26 
Prof. C. 8. Sherrington. On Reciprocal [ Nov. 4, 
Fia. 2. 
For the contraction-phase to ensue, not only 
must the external stimulus have ceased, but 
that stimulus must have possessed an intensity 
of above a certain minimal degree. 
In fig. 1, the lower reaction B, showing no 
atter-phase of contraction, was given by a 
weaker repetition of the faradic stimulus which 
in the same reflex preparation had shortly 
before caused the upper reaction A, showing 
marked after-phase of contraction. This influ- 
ence of intensity is well shown when the 
stimulus is very brief, 7.2, a single make or 
break induction shock. When the shock is a 
weak one, inhibitory relaxation of the muscle 
is its only effect obvious in the preparation 
(fig. 2). When the shock is more intense, a 
phase of contraction follows the phase of 
relaxation (fig 2), and the contraction-phase 
becomes more marked as the intensity of the 
shock is made greater (fig. 2). With a quite 
strong induction shock, eg., a break shock 
unpleasant to the observer's tongue, the 
contraction-phase ensuing on the relaxation 
may reach a height much exceeding that of 
the contraction pre-existent to the inhibitory 
relaxation (fig. 2). Also, the contraction-phase 
may last several seconds, and be much longer 
than the phase of inhibitory relaxation which 
preceded it (fig. 2). 
With longer and more complex stimuli, such 
as series of double shocks at an intermittence 
of 30 per second, a similar relaxation is even 
better evident (fig. 3). When the stimulus is 
quite weak, the reflex effect in the extensor 
muscle, ¢.g., vasto crureus, is simply an inhibitory 
relaxation (fig. 3, A). As the stimulus is 
repeated with greater intensity, a phase of 
contraction follows on the phase of inhibition, 
and the contraction-phase increases with in» 
crease of the intensity of the stimulus (fig. 3, 
B, C, D). But the phase of contraction never 
