196 RALPH S. L1LLIE. 



rior and interior of the cell, such as would result from an increased 

 permeability to anions. During life such potential changes occur 

 only temporarily ; they are an apparently inseparable accompani- 

 ment of any form of stimulation. On the death of the cell, how- 

 ever, there follows a marked and permanent increase in the general 

 permeability, and this change is always associated with a perma- 

 nent fall in the potential difference between surface and interior. 

 That the normal potential difference observed during life is cor- 

 related with the normal impermeability of the plasma-membrane 

 thus appears highly probable. The plasma-membrane is hence 

 to be regarded as the seat, during life, of a permanent electrical 

 polarization which is diminished during stimulation, and disappears 

 or is greatly diminished at death, when the vital semi-perme- 

 ability is lost. The cation to whose penetration the external 

 positivity is due was at first supposed to be potassium, which is 

 present in relatively large proportion in the interior of vertebrate 

 muscle cells ; but this is now known not to be the case. Were 

 it so stimulation would involve a loss of potassium salts from the 

 cell, of which there is no evidence ; moreover Hoeber has shown 

 that increasing the external concentration of potassium ions does 

 not reverse the direction of the demarcation current. The indi- 

 cations point strongly to the rapid and highly penetrating hy- 

 drogen ion as the source of the outer positive potential of cells ; 

 and since acids, particularly carbonic acid, are known to be formed 

 in metabolism and to leave the cell during stimulation, there 

 exists in fact within the cell a constant source of hydrogen ions 

 to account for the characteristic surface polarization. That such 

 a system should show a surface polarization of the above kind is, 

 in fact, not surprising. The depolarization associated with stimu- 

 lation and with the death process implies the loss of the polariz- 

 ing electrolyte from the cell ; it has long been known that car- 

 bonic and possibly other acids are evolved at such times in greatly 

 increased quantity. 



Proofs that the surface layer of cells is unequally permeable to 

 the anions and cations of many electrolytes have been furnished 

 by the investigations of Hamburger, Koeppe, and Hober. 1 



1 An account of these investigations is given in Hober's " Physikalische Chemie 

 der Zelle," pp. 303 seq. 



