IX ELECTRICAL EXCITATION OF NERVE 209 



excitation alone is discharged by induced currents of a certaii 

 intensity. Analogous experiments to those with frogs' nerves 

 can easily be demonstrated on warm-blooded nerves, immediately 

 after division without previous injury. 



If two unpolarisable electrodes are applied respectively to the 

 fresh cross -section, and to a point lying about 1 cm. below on 

 the rabbit's sciatic, it will be found on exciting with single 

 induction shocks of moderate strength that a twitch is discharged 

 only when the current is descending in the nerve. Under certain 

 conditions this striking reaction has a methodic value also, for it 

 is clear that when any section of the nerve, within which approxi- 

 mately equal excitability may be predicated at every point, is 

 excited with alternating currents, each single make as well as 

 break shock must take effect at a certain distance of coil. This 

 is no longer the case on stimulating the cut end of a warm- 

 blooded nerve. Only the descending direction of current will 

 then discharge an excitation, i.e. according to the direction of the 

 primary current, the break shock or make shock only. With 

 greater distance of coil, however, when the break induction 

 current is eventually alone effective, excitatory action can only 

 be expected when the current traverses the nerve in a descending 

 direction. With uniform position of electrodes and distance of 

 coil there will thus in the one case be a visible effect of excitation, 

 in the other complete absence of effect, according to the direction 

 of the primary current. And when Tick (52) observed that the 

 action of an induction shock can only be augmented when its 

 kathode, and not when its anode falls in the katelectrotonic 

 region of a polarising constant current, this must be viewed as 

 direct evidence of the polar kathodic action of induced currents. 

 It was formerly supposed by Pflliger that the total excitability of 

 the intrapolar tract could be measured by sending an induction 

 shock through it during the passage of the constant current ; but 

 this could only be correct under the presumption that the induction 

 current excited the whole tract simultaneously. Pfliiger always 

 made the induction current in the same direction as the polarising 

 current, and therefore tested excitability each time at the kathode 

 (which coincided with the kathode of the induced current) ; his 

 conclusion, that weak polarising currents strengthen the effect of 

 the (homodromous) induction current, while stronger currents 

 diminish or abolish it, must therefore be interpreted like the 



VOL. II P 



