THE NATURE OF THE EXCITATORY PROCESS 287 



where i is the smallest current which will excite, 



t is duration of the current; 

 while y, ^, are constants which depend on : 



(1) The distance between the membranes. 



(2) The distance from the membrane at which the concentration 



changes are being considered. 



(3) The diffusion constant of the ion. 



(4) The number of ions by which a given quantity of electricity is 



carried. 



(5) A constant expressing in general terms the .ease with which a 



propagated disturbance is set up. 



Investigation on these lines may give us in future sufficient infor- 

 mation to form a material conception of the factors involved in excitation, 

 factors which in the above formula have only a symbolic existence. Thus 

 a determination of the distance between the membranes would give us some 

 clue to the size of the ultimate excitatory units in the tissue involved.* The 

 constant u has reference only to the position relative to the membranes at 

 which the changes of concentration are effective. From Lucas's experiments 

 it would seem that the changes of concentration occur in the immediate 

 neighbourhood of one of the membranes. Macdonald has brought forward 

 evidence that the passage of a current through a nerve involves the setting 

 free of certain inorganic ions. The subsidence of the excitatory state 

 depends on the rate of diffusion of ions. If however we compare the rates 

 of subsidence of the excitatory state in different tissues, we find much greater 

 divergence than would be possible on the assumption that the diffusion is one 

 affecting inorganic ions. Thus between the substance ft (the intermediate 

 substance) of the frog's sartorius and the ventricular muscle fibre of the 

 same animal, the rate of subsidence of the excitatory state changes in the 

 ratio 4CCO : 1 . If the ions concerned were simple ions, such as H-, Ca-, Na-, 

 Cr, <fec.. it would be impossible to account for this wide variation, since their 

 velocities differ in the ratio of 10 : 1 at most. Moreover the effect of rise of 

 temperature on the rate of subsidence is greater than the effect of a similar 

 rise on ionic velocities. It is evident therefore that the theory is one for use 

 as a working hypothesis only. That excitation is associated with accumu- 

 lation of ions in the region of the exciting electrode, that the subsidence of 

 the excitatory state is due to disappearance by diffusion or otherwise of 

 these ions, there can be little doubt. But the questions as to the nature of 

 these ions, and their relation to the colloidal constituents of the excitatory 

 tissue, or to other possible substances, changes in which may form an integral 

 purt in the excitatory state, must be left for future investigation. 



* It would be premature at present to give any histological significance to Hill 

 and Lucas's diagrammatic cylinder. As Hardy has pointed out, the nerve cannot 

 consist of a row of such cylinders, otherwise excitation would occur throughout the 

 whole intrapolar region, and not be confined to the cathode at make and the anode 

 at break. It may be that we are dealing here again with the polarisable sheath of the 

 ' Kernlfitfr,' and that the membrane A corresponds to the surface of the axis cylinder 

 or of its neuro-fibrils. 



