CHAP, ii.] THE CONTRACTILE TISSUES. 103 



the cause of a spasm in the muscle of B; and the stimuli following 

 each other rapidly, as being produced by the tetanus of A, they 

 must do, the spasms in B to which they give rise are also fused into 

 a tetanus in B. B, in fact, contracts in harmony with A. This 

 experiment shews that the negative variation accompanying the 

 tetanus of a muscle, though it causes only a single swing of the 

 galvanometer, is really made up of a series of negative variations, 

 each single negative variation corresponding to the single spasms 

 of which the tetanus is made up. 



But an electrical change may be manifested even in cases when 

 no currents of rest exist. We have stated ( 63) that the surface 

 of the uninjured inactive ventricle of the frog's heart is isoelectric, 

 no currents being observed when the electrodes of a galvanometer 

 are placed on two points of the surface. Nevertheless, a most 

 distinct current is developed whenever the ventricle contracts. 

 This may be shewn either by the galvanometer or by the rheo- 

 scopic frog. If the nerve of an irritable muscle-nerve preparation 

 be laid over a pulsating ventricle, each beat is responded to by a 

 twitch of the muscle of the preparation. In the case of ordinary 

 muscles, too, instances occur in which it seems impossible to regard 

 the electrical change manifested during the contraction as the 

 mere diminution of a preexisting current. 



Accordingly those who deny the existence of ' natural ' muscle- 

 currents speak of a muscle as developing during a contraction a 

 ' current of action,' occasioned as they believe by the muscular sub- 

 stance as it is entering into the state of contraction, becoming 

 negative towards the muscular substance which is still at rest, or 

 has returned to a state of rest. In fact, they regard the negativity 

 of muscular substance as characteristic alike of beginning death 

 and of a beginning contraction. So that in a muscular contraction 

 a wave of negativity, starting from the end-plate when indirect, or 

 from the point stimulated when direct stimulation is used, passes 

 along the muscular substance to the ends or end of the fibre. 

 If, for instance, we suppose two electrodes placed on two points 



(Fig. 20), A and B, of a fibre about 

 to be stimulated by a single induc- 

 tion-shock at one end. Before the 

 stimulation the fibre is isoelectric, 

 and the needle of the galvanometer 

 stands at zero. At a certain time 

 after the shock has been sent 

 through the stimulating electrodes 

 (#), as the wave of contraction is 

 travelling down the fibre, the sec- 

 tion of the fibre beneath A will 

 become negative towards the rest 

 of the fibre, and so negative towards 

 FIG. 20. the portion of the fibre under B % 



