138 CHARACTERS OF STIMULI. [BOOK i. 



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 

 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 constant 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, provided 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 current applied to a nerve must have 

 a duration of at least about '0015 sec. to cause any contraction at 

 all, and needs a longer time than this 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 



