138 COMPARATIVE PHYSIOLOGY 



These peculiarities of the excitation process and other 

 considerations based on the study of currents of varying 

 intensity point in one direction. Stimulation results in a local 

 change which is associated with the migration of ions to or 

 from a surface in the neighbourhood of the point of application 

 of the stimulus. The conditions of duration and intensity 

 suggest that a certain minimal concentration of ions at this 

 surface is an essential feature of the process. In order that 

 such a minimal concentration of ions may be reached there 

 must naturally be a minimal quantity of electrical energy 

 imparted, and there must also be a minimal time during which 

 the directive force of the electrical current may influence the 

 migration of ions to and from the surface concerned. The 

 consequences of such a hypothesis are susceptible to mathe- 

 matical treatment, as was first suggested by Nernst, later 

 elaborated on the theoretical side by Hill and subsequently 

 put to experimental test by Keith Lucas. We are thus in 

 a position to construct a working hypothesis of the excitation 

 process. If, through a solution enclosed between tw^o 

 membranes impermeable to ions of a particular kind, a galvanic 

 current is for a while allowed to pass continuously in one direc- 

 tion, there will be a local concentration of such ions at one of 

 the membranes, reaching a Hmit conditioned by their diffusion 

 constant. A finite time must be allowed to elapse before any 

 appreciable increase of concentration can take place at the 

 membrane, the duration depending upon the intensity of the 

 current. If the current is reversed before the requisite time 

 has elapsed the flow of ions will be correspondingly reversed. 

 From this consideration Nernst was led to seek an explanation 

 of the inefhcacy of alternating currents of very high frequency 

 as agents of excitation. If the membranes are indefinitely 

 separated, the relations which must exist between minimal 

 intensity of current (i), duration (t)y or frequency (n) in order 

 that an arbitrary critical concentration may be reached are 

 expressed by the equations : 



k=t\/t (for constant current) 

 i=k'\/n (for alternating currents) 



