STIMULATION AND TRANSMISSION 279 



to ions, there is during the flow of the current a movement 

 of cations with the positive stream and of anions with 

 the negative stream; at the semi-permeable membranes 

 interposed in their path the movement of ions is impeded ; 

 the cations then undergo an increase of concentration at 

 those surfaces of the membranes which face toward the 

 anode, simultaneously w^ith a decrease of concentration 

 at the opposite faces; the reverse relations hold with 

 the anions. A gradient of ionic concentration is thus 

 set up between the layer of solution in immediate contact 

 with the membrane and the layer at some distance. 

 This process of concentration at the semi-permeable 

 surface continues until a condition of equihbrium is 

 reached at which the rate of diffusion back from the 

 membrane into the interior of the solution is equal to 

 the rate at which the ions are transported to the mem- 

 brane. Assuming the existence of these two opposed 

 processes, transport to the surface by current and back- 

 diffusion, it can be shown that to produce a definite 

 change in concentration at the surface, the product of 

 the current-intensity into the root of its time of flow 

 should be constant {i\/t=K). 



This result was reached by Nernst from the considera- 

 tion of the case of a single membrane interposed in the 

 path of a current. It is evident ,that such a system 

 offers conditions much simpler than those of an irritable 

 tissue, which typically consists of a bundle of cells or 

 fibrils. HilP and Keith Lucas^ have pointed out that 

 in considering the case of the living cell or nerve-fiber, 

 with its small linear dimensions, it is necessary to take 



^Journal of Physiology, XL (1910). 190. 

 ' Ibid., p. 225. 



