CHANGE OF FORM IN MUSCLE DURING ACTIVITY 



117 



twitch helps the first, pretty much according to Helmholtz's law, 

 while in the case where the second contraction rises from the 

 summit of the first, the height of the summated contraction is 

 always less than would correspond with the rule. 



We have already considered the effect, where, on repeated 

 excitation with equal, maximal, induction currents, the height of 

 twitch grows in the form of a " staircase." The significance of 

 this fact to the consequences 

 of summation has been 

 pointed out by Ch. Richet 

 (4) in particular. He 

 chiefly investigated the 

 striated muscle of crab, 

 in which the increase of 

 excitability with repeated 

 and uniform stimuli is 

 very marked. Even in 

 the case in which the 

 single stimuli individually 

 excite only sub -maximal 

 twitches, and exhibit hardly 

 any perceptible change of 

 form (are " subliminal "), 

 they may, on repeated 

 application, become effec- 

 tive, because each single 

 excitation increases the 

 muscular response to the 

 next stimulus (addition 

 latente). Fig. 49 demon- 

 strates very forcibly this 

 effect of repeated uniform stimuli, each per sc ineffective, upon 

 the muscle. 



The two first stimuli had no perceptible action, the third 

 stimulus produces a minimal contraction, the fourth, one some- 

 what greater, while the three subsequent stimuli produce very 

 marked contractions, which are fused into an incomplete tetanus. 

 It is clear that such a dependence of excitability upon a previous 

 excitation must sensibly affect the height of a summated twitch, 

 as well as the magnitude of the tetanus shortening. And thus 



FIG. 49. "Addition latente" ; muscle of Crab ; increas- 

 ing effect of seven consecutive single stimuli (induc- 

 tion shocks), each ineffective per se. (Ch. Richet.) 



