500 



Mr. K. Lucas. 



[June 6, 



method of measurement in terms of the effect produced by the disturbance 

 after it has emerged into a normal region of the tissue can probably detect 

 only changes so gross that they have led to complete extinction. To 

 measure in terms of the electric response produced in the nerve (a method 

 not infrequently used) is to beg one of the most fundamental questions in 

 the analytical problem with which we are dealing ; such a method assumes 

 a. knowledge of the relation between propagated disturbance and electric 

 response which we do not possess. It chances, however, that a method 

 which seems capable of detecting extremely small changes in a propagated 

 disturbance has grown out of some experimental work which Adrian and 

 I have recently published.* We found that if two propagated disturbances 

 are sent down a nerve in close succession, the shorter the time-interval by 

 which they are separated the less is the length of slightly impaired nerve 

 which the second disturbance is able to face without extinction.! In fact, 

 the second disturbance is continuously less in intensity the shorter the time- 

 interval separating it from its predecessor. It becomes possible, therefore, so 

 to time two disturbances that the second is only just able to propagate itself 

 successfully along a perfectly normal nerve ; such a minimal second disturb- 

 ance will at once be extinguished if the " conductivity " of the nerve is in the 

 least degree impaired. 



This method is now being applied in the Cambridge Laboratory to test 

 whether during the earliest stages of narcosis the slight changes of excita- 

 bility usually observed to precede any gross change of conductivity are in 

 reality accompanied by a small but measurable diminution of the propagated 

 disturbance in the course of its travel. When the investigation is complete 

 we hope to have an answer to this long-standing riddle of the interpretation 

 •of Griinhagen's experiment. 



As a means of analysis then this method of dissimilar alteration has failed 

 us for the present at least. But it does not follow that no differentiation of 

 two processes can be found. I believe, in fact, that along quite other lines 

 evidence has really been available for some years past, merely waiting to be 

 put together in a connected argument. 



It is a fact long ago established in physiology that if a short current just 

 too weak to set up a propagated disturbance is applied to a nerve, then for 

 a short time after this current has ceased to flow a second current, otherwise 



* Adrian and Keith Lucas, ' Journ. Physiol.,' 1912, vol. 44, p. 68. 



t This elementary fact was probably at the root of the observation of Wedensky 

 mentioned above, that a narcotised nerve may appear to be conducting normally when 

 tested with a single stimulus, but may, nevertheless, appear abnormal when tested with 

 periodic stimuli of high frequency. 



