274 PROTOPLASMIC ACTION AND NERVOUS ACTION 



polarization. And the semi-permeability of the plasma 

 membranes of irritable cells (e.g., muscle-cells) with 

 reference to the inorganic salts of the medium — which, 

 without penetrating the cell, nevertheless influence 

 profoundly its properties and activity — shows further 

 how closely cellular activities are dependent on surface- 

 conditions. Among these surface-conditions the state 

 of electrical polarization appears to be of primary 

 importance. 



The facts of electrical stimulation, now to be con- 

 sidered, show that variations in electrical surface- 

 polarization have a far-reaching control over the meta- 

 bolic and other processes occurring in the cell interior. 

 The means by which this polarization may be altered 

 are of three chief kinds: (i) changes in the structure or 

 composition or permeability of the surface-film; (2) 

 external electrical influences, especially the influence of 

 electric currents traversing the cell or its medium; and 

 (3) changes in the composition of the medium or internal 

 protoplasm. Since, according to the present view, an 

 inseparable feature of stimulation, the transmission of 

 the excitation-state, is a direct result of electrical activa- 

 tion by the currents of local bioelectric circuits, it is 

 clear that the problem of stimulation resolves itself 

 largely into the problem of the general conditions under 

 which the electric current stimulates living matter. 

 Electrical stimulation is in fact the primary form of 

 stimulation. 



CONDITIONS OF ELECTRICAL STIMULATION 



In general the physiological studies of the last two 

 decades have shown that the stimulation of an irritable 



