306 THE BODY AT WORK 



wants to see filled in. How is the impulse passed from the 

 arborization of axon No. 1 to the dendrites of neurone No. 2 ? 

 By what structural arrangement is resistance introduced, and 

 how is it regulated, if it varies ? Supposing the resistance to 

 be higher in one path than in another, or supposing that more 

 force is needed to enable an impulse to invade a wider field, 

 how is additional energy supplied ? To the first question no 

 answer can be given at present the mechanism by which 

 impulses are transferred from one neurone to another is un- 

 known ; yet it is convenient to find a name for the junction of 

 axon-endings and dendrites. It is termed a " synapse," on 

 the understanding that the word involves no hypothesis as 

 to its structural nature. It is generally held that resistance is 

 introduced into nerve-circuits at synapses ; although this again 

 is a provisional statement. The phenomena for the explana- 

 tion of which the idea of synaptic resistance was introduced, 

 may be accounted for on a purely anatomical basis of distribu- 

 tion. The extent to which one neurone influences another may 

 depend upon the size of the brush of fibrils with which its axon 

 touches it. If a certain force is needed to discharge a neurone, 

 a nerve-current must either have a sufficiently high potential 

 when it reaches it, or it must act upon it for a sufficient length 

 of time. There is little to choose between the arguments which 

 place the resistance at the synapse and those which transfer 

 it to the nerve-cell body. 



As a mechanism the nervous system is unthinkable, unless 

 we picture its units as independent, yet capable of forming 

 associations ; as functionally discrete, yet entering into func- 

 tional continuity. When acting, they act as chains. Impulses 

 run from link to link, from the end-twigs of an axon of one cell 

 to the dendrites of the next. Neurones are so arranged as to 

 make it impossible for impulses to escape backwards out of 

 dendrites into axon-twigs. In this respect the system is 

 valved. But there is no reason for thinking of the substance 

 of the neurone as polarized in any way. The physical accom- 

 paniment of an impulse the electric variation travels with 

 equal facility up and down its axon. 



There is no evidence of any specificity of neurones ; on the 

 contrary, it is clear that impulses of every kind that is to 

 say, from every source, for we recognize no specificity of im- 



