318 ELECTRO-PHYSIOLOGY CHAP. 



arrival of excitation at the muscular end of the nerve occurs 

 at the two moments in which the progressive change of position 

 in the molecules of the nerve reaches the molecules of that end of 

 the muscle. We have seen that a twitch of the muscle accom- 

 panies every sudden alteration of density in the exciting current ; 

 here too, as may easily be shown, the twitch which notifies the 

 state of excitation coincides with a motion propagated in the 

 nerve from molecule to molecule in succession till it reaches the 

 muscle. The pile-like arrangement in electrotonus is not quite 

 perfect, i.e. the molecules directed contrary to the direction of 

 the pile are turned not quite at an angle of 180, but only at 

 fractions of the half-circle, of different magnitudes. The size of 

 this revolution depends, apart from other conditions, upon the 

 density of the exciting current. If this density is suddenly 

 increased to a certain degree, then each molecule in order turns 

 a fraction farther, as directed by its neighbour ; conversely each 

 molecule turns back a fraction on the momentary diminution 

 of the current density. This progressive and partial revolution 

 of the molecules already deflected out of the peripolar disposi- 

 tion is, like the original rotation at the beginning of electro- 

 tonus, accompanied by a twitch of the muscle ; while no twitch 

 occurs so long as the molecules are at rest, no matter whether 

 they are peripolar or more or less dipolar in arrangement, or 

 whether the rotation of the single molecules follows gradually at 

 greater intervals, as is the case in gradual increase or decrease 

 of current density. The more considerable the instantaneous 

 rotation communicated from neighbour to neighbour, the greater 

 will be the state of excitation expressed by the degree of the 

 muscle twitch." 



"A persistent and apparently constant state of excitation in the 

 nerve is produced by a current interrupted at short intervals (either 

 by homodromous or by alternating and heterodromous shocks) : 

 here we must picture each of these short currents as accompanied 

 at its commencement and termination by movements of the mole- 

 cules of the nerve, so that during electrical tetanisation the mole- 

 cules undergo a rapid series of rotations in different directions. 

 Under these circumstances, therefore, the transmitted movements 

 of the electromotive molecules, and the excitatory condition, co- 

 incide in the electrically-excited nerve ; neither is seen without 

 the other, neither outlasts the other. The two must therefore be 



