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Sherriiipfton 



contrast. Superposition of the mechanical results of single stimuli evoking 

 the inhibitory reflex occurs readily when the time interval between the 

 stimuli extends to 4000cr-5000(T ; in some records the interval is very much 

 longer still. 



Stimuli which, taken singly, produce no perceptible or scarcely percept- 

 ible relaxations, produce on repetition relaxations of large extent by 

 summation. P'ig. 9 exemplifies this. On the signal line each descent 

 marks a break of the primary circuit. Fig. 9, A, gives the effect of sixteen 

 feeble break shocks, each of 20 units, on the Kronecker scale, delivered 

 in the course of 4-4 seconds. The total relaxation is considerable. Fig. 9, B, 

 shows the degree of correspondence between the rate of succession of the 

 single weak stimuli and the incidence of the separate reflex relaxations. 

 The relaxation produced by a single occurrence 

 of the weak stimulus (break shock of 15 units 

 of the Kronecker scale) is seen at the top left- 

 hand corner. Then follow twenty-two similar 

 break shocks of slow and irregular repetition. 

 The effect of each of these is evident on the 

 graphic record. Next ensues an interval of 

 nearly one second without stimulus, and no 

 further relaxation occurs during that time. Then 

 follow twenty-four more of the break shocks at 

 more irregular intervals, and these again sum 

 as to the mechanical effect of their reflexes on 

 the muscle. The later members of the series of 

 stimuli succeed each other rather more rapidly 

 than do the earlier, and their combined effect 

 is seen to be greater. The greater effect of the 

 summation when the individual stimuli follow 

 each other more rapidly is seen better in fig. 10. 

 Here sixty feeble break shocks (intensity 10 units of the Kronecker scale) 

 were delivered. The first twenty-five of these were delivered at the 

 average rate of 4"5 per second, and produced relatively little relaxation ; 

 the last thirty-five shocks of the series were delivered at an average 

 rate of sixteen per second, and they produce much more relaxation — 

 more than four times as much. The more quickly delivered shocks 

 were, owing to the quicker rotation of the interruption key, given bj^ 

 more sudden opening of the primary circuit, and were individually there- 

 fore somewhat more intense ; and this applies also to the reflex of fig. 9. 

 But the much greater reflex eflfect of the more frequent than of the less 

 frequent series suggests that with the former their central reactions, 

 and not merely their mechanical elongations of the muscle, summed. 

 With summations such as that shown in fig. 8, an interpretation which 

 obviously can be put upon the result is, that the eflfect of each stimulus 

 may be divisible into two successive parts. The flrst part appears to be 



Fig. 8. 



