146 THE POPULAR SCIENCE MONTHLY 



decidedly more freely permeable to cations as a class than to anions, 

 and it is possible that this condition is typical for the plasma-mem- 

 branes of cells. The membranes of irritable tissues, however, may 

 belong to another type; certain membranes (consisting of thin films of 

 glass) whose electrical polarization depends on the relative hydrogen- 

 ion concentrations in the solutions which they separate, have recently 

 been investigated by Haber; and in some respects the phenomena pre- 

 sented by these membranes appear to correspond more closely to the 

 conditions in irritable tissues. Hydrogen-ions would be the polarizing 

 cations in the case of these membranes; and in fact irritable tissues 

 are as a rule remarkably sensitive to changes in the H-ion concentra- 

 tion of their medium. We are not yet in a position to decide between 

 such alternatives. But for the present purpose it is sufficient to recog- 

 nize that a membrane which interferes unequally with ionic diffusion 

 may become the seat of a potential-difference when it separates two 

 solutions; and the evidence that plasma-membranes and other cell- 

 membranes are of this kind appears very strong, even at the present 

 time. In general, phase-boundaries are the seat of electrical energies, 

 and these largely depend on the ionic content of the adjoining media. 

 Membrane-polarization is a special instance of this general class of 

 phenomena. The precise conditions of the normal physiological polar- 

 ization in irritable tissues have to be determined by future investigation. 



Membranes in their electrochemical aspect are to be regarded, on 

 the present theory, as ion-transmitting surfaces, just as the metallic 

 plates in ordinary electric batteries are ion-forming or ion-combining 

 surfaces. The electrical properties exhibited by all of these surfaces 

 are conditioned in essentially the same manner, and Nernst's theory 

 applies to all. A system composed of solutions separated by mem- 

 branes may thus, under the proper conditions, show the same essential 

 properties as a system of batteries connected in series. The potential- 

 differences of the individual elements may be summed by appropriate 

 arrangement so that the electric tension between the terminals may be 

 very large. In the electrical organs of Gymnotus and other fish, sys- 

 tems of this kind have actually been realized in nature, and have been 

 applied to defensive or other purposes. 



Let us now consider in a little more detail the conditions of stimu- 

 lation of an irritable tissue by an external electric current. The sur- 

 face-film of the muscle-cell or the nerve fiber is to be regarded as 

 electrically polarized in the sense already indicated. Why does the 

 tissue respond in its characteristic manner to the electric current? 

 The first fundamental suggestion as to the mode of action of the cur- 

 rent was made by Nernst in 1899. He pointed out that the current 

 in passing through a living tissue — a system equivalent to a solution 

 containing electrolytes and subdivided by semi-permeable membranes — 



