INORGANIC SALTS 159 



intimate union between the nerve end-plate and muscle 

 cell, or for the normal properties of the synaptic junc- 

 tions. Overton suggests that when the tissue is trans- 

 ferred to pure NaCl solution, the Ca in this compound 

 is replaced by Na, producing a compound of greater 

 water-absorbing properties, which swells and interrupts 

 the union. Such an effect is reversed by a return to 

 Ca-containing solutions. 



Some of the more general inferences to be drawn from 

 these and recent experiments of a similar kind are as 

 follows: Since irritability disappears in isotonic sugar 

 solutions, which maintain the osmotic balance without 

 apparently changing the crystalloid content of the cell, 

 it seems clear that the action of the salts of the external 

 medium must be superficial; i.e., is a surface-action 

 exerted upon the plasma membrane. This action is 

 probably of a twofold nature : first, the normal composi- 

 tion and physical properties of the colloidal . materials 

 composing the surface-film are preserved only when a 

 certain combination of ions is present in the medium; 

 and, second, the normal state of electrical polarization of 

 the membrane is also dependent on the presence of salts 

 in the medium as well as in the cell-interior; this effect 

 is important because the electrical polarization of the 

 membrane is a factor determining its permeability. 

 These two effects, however, cannot be regarded as 

 independent, since changes in the physical state of the 

 membrane must alter its permeability and (in so doing) 

 its electrical polarization; and, conversely, changing the 

 electrical polarization influences the permeability. 



The essential conclusion, however, is that salts may 

 exert physiological action without penetrating into the 



