io8 PROTOPLASMIC ACTION AND NERVOUS ACTION 



permeability with reference to substances which are 

 unequally soluble in its different chemical components. 

 Substances soluble in a given component (lipoid-soluble 

 substances) may thus pass a protoplasmic membrane, 

 while chemically similar but lipoid-insoluble substances 

 may not. Overton's experiments on the differences 

 between the rates of diffusion of various organic com- 

 pounds and salts into living cells illustrate selective 

 permeability of this type.' 



It is important to recognize that the plasma mem- 

 brane is not a dead structure or a purely passive partition, 

 but represents in reality a portion of the living proto- 

 plasm, characteristically modified in its structure and 

 physical properties by surface conditions.^ Hence it 

 is the seat of metabolic and other activities which influ- 

 ence its physical properties. Evidently it is that part 

 of the cell which comes into the most direct relations 

 with the surroundings. Hence many changes in the 

 surroundings influence the cell primarily through their 

 action on the plasma membrane, and there is evidence 

 that in irritable cells this structure plays the part of a 

 specially sensitive and reactive intermediary between 

 the living protoplasm and the external world; it thus 

 exerts a far-reaching control over the metabolic and other 

 processes occurring in the cell-interior. The relations of 

 the plasma membrane to stimulation will be considered 

 in detail later. 



^ Cf. Overton's summary of his work in Nagel's Handhuch der 

 Physiologie, II (1907), 744. 



^ Cf. my paper in American Journal of Physiology, XLV (1918), 406, 

 for a more complete discussion of this phase of the problem of perme- 

 ability. 



