122 ELECTRO-PHYSIOLOGY CHAP. 



muscular twitch during their action upon medullated, motor 

 nerves. With increased duration of current beyond a certain 

 point the twitches increase also, and reach their maximum with a 

 relatively short closure, after which they cannot be augmented by 

 any further prolongation of current or increase of intensity (pro- 

 vided an adequate strength of current is employed at the outset). 



If we compare the susceptibility of different excitable sub- 

 stances to currents of brief duration, it will be found lowest in the 

 non-fibrillated plasma of protozoa and in smooth muscle-fibrils, 

 highest in medullated nerve-fibres ; midway are cardiac muscle 

 and striated skeletal muscle. This is well shown in experiments 

 with single induction shocks, the effect of which agrees in the 

 main with that of excessively brief constant currents. The 

 medullated nerve of striated vertebrate muscle is peculiarly 

 sensitive to this method of stimulation, the latter itself less so 

 (especially when curarised), and smooth muscle still less, where in 

 order to excite by single induction shocks an enormous intensity 

 is often required. It is remarkable that the same graduated 

 difference appears to exist between medullated and non-medullated 

 nerve-fibres in regard to their susceptibility to single, brief currents 

 (more particularly to induced currents), as between striated and 

 smooth muscle. It is far less easy to elicit twitches from the 

 claw -nerves of the crayfish with single induction shocks than 

 with the constant current. 



We must now consider the second postulate in du Bois' 

 " universal law of excitation," by which he affirms that a positive or 

 negative variation of current must always be of a certain abruptness 

 in order to excite, and that (with otherwise uniform conditions) 

 the excitation appears more certainly, and within a certain range 

 more strongly, in proportion as the variations of intensity are 

 more sudden in their onset. 



Whatever application this may have to medullated nerves in 

 connection with twitching, striated, vertebrate muscles, it is by 

 no means a universal law, appropriate to all excitable tissues. 

 Nothing is easier than to show in the usual frog's nerve-muscle 

 preparation (the " physiological rheoscope ") that even the weakest 

 electrical currents may excite, provided they are adjusted 

 sufficiently sharply, i.e. that the intensity of variation is as steep 

 as possible. The peculiar susceptibility of these nerves towards 

 even a trace of frictional electricity is due to the presence of 



