CHAP, ii.] THE CONTRACTILE TISSUES. 121 



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 pro'duce 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 differ- 

 ent 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 contrac- 

 tion 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 the more slowly acting, breaking and making of a 

 constant current. 



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

 a stronger contraction when it is sent along the nerve than when 

 it is sent across the nerve ; indeed it is maintained that a current 

 which passes through a nerve in an absolutely transverse direction 

 is powerless to generate impulses. 



75. We have seen that when single stimuli are repeated 

 with sufficient frequency, the individual contractions are fused 

 into tetanus ; as the frequency of the repetition is increased, the 

 individual contractions are less obvious on the curve, until at 

 last we get a curve on which they seem to be entirely lost and 

 which we may speak of as a complete tetanus. By such a tetanus 

 a much greater contraction, a much greater shortening of the 

 muscle is of course obtained than by single contractions. 



The exact frequency of repetition required to produce com- 



