314 ELECTRO-PHYSIOLOGY CHAP. 



lower excitability and slower reaction of the protoplasm in 

 question. And as at make of the current the excitation by 

 no means accompanies the moment of closure only, so too at break 

 the opening excitation considerably outlasts the disappearance of 

 the current. 



The capital importance attaching to the manifestation of 

 the make and break twitch in striated muscle, with regard 

 to the entire theory and discussion of current action, renders 

 it advisable once more to raise the question of what con- 

 ditions are essential to the initiation of a wave of contraction. 

 We know from experiment that in order to produce a wave, i.e. 

 a perceptible twitch of the entire longitudinally traversed muscle, 

 the magnitude of the stimulus must exceed a certain minimal 

 limit. If the stimulus is too weak the contraction either remains 

 localised, or spreads over a limited area only of the muscle by 

 conduction from section to section, until it finally dies away in 

 consequence of the " decrement." The second condition essential 

 to the propagation of excitation is a certain rapidity of process 

 of the required magnitude. The changes at the seat of direct 

 excitation must suddenly reach a corresponding magnitude, after 

 which excitation transmits them to the neighbouring sections, 

 and these in turn produce the same effect upon their neighbours. 

 That this is so is proved directly by the fact that it is easy to 

 pass a strong galvanic current into a muscle without any visible 

 excitation phenomena, which no doubt is partly due to the 

 gradual increase of local fatigue at the kathode. This applies 

 not merely to the electrical make and break twitch, but to 

 many other experiments. We need only refer to the fact that 

 mechanical excitation caused by pressure does not produce a 

 muscle twitch if it is increased gradually. 



This all throws light upon the true significance of du 

 Bois' law of excitation, since it shows that not merely the 

 local changes at the seat of excitation, but still more the 

 propagation of the excitatory process, i.e. the discharge of a 

 wave of excitation (contraction), are dependent upon the varia- 

 tions of current intensity, and the steepness of the same, in the 

 case of tissues in which conductivity is adequately developed. 

 The " universal law of excitation " refers therefore less to 

 the manner of the excitatory process, and effectuation of the 

 changes of the excitable substance fundamental to it, t f//< : 



