid to escape a threat- 

 ened injury, the stimulation of the nerve of sight (optic nerve) 

 causes an unconscious reflex in the brain back to muscles 

 which move the eyelids. There are many other possible 

 reflexes through nerves in the head which are directly con- 

 nected with the brain. 



