Manchester Memoirs, Vol. xlii. (1898), iVb. \%. ly 



collateral, and if so, may, under favourable circumstances, 

 excite to motor activity the motor unit with which 

 the collateral is linked So also, artificial impulses 

 passing down the same axon from higher parts of the spinal 

 cord towards the ganglion and periphery, and flowing 

 into the same collaterals, may produce just the same 

 motor effects. Any impulse, however produced and in 

 whatever direction flowing, can pass from unit to unit 

 across the linkages in the direction from afferent to effe- 

 rent. But in no single case has it been found possible to 

 drive an impulse across a linkage in the contrary direction, 

 from the efferent to the afferent unit ; or, speaking more 

 generally, from the efferent unit to any of the other units 

 with which it is linked. This general feature is known 

 under the name of "Bell's Law." Stimulate a motor nerve 

 fibre as much as you please, you will obtain abundant evi- 

 dence that the nervous impulses, as indicated by currents 

 of action, are flowing inwards towards the cell from which 

 the fibre arises, towards the perikaryon of which the fibre 

 is the axon. You may be sure that the impulses reach 

 the nerve cell, but there they stop. You will not find the 

 slightest evidence of any other units being in any way 

 influenced. So far, at least, as gross nervous impulses are 

 concerned, the linkage is a valve admitting passage in one 

 direction only. 



Further, at the linkage there is a change in the nature 

 and character of the impulse. As I have already said, 

 all the evidence goes to shew that an impulse as it travels 

 along a nerve fibre undergoes no material change. So 

 long as it merely passes along an axon, so long as it is 

 confined to a unit, it may perhaps be slightly diminished 

 in intensity, but otherwise is not changed. When, how- 

 ever, it leaps from one unit to another, a change does 

 take place. This is seen in the relatively simple action of 

 unit upon unit, commonly called a reflex action. An 



