THE NERVOUS SYSTEM 309 



side than it does about neurone D in the anterior horn of 

 the opposite side. Hence the impulses generated by the 

 vinegar stimulate C, sufficiently to discharge it, so long as that 

 road is open, more quickly than they stimulate D. That C 

 should be dischargeable only so long as the foot is free implies 

 that the activity of the neurone is in some way conditioned 

 by its relation with the muscles which it innervates. When 

 the foot is held this relation is interfered with, giving to the 

 impulses generated by the continued action of the vinegar time 

 to overcome the resistance of D. 



The simile of the opening up of paths is fairly applicable to the 

 results which follow the use of artificial stimuli. Neurones seem 

 to link up in series under the influence of the impulses which 

 bombard them, popping like fireworks united by a common fuse. 



Experimental evidence points to the following conclusions : 

 (1) Resistance is offered at a synapse. This resistance must 

 be overcome before an impulse can get through from neurone 1 

 to neurone 2. (2) The impulse does not, properly speaking, 

 pass from 1, through 2. It infects 2, causing it to discharge a 

 fresh impulse. (3) Time is of the essence of this process. 

 Either the impulses head up at the synapse, or, passing through 

 into the neurone, they produce a cumulative effect within it, 

 which provokes it to discharge. (The latter hypothesis, which 

 is the less likely of the two, transfers the resistance from the 

 synapse to the neurone to be infected.) These conclusions are 

 based upon experiments of the following kind : The minimal 

 stimulus which will evoke a reflex action is determined. A 

 stronger stimulus is then applied. The reflex occurs more 

 promptly, and is more pronounced. But on further increasing 

 the stimulus, it is found that the limit of effectiveness is soon 

 reached. The proportional relation of response to stimulus 

 is much less evident than it is when the experiment is tried 

 with a nerve-muscle. Choosing a reflex action easily pro- 

 voked, the afferent path is stimulated with an electric current 

 interrupted fifty times a second. The impulses which flow 

 down the efferent path to the muscle follow one another at the 

 rate of about ten a second. A column of nerve-fibres within 

 the spinal cord is stimulated fifty times a second. Again, the 

 discharge into anterior roots has the natural rhythm of about 

 ten. The cortex of the " motor area " of the great brain is 



