ELECTROPHYSIOLOGY 349 



exactly balance each other. One always checks them against 

 each other to see that they really cancel out and that the current 

 flow is zero. If they thus cancel each other, then any potential 

 observed in the system can be safely referred to some other 

 site than the metal-electrolyte junctions. Junction potentials 

 are minimized by the use of saturated potassium chloride cells 

 or bridges or both. The mobility of the potassium and chlorine 

 ions is so nearly identical that no appreciable electromotive force 

 arises at such junctions. There remain only the electrical 

 stresses across the membrane to produce an electromotive force 

 which is balanced by the potentiometer.) 



The potential difference between the two sides of anatomical 

 membranes involves not only the electrokinetics of the unequal 

 distribution of ions but also the mechanics of the membrane. 

 Numerous hypotheses have been advanced (see pages 275, 290). 

 Here we shall mention but one of them— that which ascribes the 

 selective ionic permeability of membranes to an intrinsic electric 

 charge. 



If a membrane — to be specific, an apple skin — possesses a 

 slight initial charge which is negative, then anions, such as 

 Cl~, will be repelled by the membrane and cannot pass through, 

 while cations, such as K+, will not be repelled and can pass 

 through. The apple skin should therefore be more permeable to 

 cations than to anions, as Michaelis found to be true. This 

 unequal distribution of ions across the membrane establishes a 

 potential which increases the initial charge of the membrane. 

 It is this final potential that we measure. 



Wilhelm Ostwald suggested that the electric forces in muscles 

 and nerves have their origin in the selective permeability of 

 living membranes, in that the membranes are permeable only 

 to anions or cations. The theory has proved to be a very useful 

 and significant one, in that it places the seat of vital electro- 

 motive forces at phase boundaries or membranes. Donnan 

 interpreted the theory of Ostwald mathematically. Equilib- 

 rium (page 203) brings about an unequal distribution of ions 

 which results in a potential difference between the two sides of a 

 membrane. 



We may classify membrane potentials, in the main, into two 

 groups — those resulting from ionic (a Donnan) equilibrium and 

 those resulting from the unequal diffusion of ions through the 



