EVOLUTION THE MASTER-KEY 

 b. Reflex Action 



The amoeba, like the human germ, is a single cell, 

 complete in itself. When it withdraws from a 

 dangerous object it performs the functions of the 

 sensory nerve, the nerve-centres, the motor nerve, 

 and the muscles which enable you to perceive 

 and avoid an imminent vehicle. In each case the 

 action is essentially reflex; but in order to under- 

 stand what is meant by this term we must know 

 what is meant by a reflex arc. This typically 

 consists of a sensory nerve-fibre, such as, let us say, 

 the optic nerve ; a sensory cell, such as those from 

 which the fibres of the optic nerve are derived; 

 and a motor cell and fibre, such as those which 

 control the muscular tissues of the iris. When a 

 beam of light enters the eye, the reflex arc is called 

 into action, the iris is stimulated and the pupil con- 

 tracts. This is one of the hundreds of reflex 

 actions which are constantly taking place in us. 

 It is entirely independent of consciousness, the 

 centre for visual consciousness, at the back of the 

 brain, not being concerned in the process. Now 

 from a pure reflex action such as this we may go a 

 stage further. Consciousness of an approaching 

 fist may be aroused by the beam of light, and in 

 this case the reflex arc will be slightly different. 

 The sensory half of the arc will be similar, but the 

 motor half will consist of the nerve that runs to 

 the eyelid, and you will blink or wink. Yet 

 though your consciousness is involved, the action 

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