212 Prof. C. S. Sherrington and Miss S. C. M. Sowton. [June 29, 



observations, where the inhibitory stimulus is employed intercurrently against 

 a background of contraction, this rebound manifests itself as increase of the 

 background contraction to a height above that which it had prior to its 

 depression by the intercurrent inhibition (fig. 9, also fig. 1, c, and fig. 6). 



Circumstances favouring the exhibition of the flexor rebound are, just as in 

 the case of extensor rebound, (1) considerable intensity of the intercurrent 

 stimulus, (2) a somewhat brief duration of the intercurrent stimulus, 

 (3) considerable intensity of the background stimulation. 



Flexor rebound occurs not only when the background contraction is 

 provided by electrical stimulation, but also when a "natural" reflex is in 

 progress (fig. 6). Thus it follows intercurrent inhibition of a reflex 

 contraction evoked by pinching the pinna of the ear, or when the reflex 

 interrupted by the inhibition is of some source not clearly traceable in the 

 experiment and arises apparently " spontaneously." 



Marked rebound may ensue although the amount of elongation caused by 

 the inhibitory stimulus may have been very small (fig. 10, b), owing to the 

 length of the muscle at the time when the inhibitory reflex was evoked being 

 already great. The flexor rebound in our experience does not present the 

 prolonged tonic character which extensor rebound so often exhibits in the 

 decerebrate preparation. The rebound contraction is short-lasting ; when at 

 all prolonged it frequently has a somewhat rhythmic form (fig. 10, b). In a 

 series of somewhat quickly repeated elicitations it, like extensor rebound and 

 even more markedly than that, diminishes rapidly. In other words, when 

 provoked a number of times in rather rapid succession it soon tires out (fig. 6). 



A point of interest in regard to the rebound is the following : As shown 

 above, the contralateral nerve, although its predominant reflex effect on the 

 flexor is inhibitory, does, under certain circumstances, produce instead of 

 inhibition a weak contraction of the muscle. When this latter is its result, 

 on withdrawal of the stimulus which has excited the weak contraction there 

 not infrequently ensues increase of the contraction to beyond that already 

 excited (fig. 10, a). In other words, rebound seems to ensue although the 

 stimulus has excited no apparent precurrent inhibition. Possibly in these 

 cases an inhibitory effect is really produced during the stimulus, but remains 

 masked by concurrent excitation due to pressor fibres mixed with the 

 inhibitory in the afferent nerve. On that supposition the rebound might 

 still be post-inhibitory, although the inhibition was not apparent in the total 

 result on the muscle. 



