CHAP, ii.] THE CONTRACTILE TISSUES. 85 



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 con- 

 traction 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 continuing to slide 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. Sometimes however, after the contractions have for some 

 time remained of the same height, in spite of the stimulus, at 

 each fresh stimulation, being increased in strength, a point is 

 reached at which, with a further increase in the strength of the 

 stimulus, a new increase of contraction sets in; but we must not 

 attempt to explain here this paradoxical super-maximal contraction 

 as it is called. 



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 

 we have seen that the constant current when it is passing through 

 a nerve with uniform intensity does not give rise to a nervous 

 impulse and that it may be increased or diminished to almost any 

 extent without generating nervous impulses, provided that the 

 change be made gradually enough ; it is only w r hen there is a 

 sudden change that the current becomes effective as a stimulus. 

 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 steeper rise than the current which appears 

 when the primary circuit is made ; and accordingly we find that 

 the breaking induction shock is more potent as a stimulus than 

 the making shock. Similarly a sharp tap on a nerve will produce 



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

 sometimes one and sometimes the other, for the two kinds of shock differ in efficiency, 

 the breaking being the most potent. 



