126 MUSCULAR CONTRACTION. [CH. x. 



is represented diagrammatically in the figure. It is stimulated 

 at the end, where the two wires, + and , are placed ; it is 

 grasped in two places by pincers, which are opened by the wave 

 of thickening ; the opening of the first pair of pincers (i) presses 

 on a drum or tambour connected to a second tambour with a re- 

 cording lever (i'), and this lever goes up first ; the lever (2') of the 

 tambour connected with the second pair of pincers (2) goes up later. 

 If the length of muscle between the pairs of pincers is measured, 

 and by a time-tracing the delay in the raising of the second lever 

 is ascertained, we have the ai'ithmetical 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, so that the 

 muscle receives the second stimulus before it has finished con- 



Fig. 150. Tracing of a double muscle-curve. To be read from left to right. While the 

 muscle was engaged in the first contraction (whose complete course, had nothing inter- 

 vened, is indicated by the dotted line), a second induction-shock was thrown in, at such 

 a time that the second contraction began just as the first was beginning to decline. 

 The second curve is seen to start from the first, as does the first from the base line. 

 (M. Foster.) 



tracting under the influence of the first, a second curve will 

 be added to the first, as shown in the accompanying diagram 

 (fig. 150). The third little curve is only due to elastic after- 

 vibration. This is called super-position, 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 sub- 



