308 THE BODY AT WORK 



which the study of electricity has rendered us familiar ; but 

 it must be evident from the experiment just described that 

 these terms are not really applicable to nervous phenomena, 

 convenient though they may be for use in an allegorical sense. 

 Holding the foot does not, by any mechanism which we can 

 recognize, switch off the shorter circuit, yet the impulses 

 abandon it for the longer path. There is no evidence of a 

 struggle to free the foot that has been fixed, coincident with 

 the spread of impulses, as they gather sufficient strength to 

 reach the nervous mechanism of the other leg. The right foot 

 not being available, the impulses choose the route to the left 

 foot. Any attempt to explain this in terms of resistance and 

 potential involves the formulation of a number of subsidiary 

 hypotheses ; easy to devise, no doubt, but stultifying to the 

 explanation exactly in proportion as they complicate it. Yet 

 the hypothesis of lines of greater and of less resistance (keeping 

 as far away from electrical analogies as possible) is essential 

 to any explanation of nervous phenomena, and is, moreover, 

 justified by the evidence available. There are two causes 

 in chief upon which it depends : (1) The greater -the number 

 of neurones in a linear chain, the greater is the number of 

 synapses to be traversed. If A, B, C are in the same circuit, 

 the sum of their resistance has to be overcome. (2) The 

 greater the number of neurones amongst which a nerve-current 

 has to be subdivided, the smaller the charge available for each 



T> 



of them. Imagine A~ so placed as to divide B and C, 



the charge delivered by A between. This arrangement has, 

 probably, an anatomical expression which accounts for the 

 relative ease or difficulty of a path, even on the supposition 

 that impulses do not open out as they advance do not spread 

 along all the branches into which an axon divides but keep 

 to a given line. The axon of neurone A divides, to branch 

 about B, C, and D ; but its representation in the several 

 pericellular nets (the expression may pass for the sake of the 

 simplicity which it introduces into the picture) is unequal. 

 In the vinegar experiment the impulses delivered to the spinal 

 cord by the root-ganglion neurone A pass to neurone B of 

 the posterior horn. B's axon arborizes more freely about 

 the cell-body of neurone C in the anterior horn of the same 



