308 



COORDINATION AND SENSATION 



In this way the effect of an external stimulus may reach 

 and bring about action in any part of the body. This is in 

 brief the general plan of inducing action in the various 

 organs of the body. This plan, however, is varied according 

 to circumstances, 'and at least three well-defined forms of 

 action are easily made out. These are known as reflex 

 action, voluntary action, and secondary reflex action. 



Reflex Action. When some sudden or strong stimulus 

 acts upon the nerve terminations at the surface of the 

 body, an immediate response is frequently observed in 



MUSCLE 



SPINAL CORD 

 FIG. 138. Diagram illustrating reflex action of an external organ. 



some quick movement. The jerking away of the hand 

 on accidentally touching a hot stove, the winking of the 

 eyes on sudden exposure to danger, and the quick move- 

 ments from slight electrical shocks are familiar examples. 

 The explanation of reflex action is that external stimuli 

 start impulses in neurons terminating at the surface of the 

 body and these, in turn, excite impulses in neurons which 

 pass from the spinal cord or. brain to the muscles (Fig. 

 138). Since there is an apparent turning back of the 

 impulses by the cord or brain, the resulting movements 

 are termed reflex?- 



Reflex Action and the Mind. If one carefully studies 

 the reflex actions of his own body, he will find that they 



1 Derived from the Latin re, back, &n&Jlectere, to turn or bend. 



