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SOME COMPARISONS BETWEEN REFLEX INHIBITION AND 

 REFLEX EXCITATION. By C. S. Sherrington. (From the 

 Physiology Laboratory, University of Liverpool.) 



I. Grading of Intensity of Reflex. 



Opinion regarding relation between strength of the stimulus exciting a 

 reflex action and the intensity of the resulting reflex is undergoing change. 

 It was thought that something like the " all or nothing " rule observable 

 for the relation between stimulus and response of the vertebrate myo- 

 cardium held good for spinal reflex arcs. The statement still often is that 

 within but a narrow range does variation of intensity of external stimulus 

 aflect the intensity of the spinal response. Internal condition of the reflex 

 arc does certainly enormously influence the intensity of the arc's reaction. 

 But recently instances have been forthcoming to show that grading of 

 reflex eftect follows closely the grading of the external stimulus/ and in 

 some cases through a wide range of intensity of stimulus.^ 



There has to be remembered, in dealing with this question, the property 

 of temporal summation so transcendently displayed by reflex arcs.^ A 

 series of weak stimuli may by summation become more potent than a 

 stimulus of much greater physical intensity, but single or of relatively few 

 or infrequent repetitions. Mere duration of the stimulus comes, therefore, 

 to be equivalent to intensity. A simple waj^ of eliminating this source of 

 confusion is to employ as external stimulus an agent of variable intensity, 

 but of duration practically infinitely brief ; its period then becomes 

 negligible. A single induction shock may be regarded as furnishing such 

 a stimulus. The single induction shock has, however, figured very rarely 

 as the stimulus for evoking a reflex reaction. There has existed a belief 

 that a single induction shock must, in order to excite a reflex reaction, be em- 

 ployed in very high strength. Indeed, authorities have questioned whether a 

 single induction shock can excite a reflex at all. Some reflexes are, it is 

 true, extremely difficult to evoke by a single induction shock ; thus, the 

 " scratch-reflex " of the spinal dog I was on no occasion able to elicit by a 



1 Merzbaclier, L., Pfliiger's Arch., Ixxxi., 1900. Langelaan, J. W., Aivh. f. Physiol., 

 Suppl. Bd., 1903. Sherrington, C. S., Proc. Physiol. Soc, .March 1904; Joiirn. of 

 Physiol., xxxi., p. xvii. ; Journ. of Physiol., xxxiv., p. i. ; Intograt. Action of the 

 Nervous System, p. 70, 190(i. Pari, G. A., Zeitschr. f. allgeni. Physiol., iv., 1904 ; Arch, 

 italiennes de biolog., 42 ; Atti. Instit. Veneto, 65, 1906. Baglioni, S., Analyse d. 

 Reflex-funktion., 1907. 



2 Sherrington, C. S., op. cit. ^ Stirling, W,, Ludwig's Arbeiten, 1874, p. 245. 



