514 



Mr. K. Lucas. 



[June 6, 



process, according to the view which I have laid before you, is a change 

 localised at the point of stimulation, produced as the direct consequence of 

 the passage of the stimulating current, and providing when of adequate 

 intensity the conditions within a tissue which are necessary for starting the 

 propagated disturbance. If this account of the process is correct we cannot 

 doubt the importance of its recognition. We are able to vary in many ways 

 the character of the stimulus used to provoke this change. If we are using 

 an electric current we can vary its direction, its strength, its duration, and 

 the time-relations of its rise and fall. From the conditions which these 

 various factors must fulfil in order to produce an adequate local excitatory 

 change we obtain the data by which to test any hypothesis of the physico- 

 chemical nature of that change. In such an hypothesis, supposing it to have 

 been eventually established, we have a knowledge of the conditions within 

 a tissue which are necessary for producing a propagated disturbance, and so 

 we have fresh data for testing any hypothesis as to the nature of the latter. 

 Perhaps by investigation along such lines we may be brought to an 

 hypothesis of the propagated disturbance whose acceptance will carry with it 

 a solution of that riddle of the electric response which seems to baffle direct 

 experimental observation. 



In sketching such a possible course I fear that I am outrunning too far 

 the sober limits of accomplished work. Nevertheless, some progress has 

 been made towards the first step, the physico-chemical identification of the 

 local excitatory process. In the time that remains to me I shall attempt 

 to show what this progress has been, and to lay before you both the hopeful 

 signs and the very real difficulties which have to be met. 



It is an old idea that the essential effect of an exciting current on a 

 nerve or muscle is to produce a polarisation within the tissue. But the 

 birth of this idea as a definite hypothesis open to verification by quantitative 

 measurement begins with the work of Nernst.* Attracted by the fact that 

 alternating currents of extremely high frequency can be passed through 

 the human body without exciting the nerves or muscles, he was led to test 

 the old suggestion that such currents pass only through the surface layer 

 of an electrolytic conductor. He found this view untenable, and proposed 

 that the essential condition which an effective exciting current must fulfil 

 might be the production, at some membrane within the excitable tissue, of 

 a definite concentration of certain ions unable to pass the membrane. This 

 supposition would explain the failure of high frequency currents, because 

 the rapid alternation of direction would carry the ions away from each 

 membrane again before they had sufficient time to reach a considerable 

 * Nernst, 'Gott. Nach. Mathem. physik. Klasse.,' 1899, p. 104. 



