THE NERVOUS SYSTEM 307 



pulses can travel equally well through neurones of all 

 forms. At every junction, in passing through each synapse, 

 they are delayed. It takes at least 0-01 second (less if the 

 knee-jerk be a true reflex action) for a message delivered to the 

 cord by a sensory root to reach a motor root. This hundredth 

 of a second the sum of the delays entailed in fording two 

 or three synapses is regarded as the minimum reflex time. 

 To it must be added, in considering any particular reflex action, 

 the time taken in travelling up sensory and down motor 

 nerves. Delay indicates resistance. If a sensory stimulus be 

 not sufficiently pronounced to provoke a reflex action, the 

 reflex may be obtained on intensifying it. Prolonging or re- 

 peating the stimulus really the same thing, since sensory 

 impulses are rhythmic, not continuous has a far more potent 

 effect than increasing its force. The resistance of synapses 

 gives way after a number of impulses have bombarded them. 

 The desire of brushing a fly from the skin, if resisted, becomes 

 intolerably urgent after a time. A persistent outflow of im- 

 pulses produced by the irritation of a spot in the cortex of the 

 brain overwhelms the nerve-muscle system in an epileptic fit. 

 The following is an experiment illustrating the spread of im- 

 pulses from their customary path to another less often used : 

 A piece of blotting-paper, wet with vinegar, is placed on the 

 inner side of the thigh of a brainless frog. There is no use in 

 trying the experiment on a frog which retains its brain ; the 

 substitution of one action for another would be an exhibition 

 of the adaptation of means to end a demonstration of the 

 animal's right of choice. Besides, the frog might choose not to 

 act, and so the experiment would fail. The brainless frog wipes 

 off the blotting-paper with the foot of the same side. This foot 

 is then fixed so that the action cannot be performed, and the 

 blotting-paper replaced. After a longer interval the frog 

 removes it with its other foot. Evidently it is more difficult 

 for the impulses generated by the irritation which the vinegar 

 causes to get across the cord than it is for them to reach 

 motor neurones on the same side. Evidently, too, the con- 

 tinued irritation of the vinegar adds to the travelling power of 

 the impulses. They are strengthened until they are capable 

 of overcoming the resistance in the longer path. " Resistance 

 in conductors " and " potential of current " are terms with 



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