EXCITATION AND TRANSMISSION 75 



this ability decreasing from a through b, c, d, etc., to a 

 point x where the electrical change becomes so weak 

 that it is incapable of acting as a stimulus. Moreover, 

 the current resulting from excitation at a tends at the 

 same time to re-establish the polarization at a and so 

 to reverse the depolarization and restore the original 

 condition of polarization at a. 



Each new point b, c, d, etc., excited by the current 

 arising from a, becomes the source of a current which, 

 as Lillie has shown, may produce excitation at further 

 points and might also produce re-excitation at a if the 

 refractory period following excitation did not prevent. 

 It is evident that according to this conception the 

 velocity of transmission depends upon the velocity of 

 the local processes in the membrane at each excited 

 point and is entirely independent of the velocity of elec- 

 trical transmission which is practically instantaneous. 

 Moreover, in any case in which excitation varies directly 

 (not necessarily proportionally) with stimulus, a decre- 

 ment must occur in the course of transmission. Only 

 in certain highly specialized protoplasms for which the 

 "all-or-none law" holds, i.e., in which any stimulus 

 above the threshold produces maximal excitation, is 

 transmission without a decrement possible. 



PRIMITIVE EXCITATION PROCESSES 



The question might be raised whether the assump- 

 tions concerning the semi-permeability and electric 

 polarization of the plasma membrane and its depolari- 

 zation in excitation are the only possible basis for the 

 electrical phenomena associated with excitation. It 

 has been pointed out by Lillie and others that acid 



