ii CHANGE OF FORM IN MUSCLE DURING ACTIVITY 115 



the line (a, y, h, i, k). We can readily see that the height of 

 the summated twitches must be greatest, i.e. doubled, when the 

 interval of both stimuli, like the stage of rising energy, is a 

 simple contraction. This rule naturally loses its significance when 

 several uniform stimuli follow successively at equal intervals, 

 since a maximum of contraction is soon reached and cannot be 

 exceeded. On the other hand, it is possible that each individual 

 stimulus in incomplete tetanus may produce an equivalent 

 period of rising energy. V. Kries, however, showed that this 

 does not occur, even with summation of only two twitches. 

 As is obvious from the above scheme, the apex of the summated 

 curve must coincide with that of the second single twitch, or lie 

 vertical to it, if Helmholtz's law is of general application. 

 According to v. Kries (2), however, this is not the case. In 

 1886 he pointed out that in summated contractions the maxi- 



FIG. 47. Schema of superposition of two twitches. (Helmholtx.) 



mum of shortening was reached much sooner after the second 

 excitation than in a single twitch, i.e., in other words, the period 

 of rising energy is shorter in the second twitch than in the 

 first. If, with v. Kries, we denote the interval at which the 

 apex of the summated twitches succeeds the second stimulus, the 

 " apex-time," and the magnitude of the ordinates of the sum- 

 mated twitch the " apex-height," we find that (as above) in a 

 series of " rising " or " falling " summated contractions (i.e. in the 

 period of ascending or diminishing energy), discharged by two 

 maximal induction currents, the " apex-time " decreases with a 

 rising "apex-height" (Fig. 48). This is expressed in the accom- 

 panying series of curves, in which the place of the second 

 stimulus remains unaltered, while that of the first can be moved 

 to any distance ; the apex of the summated twitch falls so much 

 farther from the first stimulus in proportion as it lies higher. 

 If we compare a rising and falling summated twitch, it will be 

 found that the " apex-time " of the first is higher than that of the 



