1 6 Foster, Physical Basis of Psychical Events. 



being, at least in the majority of cases, obvious and 

 distinct. 



The nerve fibres which compose the nerves are, as \ve 

 have seen, parts of units of two different functions, parts of 

 afferent or sensory units and parts of efferent or motor units. 

 Now, when we study nerve fibres by the help of artificial 

 stimulation, and take the current of action as the token of 

 the phase of nervous activity of the nervous impulse, we can 

 find no differentia between the sensory and motor unit. 

 In both, the nervous impulse travels with equal facility in 

 either direction, and with the same features. We can 

 with ease drive a nervous impulse backwards towards 

 the periphery along an afferent fibre, and forwards 

 towards the spinal cord along an efferent. We can, 

 in certain cases, do the same thing along fibres running 

 within the central nervous system, and the nervous 

 impulse thus propagated in what appears to be an inverse 

 direction, may manifest itself by other effects than the 

 mere current of action. 



Nevertheless, when we deal not with parts of units but 

 with units linked together, the nervous impulses, nervous 

 changes, nervous influences, call them what you will, pass 

 across the linkages in one direction only, and that may 

 be broadly defined as the direction from the sensory or 

 afferent unit to the motor or efferent unit. Not only 

 natural but artificial impulses readily pass from the 

 afferent axon to the motor units, with which that axon is 

 linked. They do this irrespective of the direction in 

 which they are passing along the axon. The afferent 

 axons of the cells in posterior spinal ganglia pursue a long 

 course within the spinal cord, constituting definite strands. 

 As each axon passes along, it gives off collaterals, which, 

 in many cases, form linkages with motor units. Natural 

 impulses flowing up the axon from the ganglion, and 

 coming ultimately from the periphery, may pass into a 



