THE RABBIT. 93 



an impulse along its axis-cylinder, which ends in a ramifica- 

 tion among the dendrons of a nerve-cell, B, in the central 

 nervous system. Through these the impulse is transmitted 

 to the cell B itself, which in consequence sends an impulse 

 along its axis-cylinder, which ends by ramifying over the 

 surface of a muscle-fibre, 0, which is thus stimulated to 

 contract. Since in this simple case the impulse sent into 

 the central nervous system is, in a sense, sent back or 

 reflected at once, we call the whole process a re/lex action. 

 In one way this term suggests too great simplicity : the 

 motor impulse is not simply the sensory impulse reflected, 

 as a ray of light is reflected from a mirror; for at B, no 

 less than at (J and at A, we have a store of energy that is 

 unlocked by the impulse that arrives, and the result is in 

 each case disproportionate to the exciting cause. We can 

 find a better analogy by imagining a rifleman to discharge 

 a bullet at a machine which is thereby set in motion and 

 fires a cannon. 



In actual life no such simple reflex actions as we have 

 been describing occur. For no external agent can have its 

 effect confined to one nerve-fibre; nor does one fibre end 

 in a ramification about one nerve-cell alone it divides and 

 influences many nerve-cells; while finally each nerve-cell 

 in the central nervous system is surrounded by the ramifica- 

 tions of a number of nerve-fibres. Nevertheless, if we 

 substitute groups of nerve-cells, nerve-fibres, and muscle- 

 fibres for the solitary ones in our diagram, we shall 

 approximate to a notion of the simplest cases of reflex 

 action in the body of one of the higher vertebrates. Thus, 

 if we go from the light into darkness, the pupil of our 

 eye dilates without any knowledge or power of interference 

 on our own part. Similarly, a sudden loud noise while I 

 am writing will cause the muscles of my arm and hand to 

 jerk my pen forward violently, quite against my will. 

 Again, the sight of a body suddenly advancing towards the 

 eye always causes the eyelids to blink. All these are reflex 

 actions, effected in the way explained. 



12. Instinctive and Voluntary Actions. From simple 

 reflex actions like these we pass to reflex actions of a much 



