218 ELECTRO-PHYSIOLOGY CHAP. 



more complicated (owing to' the interference of electrotonic 

 alterations of excitability) than when one shock only is acting; 

 since both the modification of the central current by the 

 peripheral and that of the peripheral by the central then come 

 into play. In such cases, all the resulting phenomena are in 

 line with the laws of electrotonus. 



Kaiser (59) has recently described a special case of interference 

 between two excitations discharged at different points of a nerve. 

 He found, i.e. with simultaneous excitation of the frog's sciatic 

 at two points as far apart as possible, on the one hand by tetanising 

 alternating currents, on the other by glycerin, that the glycerin 

 tetanus was sometimes inhibited at the beginning of and during 

 the electrical stimulation. Since the same effect appears in the 

 simultaneous action of two different chemical stimuli (glycerin 

 and NaCl, or glycerin at both points), explanation by electrotonic 

 alterations of excitability is ah initio excluded. When the same 

 chemical stimulus acts upon two distinct points of the nerve, 

 there is at most a very moderate tetanus, the sudden strengthening 

 of which after amputating the upper seat of excitation is very 

 striking. It seems tolerably certain that the inhibition that 

 occurs in nerve-fibres, subjected simultaneously at two distinct 

 points to tetanising stimuli, is due to processes which run 

 their course in the nerve itself. Since the negative varia- 

 tion (as shown by the capillary electrometer) is invariably 

 augmented under these conditions, instead of diminishing, as 

 might a priori be expected, there cannot be merely an inter- 

 ference effect of the electrical waves of variation in the nerve, in 

 the physical sense that they are neutralised by the coincidence 

 of unequal phases. According to Kaiser, there must, whenever 

 an excitatory wave is overtaken and submerged by the following 

 wave, be " summation of negativity in the coincident points," so 

 that in the given case " the waves of excitation resulting from 

 the two stimuli are more or less fused together, and the amplitude 

 of the variation sinks below the limen required to evoke action 

 from the muscle." 



We already know from innumerable examples that simple 

 summation of excitatory conditions may occur at any point of 

 the nerve, inasmuch as the excitability of any part of the 

 nerve appears to be heightened when it is the seat of a weak 

 and intrinsically inadequate (latent) excitation. 



