PERMEABILITY OF SURFACE LAYER OF CELLS. 1 97 



These have yielded the result that under the influence of carbonic 

 or other weak acids (/. e., of H-ions in low concentration) blood 

 corpuscles give evidence by showing an exchange of anions 

 with the surroundings or by characteristic behavior in electrical 

 convection of becoming much more freely permeable to various 

 anions (Cl, Br, SO 4 , HCO.^, NO 3 ) than before, while the perme- 

 ability to cations appears unchanged. This increase in permeability 

 to anions is readily reversible on removing the carbon dioxide. 

 Proof is thus afforded that the permeability of cells to ions is a 

 variable quantity, which may undergo increase or decrease ac- 

 cording to circumstances. The theoretical importance of a 

 demonstration of reversible changes in ionic permeability is evi- 

 dent when we consider the essential role which, on the mem- 

 brane theory, such changes play in stimulation. 



The permeability of the living cell is thus not a constant factor, 

 but one subject to alteration under a variety of conditions. The 

 reversible changes induced by stimulating or inhibiting agencies, 

 whose respective action appears to consist mainly in increasing or 

 decreasing the normal permeability, are of particular physiologi- 

 cal interest. Alterations of permeability are also induced by a 

 large class of foreign substances which act directly upon the 

 plasma-membrane. Such are in general : (i) The class of lipo- 

 lytic substances or organic fat-solvents ; these directly affect the 

 lipoids of the plasma-membrane ; if present in sufficient concen- 

 tration they produce marked increase in the permeability of the 

 surface layer of cells. The effect is conspicuous in pigment- 

 containing cells (erythrocytes, egg-cells, etc.) ; the pigment leaves 

 the cells and colors the solution (haemolytic auction of fat-solvents). 

 Increase of permeability sufficient to allow the exit of colloid 

 substances is usually irreversible and destructive to the cell. (2) 

 Various electrolytes ; these affect primarily the aggregation-state 

 of the colloids. Neutral salts of alkali and alkali earth metals 

 produce largely reversible changes in permeability ; with salts of 

 heavy metals, and with acids and alkalis above low concentrations 

 the effects are irreversible, hence (in part at least) the greater 

 toxicity of this class of electrolytes ; alkalis may have also a 

 saponifying action. (3) A large and miscellaneous class of poi- 

 sonous substances cytotoxins, haemolysins, alkaloids and gluco- 



