ix ELECTRICAL EXCITATION OF NERVE 217 



or opening of the battery circuit will then discharge absolutely 

 simultaneous induction currents in all the secondary coils, 

 and these can be led off to points of the nerve by means of un- 

 polarisable electrodes. The method is shown in the accompanying 

 diagram (Fig. 200) from Werigo (58). 



Four combinations are possible, relatively to the direction of 

 the two exciting currents. These may either be homodromous 

 (ascending or descending), or, with opposite direction, may flow 

 to or from each other, so that with the first the kathodes, with the 

 second the anodes, are in juxtaposition. If current intensity 

 is so adjusted that one current in itself gives minimal, the other 

 no contraction, both shocks being in the ascending direction, a 

 reciprocal effect appears ; i.e., with riot too great a distance 

 between the exciting electrodes, the ascending stimulus directed 

 towards the muscle (peripheral), and of itself inadequate (infra- 

 minimal), perceptibly augments the action of the central, 

 ascending current, while conversely a central, ascending, infra- 

 minimal stimulus inhibits the already minimal effect of an 

 ascending, peripheral current. If both induction currents are 

 descending, the same effects as in peripheral, inframinimal 

 excitation hold good of the central, and vice versa. If the 

 currents flow towards each other, the final effect is that of 

 mutual augmentation, until a maximal twitch may arise from 

 two stimuli, each perse ineffective; while with not undue distance 

 between the excited parts an antagonistic inhibition may be 

 detected. 



These results coincide with those of Sewall (58), and are 

 easily reduced to the polar action of the current. " Augmentation 

 will invariably be found on applying the exciting current in the 

 vicinity of the kathode of the modifying current, with the 

 converse diminution when it is sent in near the anode." Always, 

 however, the increase of excitability in the katelectrotonic region 

 is more strongly marked than its diminution in that of anelectro- 

 tonus. If both shocks are effective, even if unequally, the 

 distance between the two excited parts being such that electro- 

 tonic effects are absolutely excluded, the muscle reacts to 

 the stronger excitation only and that as if this alone were 

 present. There would thus seem to be no real interference in 

 the sense of addition or subtraction of stimuli. If the distance 

 between the excited parts is reduced, the effects become much 



