254 Prof. C. S. Sherrington. Break-shock 



do the moderate or weak. And since break-shock stimuli of lower strength 

 than these strong ones already produce the maximal twitch, i.e., are by 

 definition " maximal " and excite all the motor nerve-fibres to the muscle, 

 these latter strong ones cannot excite a greater number of the motor nerve- 

 fibres than do those former. The seat of the initial causation of the " supra- 

 maximal " response seems, therefore, excluded from being either at the 

 neuro-myal junction or in the muscular fibres proper. 



Bearing in mind, however, features of the " local excitatory process," the 

 process recognised by Adrian and Keith Lucas (5), (6), to occur at the seat of 

 stimulation of an excitable tissue, and elucidated by them, there seems no 

 inherent unlikelihood that a single shock applied to a nerve should, if 

 strong, evoke a short series of impulses in individual nerve-fibres. Forbes 

 and Gregg (4) have already called attention to this probability. Persistence 

 of the excited state at the locus of application of the strong stimulus should, 

 if suitably prolonged, result in initiation thence of a succession of propagated 

 disturbances, impulses, along the fibres. This view merely demands that 

 the nerve-fibre reacts essentially similarly to cardiac muscle-fibre. Garten (9) 

 has shown that under continued stimulation by a strong voltaic current 

 the response of the mammalian nerve is a rhythmic series of action-currents. 



But if motor nerve-fibres can be excited by a single induction shock so as 

 to react repetitively, so also presumably can afferent nerve-fibres. The 

 evidence of repetitive discharge in the break-shock reflex may, therefore, be 

 referable to repetitive response on the part of the directly stimulated afferent 

 fibres themselves rather than to repetitive response developed in the refiex 

 centre. To this possibility the present observations supply no entirely 

 decisive answer. It is noteworthy, however, that the motor nerve-fibres 

 indicate tetanic response only when the break-shock stimulus is of a far 

 higher strength than are break-shocks fully sufficing to evoke reflex con- 

 tractions of tetanic character when applied to the afferent nerve. These 

 strong break-shock stimuli are presumably of a strength corresponding with 

 that named by Forbes and Gregg (4), in their work with action-currents, 

 " the maximal limiting value." If the tetanic character of the reflex 

 contraction is attributable to repetitive response in the afferent nerve-fibres 

 themselves, these latter must differ from motor nerve-fibres by reacting 

 repetitively to stimuli sometimes a hundred-fold weaker than those required 

 for making ordinary motor nerve-fibres so react. Again, the differences 

 (figs. 4 and 8a) are striking between the myogram forms of, on the one 

 hand the " supra-maximal responses " produced by break-shock stimulations 

 of motor nerve, and on the other hand the break-shock reflex contractions 

 evoked by stimulation of the afferent nerve. Even in instances wherein 



