58 Prof. C. 8. Sherrington. On Reciprocal [ Nov. 4, 
reflex by a single induction shock as stimulus than by a faradic stimulus, is 
explicable by the influence of mere duration of the stimulus upon the efficacy 
of a stimulus to induce the contraction-phase. The rule holds generally in 
reflex reactions that frequently-repeated small stimuli sum to a total effect 
equal to that of a single stimulus of much greater individual intensity. The 
above results amount, therefore, simply to this, that the phase of inhibition 
has to exceed a certain minimal value in order that a phase of contraction 
ensue from it. 
In fig. 3 it will be noticed that the amount of lengthening of the muscle 
in the inhibitory phase of the reflex increases with the strength of the 
stimulus, as does the ensuing contraction-phase itself. Yet itis not at all 
necessary, in order to obtain the contraction-phase, or to obtain a marked 
contraction-phase, that the lengthening of the muscle in the precurrent 
inhibition-phase should have been extensive. The intensity of contraction- 
phase bears no close relation to the amplitude of the lengthening in the 
inhibitory phase. The amount of contraction-phase is independent in great 
measure of the extent of the fore-running inhibitory lengthening of the 
muscle. Thus, in figs. 4, 5, 6, and 7, the amplitude of the lengthening 
registered in the inhibition-phase was very small, yet the ensuing contraction- 
phase is marked in fig. 4, and is very great in figs. 5 and 6. The amplitude 
of the lengthening of the muscle, which takes place during the inhibition- 
phase, depends much on the degree of contraction present at the moment 
when the inhibitory stimulus is applied. If the muscle be then already 
fairly at relaxation length, the amount of further lengthening which ensues 
during the inhibition-phase may be small. The stimulus will then, 
nevertheless, if it have sufficient intensity or duration, be followed by large 
contraction-phase (figs. 5, 6, and 7). This seems important, because it 
indicates that the main predisposing factor for the after-coming contraction is 
not the peripheral relaxation of the muscle, but the central process of 
inhibition, and this the muscle only fully expresses under suitable 
mechanical conditions. We may suppose that the central change underlying 
this central inhibition increases with increasing intensity of stimulus— 
‘intensity including, as was argued above, duration (within limits) of stimulus. 
The amount of the after-coming contraction-phase seems, therefore, 
proportioned to the amount of the precurrent inhibition which takes place 
centrally. This argues in favour of regarding the contraction-phase as due 
to a central rebound from inhibition to excitation.* 
It was said above that a stimulus, too weak when brief to evoke an after- 
phase of contraction, becomes effective when prolonged (fig. 4). The range of 
* Of. “Integrative Action, &c.,” p. 212 ; ‘Journ.-of Physiology,’ vol. 36, p. 191. 
