THE MUSCLES. 



553 



If the two stimuli be nonmaximal, the effects of the two separate 

 stimuli will be superimposed; that is, there will be a summation of 

 the contractions. This summation occurs regardless of the time of 

 application of the stimuli. 



Summation of Stimuli. As the second stimulation was just seen 

 to add its curve to the first, so does the third add itself to the second, 



Fig. 199. Progress in Fusion of Contraction. 

 A 6, B 1, 08, per second. 



fourth to the third, etc. If the excitations occur with a rhythm 

 t is not too rapid, the various shocks are nearly equal, as shown 

 the myogram, but yet they do not mingle. These isolated shocks 

 seen when the rhythm does not exceed six per second. 

 If, now, these same excitations be repeated with a frequency of 

 enty per second, isolated shocks will not be seen. Each stimulus, 

 lasting but V 20 of a second, does not allow the muscle completely to 



