120 CHANGE IN FORM IN A MUSCLE WHEN IT CONTRACTS [CH. X. 



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 submaximal 



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 intervened, is indicated by 

 the dotted line), a second induction-shock was thrown in, at such a time that the second con- 

 traction 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.) 



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. 



Effect of More than Two 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 



FIG. 151. Curve of incomplete tetanus, obtained from the gastrocnemius of a frog, where the shocks 

 were sent in from an induction coil, about sixteen times a second, by the interruption of the 

 primary current by means of a vibrating spring, which dipped into a cup of mercury, and broke 

 the primary current at each vibration. (Tracing to be read right to left. ) 



second ; a fourth on the third, and so on. Each successive increment 

 is, however, smaller than the preceding, and at last the muscle 



