INTERFERENCE OF EXCITATIONS 219 



that the "all or none law" applies to striated muscle, then an 

 increase of the contraction from homotopic summation cannot 

 occur, because an isobolic system cannot show an increase of its 

 already maximal excitation by summation. Such being the case, 

 the tonic shortening of striated muscle can only be explained as 

 an expression of a heterotopic interference. 



If we assume that the summation of sub-threshold stimulation, 

 by increasing excitation, brings about a state of equilibrium from 

 below, as it were, so also inhibition may be assumed to be the 

 reverse, the level of equilibrium being reached from above, as it 

 were, by decrease of the primary excitation from strong stimula- 

 tion. This is expressed in our general scheme of the develop- 

 ment of summation and inhibition resulting from the effect of a 

 series of stimuli. At the same time the first part of the curve to 

 the fall of irritation to the level of the sub-threshold equilibrium 

 can be shortened to a minimum by strong stimulation or greater 

 frequency of the same, and we have then the type of inhibition 

 with primary excitation. As example of this I wish to again recall 

 the strychninized frog which was used in the fundamental experi- 

 ments for understanding of the theory of inhibition. If we stimu- 

 late a sensory nerve of a strychninized frog, in which the refrac- 

 tory period is already lengthened, with rhythmic single induction 

 shocks of slow frequency, the muscle arranged to make a graphic 

 record will show reflex contraction following each stimulus. If, 

 on the other hand, we apply a series of stimuli, consisting of single 

 stimuli rapidly repeated, contraction is produced only by the first, 

 or the first few stimuli (Figures 45 and 46, pages 202, 203). For 

 the succeeding stimuli the centers remain inhibited, because each 

 succeeding stimulus occurs in the refractory period of the former. 

 The origin of this inhibition shows us with particular clearness 

 how excitation produced by each single stimulus depending upon 

 the frequency of the same, falls rapidly or slowly beneath the 

 threshold of perceptible response. In this case, the state of equi- 

 librium is reached which is maintained by the following stimuli. 

 That a single stimulus is not entirely without effect upon this 

 state of equilibrium follows from the fact that during the con- 

 tinuation of the stimulus a recovery to the point of observable 



