146 PROTOPLASMIC ACTION AND NERVOUS ACTION 



eluded that these membranes are simple continuous 

 sheets of lipoid; the very fact that their properties are 

 complex and vary from species to species is inconsistent 

 with any such simple view, and the existence of specific 

 cytolysins may indeed be taken as proof that proteins 

 form an essential part of their composition. In fact, 

 as already pointed out, the plasma membrane is not to 

 be regarded as a simple passive layer of colloidal or 

 other material, but rather as a special living structure 

 with a characteristic metabolism of its own, and with 

 both its physical and chemical properties modified in 

 correspondence with its situation at the cell-boundary. 

 All that can safely be maintained is that its properties 

 are intimately dependent on the properties of its lipoid 

 constituents, hence vary with changes in the physical 

 and chemical state of the latter. It will be unnecessary 

 to review in further detail the large body of experimental 

 fact indicating the presence of lipoids in the surface-films 

 of cells; this evidence is discussed at length with full 

 references to the literature in Hober's and Bayliss' 

 textbooks. 



PERMEABILITY AND PHYSICAL, ESPECIALLY 

 ELECTRICAL, CONDITIONS 



Of late years electrical conditions have been shown 

 to be of great importance in determining the permeability 

 of artificial partitions (parchment, porcelain, and other 

 substances), and from general principles it seems certain 

 that such factors must play a corresponding part in 

 the protoplasmic membranes. In artificial membranes 

 two factors have been shown to be of importance: (i) 

 the potential-difference between the solutions in contact 



