decidedly more freely permeable to cations as a class than to anions, and it is possible that this condition is typical for the plasma-membranes of cells. The membranes of irritable tissues, however, may belong to another type; certain membranes (consisting of thin films of glass) whose electrical polarization depends on the relative hydrogenion concentrations in the solutions which they separate, have recently been investigated by Haber; and in some respects the phenomena presented by these membranes appear to correspond more closely to the conditions in irritable tissues. Hydrogen-ions would be the polarizing cations in the case of these membranes; and in fact irritable tissues are as a rule remarkably sensitive to changes in the H-ion concentration of their medium. We are not yet in a position to decide between such alternatives. But for the present purpose it is sufficient to recognize that a membrane which interferes unequally with ionic diffusion may become the seat of a potential-difference when it separates two solutions; and the evidence that plasma-membranes and other cellmembranes are of this kind appears very strong, even at the present time. In general, phase-boundaries are the seat of electrical energies, and these largely depend on the ionic content of the adjoining media. Membrane-polarization is a special instance of this general class of phenomena. The precise conditions of the normal physiological polarization in irritable tissues have to be determined by future investigation.
Membranes in their electrochemical aspect are to be regarded, on the present theory, as ion-transmitting surfaces, just as the metallic plates in ordinary electric batteries are ion-forming or ion-combining surfaces. The electrical properties exhibited by all of these surfaces are conditioned in essentially the same manner, and Nernst's theory applies to all. A system composed of solutions separated by membranes may thus, under the proper conditions, show the same essential properties as a system of batteries connected in series. The potentialdifferences of the individual elements may be summed by appropriate arrangement so that the electric tension between the terminals may be very large. In the electrical organs of Gymnotus and other fish, systems of this kind have actually been realized in nature, and have been applied to defensive or other purposes.
Let us now consider in a little more detail the conditions of stimulation of an irritable tissue by an external electric current. The surface-film of the muscle-cell or the nerve fiber is to be regarded as electrically polarized in the sense already indicated. Why does the tissue respond in its characteristic manner to the electric current? The first fundamental suggestion as to the mode of action of the current was made by Nernst in 1899. He pointed out that the current in passing through a living tissue—a system equivalent to a solution containing electrolytes and subdivided by semi-permeable membranes—-
