108 THE CONTRACTILE TISSUES. 



long way off the primary coil pr. c., no visible effect at all follows upon the 

 discharge of the induction-shock. The passage of the momentary weak cur- 

 rent is either unable to produce any nervous impulse at all, or the weak 

 nervous impulse to which it gives rise is unable to stir the sluggish muscular 

 substance to a visible contraction. As we slide the secondary coil toward 

 the primary, sending in an induction-shock at each new position, we find 

 that at a certain distance between the secondary and primary coils, the mus- 

 cle responds to each induction-shock 1 with a contraction which makes itself 

 visible by the slightest possible rise of the attached lever. This position of 

 the coils, the battery remaining the same and other things being equal, 

 marks the minimal stimulus giving rise to the minimal contraction. As the 

 secondary coil is brought nearer to the primary, the contractions increase in 

 height corresponding to the increase in the intensity of the stimulus. Very 

 soon, however, an increase in the stimulus caused by further sliding the 

 secondary coil over the primary fails to cause any increase in the contrac- 

 tion. This indicates that the maximal stimulus giving rise to the maximal 

 contraction has been reached ; though the shocks increase in intensity as 

 the secondary coil is pushed further and further over the primary, the con- 

 tractions remain of the same height, until fatigue lowers them. 



With single induction-shocks then the muscular contraction, and by 

 inference the nervous impulse, increases with an increase in the intensity of 

 the stimulus, between the limits of the minimal and maximal stimuli ; and 

 this dependence of the nervous impulse, and so of the contraction, on the 

 strength of the stimulus may be observed not only in electric but in all 

 kinds of stimuli. 



It may here be remarked that in order for a stimulus to be effective, a 

 certain abruptness in its action is necessary. Thus as we have seen the con- 

 stant current when it is passing through a nerve with uniform intensity does 

 not give rise to a nervous impulse, and indeed it may be increased or 

 diminished to almost any extent without generating nervous impulses, pro- 

 vided that the change be made gradually enough ; it is only when there is 

 a sudden change that the current becomes effective as a stimulus. And the 

 reason why the breaking induction-shock is more potent as a stimulus than 

 the making shock is because as we have seen ( 44) the current which is 

 induced in the secondary coil of an induction-machine at the breaking of the 

 primary circuit is more rapidly developed, and has a sharper rise, than the 

 current which appears when the primary circuit is made. Similarly a sharp 

 tap on a nerve will produce a contraction, when a gradually increasing 

 pressure will fail to do so ; and in general the efficiency of a stimulus of any 

 kind will depend in part on the suddenness or abruptness of its action. 



A stimulus in order that it may be effective must have an action of a 

 certain duration, the time necessary to produce an effect varying according 

 to the strength of the stimulus and being different in the case of a nerve 

 from what it is in the case of a muscle. It would appear that an electric cur- 

 rent applied to a nerve must have a duration of at least about 0.0015 second 

 to cause any contraction at all, and needs a longer time thanthis to produce 

 its full effect. A muscle fibre apart from its nerve fibre requires a still 

 longer duration of the stimulus, and hence, as we have already stated, a 

 muscle poisoned by urari, or which has otherwise lost the action of its 

 nerves, will not respond as readily to induction-shocks as to the more slowly 

 acting, breaking and making of a constant current. 



In the case of electric stimuli, the same current will produce a stronger 



1 In these experiments either the breaking or making shock must be used, not some- 

 times one and sometimes the other, for, as we have stated, the two kinds of shock differ 

 in efficiency, the breaking being the most potent. 



