CH. IX.] EFFECTS OF SUCCESSIVE STIMULI 101 



of the second lever is ascertained, we have the arithmetical data for 

 calculating the rate of propagation of the muscle-wave. It is about 

 3 metres per second in frog's muscle, but is hastened by warmth and 

 delayed by cold and fatigue. 



The Effect of Two successive Stimuli. 



If a second stimulus follows the first stimulus at a sufficient 

 interval of time, each will cause a twitch and two simple muscle 

 curves will be written (fig. 123, A) ; the second is a little bigger than 

 the first (beneficial effect of contraction). If the second stimulus 

 arrives before the muscle has finished contracting under the influence 

 of the first, a second curve will be added to the first, as shown in 

 fig. 123, B. This is called superposition, or summation of effects. 



If the two stimuli are in such close succession that the second 

 occurs during the latent period of the first, the result will differ 

 according as the stimuli are maximal or submaximal. If they are 

 maximal, the second stimulus is without effect ; but if submaximal, 

 the two stimuli are added together, and though producing a simple 

 muscle curve, produce one which is bigger than either would have 

 produced separately. This is called summation of stimuli (fig. 123, C). 



Effect of More than Two Stimuli. 



If a succession of stimuli are sent into a muscle, or its nerve, the 

 results obtained depend on the rate at which the stimuli follow one 

 another. If the time intervals between the stimuli are sufficiently 

 great, each stimulus will produce a simple muscular contraction, and 

 one records a succession of twitches, and the beneficial effect of 

 previous action is exhibited in what is known as a staircase (fig. 124, 

 A and B). 



If the induction shocks follow each other more rapidly, the effect 

 is a continuation of the superposition curve already described in 

 connection with two successive stimuli. Just as a second stimulus 

 adds its curve to that written as the result of the first, so a third 

 stimulus superposes its effect on the second ; a fourth on the third, 

 and so on. Each successive increment is, however, smaller than 

 the preceding, and at last the muscle remains at a maximum con- 

 traction, till it begins to relax from fatigue. 



A succession of stimuli may be sent into the nerve of a nerve- 

 muscle preparation by means of the Wagner's hammer of a coil, or 

 the vibrating reed previously mentioned (p. 92). This method of 

 stimulation is called faradisation. Fig. 124, C to F, shows the kind 

 of tracings one obtains. The number of contractions corresponds to 

 the number of stimulations ; the condition of prolonged contraction 



