118 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



It is evident that the condition of contracture which is developed in a 

 rapidly stimulated muscle will tend to maintain a condition of continuous con- 



FIG. 50. Effect of rapid excitations to produce tetanus. Experiment with a gastrocnemius muscle 

 of a frog, excited directly, with breaking induction shocks of medium strength, at the rate of 33 per 

 second. The weight was about 15 grams. The drum was moving much more slowly than in the pre- 

 ceding experiment. The time record gives fiftieths of a second. 



traction, there being no opportunity for the muscle to relax in the intervals 

 between the succeeding excitations. 



4. Explanation of the Great Height of Tetanic Contractions. We have 

 now to seek an explanation of the fact that a muscle when tetanized will con- 

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

 seen, repeated excitations cause, in the case of 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 height 

 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- 



1 Archiv fur Anatomic und Physiologic, 1888. 



