can produce decided changes of condition only at the semi-permeable surfaces, where the movement of ions is blocked; changes of electrical polarization would be produced at such surfaces; ions of a given sign would be carried against one face of the membrane by the current and would concentrate there until the back-diffusion equalled the current-transport; the same effect, with the signs of the ions changed, would result at the other face (Fig. 3). He conceived that in 
Fig. 3. Illustrating the polarization of the current on a membrane difficultly permeable to ions. The anions and cations of the electrolyte, , move in the direction indicated by the arrows. The current, passing from left to right, carries cations toward and anions away from the left face of the membrane; at its right face the conditions are reversed. The membrane thus becomes electrically polarized, with its left face at the higher potential. electrical stimulation something of the kind occurs. The essential or critical change occurs at the semi-permeable membrane, and consists in carrying to this membrane sufficient ions to produce a given ionic concentration-difference corresponding to a given electrical polarization. This is the determining condition of stimulation. A certain time will be required for the process, depending on the strength of the current, and on the specific diffusion-rate of the ions. Nernst estimated that on this hypothesis the stimulating action (S) of a given current ought to vary directly with its strength (i), and with the square root of its duration (t) ( K being a constant characteristic of the tissue). The experimental data show that a more intense current requires for stimulation a shorter time than a weaker current, and in approximately this proportion. The more recent work of Lapicque, Lucas and Hill has confirmed and amplified Nernst's theory. There is therefore strong evidence that a current stimulates by producing an electrical polarization at the membranes.
During life, however, the membranes are apparently already the seat of a pre-existent polarization, as we have seen. The polarization produced by the external current must, therefore, modify this. Now it appears that in most, if not all irritable tissues, stimulation results when the physiological polarization is diminished suddenly, but not when it is increased. This is the simple inference from the law of polar stimulation. When a current is passed through a tissue the external positivity of the irritable elements is lowered on the side directed toward the cathode and increased on the side directed toward the anode, as may be seen by reference to Fig. 3. Now it has long been known that the stimulus originates on the cathodal side of an irritable tissue when the current is made, and on the anodal side when
