EVOLUTION THE MASTER-KEY 



If you cross one leg over the other and sharply 

 tap, with the edge of your hand, the superjacent 

 knee, just below the knee-cap, the leg will be 

 jerked forward. Much more markedly will it jerk 

 if the stimulus be applied by a friend when you 

 are thinking of something else, and especially if 

 you have interlocked your fingers and are striving 

 to pull your hands apart. Now this "knee-jerk" 

 may be regarded as a typical reflex action ; but we 

 have already observed that it varies (inversely) 

 with the amount of attention which the subject 

 gives to it. The centre for this reflex is in the 

 spinal cord, and to the centre there run the voli- 

 tional motor fibres from the leg-centre on the sur- 

 face of the brain. Now, if anything has hap- 

 pened to break or press upon these motor fibres 

 in their course from the brain to the cord, or if the 

 cells from which they start have been destroyed, 

 it is found that the involuntary knee-jerk is greatly 

 exaggerated; while, of course, any voluntary jerk- 

 ing of the leg is impossible. If, on the other hand, 

 the motor cells in the brain, or their fibres, are 

 irritated that is, stimulated by anything, the 

 knee-jerk is greatly diminished. It is obvious, 

 then, that the brain-cells, in health, are constantly 

 exerting an inhibitory or restraining action upon 

 the cells in the spinal cord. What is true of this re- 

 flex is true of dozens more ; and in many cases the 

 inhibitory action of the upper centre is so power- 

 ful that no reflex action occurs save when the upper 

 centre or its conducting fibres are weakened and 

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