1912.] Induced by Concurrent Excitation and Inhibition. 



295 



Of these four possible explanations, I believe B involves less needless 

 assumptions than any of the others. On this assumption a reflex view of 

 the knee-jerk would call for a third reflex arc, one of excitatory influence 

 whose centre must be local, since it clearly does not involve the hind-brain 

 directly. 



In this connection it is of interest to consider a record obtained from the 

 knee extensor with a series of stimuli of progressively increasing strength 

 delivered to the ipsilateral sciatic nerve (fig. 4). The record is in certain 

 respects similar to those shown by Sherrington and Sowton.* It presents 

 one difference : with a stimulus of 18 units three phases occur in response 

 to a single stimulus — initial contraction on application of the stimulus, 



Fig. 4. — Time-marker records in seconds. (Liverpool.) 



inhibition during the continuance of the stimulus, and " rebound con- 

 traction " on cessation of the stimulus.f Sherrington and Sowton record 

 the first two phases with weak stimuli and the last two phases with strong 

 stimuli, but in no case more than two phases with any one stimulus. 



They have elsewherej proposed tentatively an explanation of " rebound 

 contraction" as the delayed expression of the excitatory content in the 

 ipsilateral stimulus. On the same lines it seems to me that the three 

 phases occurring in response to the single stimulus of 18 units (fig. 4) may 

 possibly be explained as the discharges of the three reflex arcs assumed 

 above on the view that the knee-jerk is a true reflex. The initial 

 * Loc. cit. 



t Cf. Graham Brown, 'Quart. Journ. Exp. Physiol.,' vol. 4, p. 366. 

 I Unpublished paper on ' Reflex Rebound,' 1911. 



