GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 121 



tract much higher than it will as a result of a single excitation. As we have 

 seen, repeated excitations cause, in the case <»('a fresh muscle, a gradual increase 

 in irritability and consequently a gradual rise in the height of the succeeding 

 contractions, but the staircase sooner or later reaches its upper limit, and will 

 not alone account for the great shortening which occurs in tetanus. 



Effect of Two Rapidly Following Excitations. — Helmholtz was the first to 

 investigate the effect of rate of excitation on the height of combined contrac- 

 tions. For the sake of simplicity, he excited a muscle with only two breaking 

 induction shocks, of the same strength, and observed the effect of varying the 

 interval between these two excitations. He concluded that if the second stim- 

 ulus is given during the latent period of the first contraction, the effect is the 

 same as if the muscle has received but one shock ; if the second shock be applied 

 at some time during the contraction excited by the first, the second contraction 

 behaves as if the amount of contraction present when it begins were the resting 

 state of the muscle, i. e. the condition of activity caused by the first shock has 

 no influence on the amount of activity caused by the second, but the h< aght 

 of the second contraction is simply added to the amount of the first contraction 

 present. Were this rule correct, as a result of this summation, if the second 

 contraction occurred when the first was at its height, the sum of the two con- 

 tractions would be double the height of either contraction taken by itself. 



Helmholtz' conclusion, that the condition of activity awakened by the first 

 excitation has no effect upon that caused by the second excitation, has not been 

 substantiated by later observers. Von Kries ' has found that the presence of 

 the first contraction hastens the development of the contraction process result- 

 ing from the second excitation ; and Von Frey 2 has ascertained that Helm- 

 holtz's rule of summation applies only to weighted muscles. In the case of 

 unweighted muscles the summation effect is greatest when the second contrac- 

 tion starts during the period of developing energy caused by the first excita- 

 tion, i. e. during the rise of the first contraction. If the second contraction 



Fig. 54.— A schema i 'i' Hi. effect Of double excitations upon the gracilis muscle of n froc, by di Hi-r- 

 ent intervals of excitation. To obtain ibis figure, the results of different experiments were super- 

 imposed (after Von Frey). 



starts during the period of relaxation of the first, the second may be not 

 even as high as when occurring alone (see Fig. 54). 



1 Archiv Jiir Anatomic und J'hysiologie, 1888. 2 Ibid., S. 213. 



