32 
able tissue, the mass influence of these ions will be sufficient 
to displace the ions already holding the proteid molecule, 
and to take their place. Hence the kations set free in one 
section of an excitable tissue by excitation may in turn dis- 
place others in the next section of ion-proteid material, which 
again may set free ions in the following section, and so on, so 
that a wave of excitation is propagated through the tissue. 
Thus we conclude that the "wave of negativity" does not pro- 
gress so much by diffusion as by a process of successive dis- 
placement. 
The evidence for this fact will come out more clearly in 
the sequel, but we may allude to some of the facts supporting 
it now. Just as in muscle, we consider that there is present 
in the axis cylinder of nerve a kation-proteid which, by its 
katabolism under stimulation, gives rise to a wave of nega- 
tivity, only, as in this case there is no elastic surface for ions 
to be deposited on, no contraction is evoked. Now, the exci- 
tatory state evoked in nerve by an intense stimulus is propa- 
gated more rapidly than that caused by a weaker one."^ We 
can easily see that this must be due to the greater mass of 
kations set free initially : they would more easily and quickly 
set free other ions in each section (for it is the principle of 
mass action that the rate of chemical change depends upon the 
masses of the re-acting substances). This will be seen more 
easily when we come to consider the genesis of the discharge 
in the heart ; but it is obvious that if the wave of negativity 
were propagated by mere diffusion, since the number of ions 
set free in no wise affects their velocity, the intensity of the 
stimulus could not affect the velocity of the excitatory wave. 
Of a similar nature may possibly be the explanation of 
the fact that nerve cells conduct more slowly than nerve 
fibres.! The cross-section of a nerve cell is much greater than 
that of its fibre ; hence at any moment the same number of 
ions would have very many more ion-proteid molecules to cope 
with than tliey had during their course in the fibre. 
Another line of evidence supporting the theory we have 
put forward is the influence of various solutions of salts upon 
the transmission of excitation. If a portion of a conducting 
excitable tissue were immersed in a solution with a negative 
stimulation efficiency, and a wave of negativity initiated else- 
where, on passing through the immersed portion (if it travels 
by displacement) should either be diminished, abolished, or 
converted into a wave of positivity, according as little or 
* Gotch : Schafer's Textbook of Physiology, vol. ii., page 458. 
t Biedermann : P]Iectro-physiol«o;y : Trans, by F. A. Wolby, 
vol. ii., page 69. 
