1912.] The Process of Excitation in Nerve and Muscle. 5 L 7 



formula can be deduced in this way, which certainly does fit the observed 

 facts of excitation by long currents. On this count, then, there is nothing 

 to be said against Nernst's hypothesis. It was worked out by Nernst for 

 stimuli of a limited range of duration, and simplifications were therefore 

 made which applied only within those limits. That the formulae so deduced 

 are not found to apply outside those limits can hardly be used, as some 

 physiologists have used it, as an argument against the validity of the hypo- 

 thesis. Physiologists were naturally anxious that any hypothesis should 

 take account of the features of excitation by long currents, because these 

 are of particular interest in relation to the differences between different 

 excitable tissues. For example, Lapicque* and If had found that various 

 excitable tissues could be differentiated by the fact that the duration at 

 which the exciting current first reached a constant minimum value was 

 longer for some tissues and shorter for others. The physical meaning of this 

 difference was a point of obvious interest in the evolutionary history of the. 

 excitable tissues. But the absence of these facts from Nernst's formula was 

 not to be set down to a failure of his hypothesis, whose consequences had 

 simply not been worked out for the current durations with which we had dealt. 



We may turn, therefore, to the second of the points which I have mentioned, 

 the failure of slowly rising currents to excite. Nernst fully realised the 

 point and suggested that the failure probably arose from a chemical change, 

 produced by the ions, and proceeding at such a rate as not to be notice- 

 able when the concentration was completed quickly 4 He went so far as to 

 set down in an equation the way in which the increase of the necessary con- 

 centration of ions might be related to the rate at which the concentration was 

 produced and to the nature of the tissue used. Naturally this disturbing 

 factor was not embodied in the treatment of currents of short duration, 

 because both theory and experiment indicate that for such currents it is 

 negligible.§ However, Hill|| has since worked out the consequences of a 

 supposition which is practically that originally offered by Nernst, and his 

 conclusion is that though the general facts of observation are predicted, it is 

 the experimental data which are not yet of a kind to allow of the comparison 

 of observed and calculated values. 



So far then it seems that the suggestions originally made by Nernst do offer 



* Lapicque, 'C. R Acad. Sc.,' 1903, vol. 136, p. 1147 ; 1905, vol. 140, p. 801 ; ' C. E. 

 Soc. Biol.,' 1903, vol. 55, p. 445 and p. 753 ; 1905, vol. 57, p. 501. 



+ Keith Lucas, ' Journ. Physiol.,' 1907, vol. 35, p. 310 ; 1907, vol. 36, p. 113. 

 | Nernst, loc. cit., pp. 280, 281. 



§ Cf. Gildemeister, ' Arch. f. d. ges. Physiol.,' 1904, vol. 101, p. 203 ; Keith Lucas, 

 Journ. Physiol.,' 1907, vol. 36, p. 273. 

 || Hill, loc. cit., p. 203. 



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