120 CHARACTERS OF STIMULI. [Book i. 



fairly suppose that in two experiments we may in the one experi- 

 ment bring the induction-shock or other stimulus to bear on a few 

 nerve fibres only, and in the other experiment on many or even all 

 the fibres of the nerve. In the former case only those muscular 

 fibres in which the few nerve fibres stimulated end will be thrown 

 into contraction, the others remaining quiet, and the shortening 

 of the muscle as a whole, since only a few fibres take part in it, 

 will necessarily be less than when all the fibres of the nerve are 

 stimulated and all the fibres of the muscle contract. That is to 

 say, the amount of contraction will depend on the number of 

 fibres stimulated. For simplicity's sake however we will in what 

 follows, except when otherwise indicated, suppose that when a 

 nerve is stimulated, all the fibres are stimulated and all the 

 muscular fibres contract. 



This being premised, we may say that, other things being equal, 

 the magnitude of a nervous impulse, and so the magnitude of the 

 ensuing contraction, is directly dependent on what we may call 

 the strength of the stimulus. Thus taking a single induction- 

 shock as the most manageable stimulus, we find that if, before we 

 begin, we place the secondary coil (Fig. 4, sc.) a 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 current 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 towards 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 muscle 

 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 corre- 

 sponding 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 contraction. 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 contractions 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 



1 Tn these experiments either the breaking or making' shook must he used, not 

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

 shock differ in efficiency, the breaking being the most potent. 



