20 PHYSIOLOGICAL CHARACTERISTICS OF MUSCLE 



Effect of Two or More Successive Stimuli. If a muscle 

 receives two successive stimuli a sufficient length of time apart, 

 two curves of contraction are produced, the second being a 

 little higher than the first (beneficial effect of contraction) . How- 

 ever, if the second stimulus arrives before the period of relaxation 

 is complete, a secondary rise is produced which is called 

 superposition or summation of effects. If the two stimuli occur 



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FIG. 20. 



7, Two successive submaximal contractions. //, A series of contractions induced 

 by 12 induction-shocks in a second. ///, Marked tetanus induced by rapid shocks. 

 (Landois.) 



close enough together, the result will be one curve which is greater 

 than either would have produced separately. This is called 

 summation of stimuli. If, instead of just two stimuli, a number 

 of stimuli are applied very close together, we get the effect shown 

 in (II). If these stimuli occur still closer together the effect 

 shown in (III) is produced which is called tetanus. When 

 the stimuli occur so as to allow partial relaxation between each 

 stimulus, (II) the effect is called incomplete tetanus, but when 

 no relaxation occurs as in (III) the effect is complete tetanus. 



