296 



Dr. A. Forbes. Reflex Rhythm 



[Apr. 11, 



contraction would then be due to the arc involved in the knee-jerk with its 

 short latent period. The inhibition following it, and persisting throughout 

 the application of the stimulus, would be due to the preponderant influence 

 of the spinal inhibitory centre. The " rebound contraction," on cessation 

 of the stimulus, would be due to the deferred action of the excitatory 

 centre in the hind-brain or mid-brain. Such an interpretation is only a 

 tentative conjecture at best, but it is quite clear that there is an excitatory 

 content as well as inhibitory in the ipsilateral stimulus. 



I have referred above to the defect in the commutator which appeared 

 to influence reflex response. I was unable to trace the cause of the defect 

 or to learn its exact nature. It was most clearly revealed by failure to 

 maintain an even tetanus in the nerve muscle preparation. The influence 

 which it appeared to have on ret! ex response was the relative accentuation 

 of the excitatory content of the ipsilateral stimulus. This effect was 

 inferred from the following facts. In three consecutive preparations with 

 which the commutator was employed for inhibitory stimuli after months 

 of regular use, the threshold of inhibition was found to be abnormally high. 

 That is, it required a stimulus so strong that it threw the muscles of the 

 trunk and fore-limbs into marked activity, and several times as strong as 

 is usually needed to produce effective inhibition. In two of these pre- 

 parations, after prolonged inhibitory stimulation, the subsequent inhibitory 

 response was improved instead of being impaired as is usually the case.* 



These facts seem to indicate that the quality of the stimulus delivered by 

 the commutator was such as to favour the excitatory content in the reflex 

 effect, and thus partly mask the inhibitory content. Possibly the reflex 

 condition of some of these animals favoured the appearance of the excitatory 

 content. 



In one of the three preparations just described, a stimulus of 32 units 

 applied to the ipsilateral sciatic nerve only just sufficed to produce inhibition 

 of the extensor. Stimulation of the sciatic nerve with 42 units produced the 

 oscillatory response shown in fig. 5 a. In a preparation which showed 

 effective inhibition with 7 units applied to the ipsilateral peroneal nerve, 

 a stimulus of 5 units also applied to the peroneal nerve caused similar but 

 less marked oscillations (fig. 5 b). 



It seems to me probable that these oscillations occurring upon stimulation 

 of a single ipsilateral nerve are akin to those which result from the stimula- 

 tion of two afferent nerves whose effects are opposed, and that they are due 

 to the opposition of reflex influences arising from the single stimulus. At all 



* Forbes, 'Reflex Inhibition,' etc., loe. cit. 



