ACTION OF VOLTAIC CURRENT ON MUSCLE. 435 



The method by which the question has now been settled is as 

 follows : 1 — 



An oblong trough is divided by upright partitions of plaster of Paris into 

 three chambers, of which the middle one is square, the two end ones being 

 narrow but of the same length as one side of the square. The middle chamber 

 contains physiological salt solution, each of the end ones saturated solution of zinc 

 sulphate. Into these last are plunged amalgamated zinc plates, in which the 

 battery circuit ends. The current flows in nearly parallel threads through the 

 salt solution from one porous partition to the other. The muscle is prepared 

 with its bony attachments intact, and supported independently of the trough, 

 and is so held that it is immersed in the salt solution with its fibres horizontal. 

 As the trough can be freely rotated, without moving the muscle, the threads of 

 current may be made to cross the fibres at any desired angle. 



It can thus be proved that the excitatory response to closure of the 

 current is strongest when the direction of the fibres is the same as that 

 of the current ; that it is much weaker when there is a difference between 

 them of 45°, and is annulled when they cross at right angles. In 

 muscles with injured ends no effect is produced unless the strength of 

 the current is excessive ; whence it may be inferred that the ends of 

 muscular fibres are much more susceptible to this kind of stimulation 

 than the other parts. This fact is analogous to that referred to above 

 — the refusal of an injured muscle to respond to cathodic excitation, 

 when the cathode is on the injured part. 



From the preceding facts we have learned that the exciting effects 

 of the current on curarised muscle are chiefly of two kinds, those 

 which are evoked by its sudden closure, and those which are dependent 

 on its continuance. 



The first are of the nature of the excitatory responses which (p. 414) 

 have been attributed to instantaneous stimulation. These are (1) 

 the excitation wave with its concomitant electrical change, and (2) the 

 wave of contraction. The intimate relation between these is shown by 

 the fact that each is propagated in the direction of the fibres at the same 

 velocity, and that both affect each muscular element for a strictly 

 limited period of exceedingly brief duration. The dependence of rapidly 

 propagated effects (Reizivellen and Contractionsiuellcn) on the suddenness 

 with which the change which produces them is accomplished, has been 

 studied with great exactness in nerve. With reference to it, du Bois- 

 Beymond enunciated many years ago his well-known "Law of Excitation," 2 

 according to which the excitatory efficiency at any moment of a current 

 led along a nerve depends on " the rate of change of the current density " 

 (i.e., of the relation between current strength and sectional area). 

 There are no experimental data which would justify the extension of du 

 Bois-Eeymond's law to the case of curarised muscle. It is, however, so 

 evident that in muscle as in nerve, rapidly propagated excitatory effects 

 are exclusively produced by sudden changes of current strength, that we 

 may fairly assume the relation in muscle between rate of change and 

 these characteristic effects, to be analogous to that which is expressed as 

 regards nerve in du Bois-Eeymond's law. This being admitted, there is 

 little difficulty in understanding why excitatory effects of the kind in 



1 Leicher, "Ueber den Einfluss des Durchstromungswinkels, etc.," Untersuch. a. d. 

 physiol. List. d. Univ. Halle, 1888, S. 1. 



2 "Untersuchungen," 1848, Bd. i. S. 258. 



