PERMEABILITY OF SURFACE LAYER OF CELLS. 195 



ration of electricities due to the passage of a certain small pro- 

 portion of ions of one sign through the membrane ; this would 

 continue until the electrostatic tension thus arising balanced the 

 osmotic pressure difference between the ions on opposite sides of 

 the membrane, when the ionic transfer would cease. A certain 

 potential difference constant (at a given temperature) with 

 unchanged permeability of the membrane and concentrations of 

 the electrolyte would then exist between opposite surfaces of 

 the membrane. The presence of membranes having such a dif- 

 ferential permeability in reference to the anions and cations of the 

 tissues might, Ostwald pointed out, be the source of the elec- 

 trical phenomena of living organisms ; and this suggestion has 

 been further developed by Bernstein, followed by Briinings, 

 Hoeber and others. The theory based on this interpretation - 

 the so-called " membrane theory " of the origin of the bio-electric 

 currents appears to-day as the most adequate explanation of 

 these complex and puzzling phenomena. Its merit consists 

 mainly in the introduction of a third variable in addition to 

 character and concentration of the ions at the boundary surface l 

 namely, the ionic permeability of the membrane itself. This 

 is subject to more or less sudden alteration either in the direction 

 of increase or decrease ; and the sudden and pronounced elec- 

 trical changes accompanying stimulation and inhibition, as well 

 as the remarkably high potential differences found in some in- 

 stances (electric fishes), become readily intelligible on this theory _ 

 The outer uninjured surface of a resting cell e. g., a muscle 

 cell 2 always proves positive relatively to the interior when- 

 ever the latter is exposed by any kind of injury or by chemical 

 or other alteration of the surface. The membrane theory there- 

 fore assumes that the plasma-membrane is freely permeable during 

 rest to the cations of a certain electrolyte or electrolytes contained 

 within the cell, but not to its anions. This condition may, how- 

 ever, undergo temporary alteration, as during stimulation, when 

 there is invariably a fall in the potential difference between exte- 



1 Assuming constant temperature. 



2 Of course observation cannot be directly made on single cells unless on certain 

 egg-cells a possibility exemplified by Miss Hyde'sinvestigations cited below. The 

 above statement assumes that what is observed for parallel bundles of cells, e.g., 

 such a muscle as the frog's sartorius, is true for single components of the bundle. 



