i44 THE POPULAR SCIENCE MONTHLY 



These processes again, like some of those already referred to, appear 

 to be a function of the changing permeability of the plasma-membrane. 

 When we take a tissue consisting of a parallel bundle of cells, like a 

 frog's sartorius muscle, cut it across, place one electrode in contact with 

 the normal uninjured surface of the muscle, and the other with its cut 

 surface, and connect the two with a galvanometer, we find that an 

 electrical current passes — the so-called demarcation-current. The ex- 

 posed interior (or cut surface) of the cells always shows a lower poten- 

 tial than the exterior; the potential-difference lies usually between a 

 tenth and a twentieth of a volt. This potential-difference depends on 

 the living condition of the cells. It is absent or insignificant in dead 

 muscle. It diminishes when the muscle-surface is treated with cytolytic 

 substances — i. e., with substances which increase the permeability of the 

 plasma-membrane. The evidence, in fact, indicates that the existence 

 of a normal demarcation-current potential is dependent on the semi- 

 permeability of the plasma-membrane. When the permeability is arti- 

 ficially increased, the potential-difference is invariably diminished; its 

 degree thus appears to be dependent on the degree of permeability of 

 the membrane; hence its increase on death or under the influence of 

 membranolytic substances. Now during stimulation the demarcation- 

 current potential always undergoes a marked decrease; this is the 

 change known as the negative variation or action-current, which is an 

 inseparable accompaniment of stimulation. Normally, this change is 

 completely reversible, and when stimulation ceases, the original poten- 

 tial-difference is regained. What is significant from the present point 

 of view is that the direction of the electrical variation accompanying 

 stimulation is the same as in that resulting from death or cytolytic 

 action and associated with an increase of permeability. The phenom- 

 enon is thus intelligible on the assumption that during stimulation 

 there is a sudden and marked increase in the permeability of the plasma 

 membrane. This permeability increase, with the accompanying electro- 

 motor variation, differs from that associated with death or cytolysis 

 chiefly in being rapidly and completely reversible. Stimulation may, 

 however, be so excessive under some conditions as to lead to irreversible 

 alterations in the membranes, or even to the death of the cell ; i. e., the 

 degree of reversibility is limited, and this consideration explains why 

 excessive stimulation is so injurious — it is in effect equivalent to a 

 cytolytic action or any other action where permeability is irreversibly 

 increased. 



Why should a change in the permeability of the membrane produce 

 electrical effects of this kind? The phenomenon becomes intelligible 

 when we remember that membranes act by limiting or preventing dif- 

 fusion, and that they may limit the diffusion of ions — the mobile, elec- 

 trically charged atoms and atomic groups present in salt solutions — just 



