154 PROTOPLASMIC ACTION AND NERVOUS ACTION 



and protozoan structures like the stalk of Vorticella 

 is independent of Na ions in the medium; cilia, in fact, 

 often exhibit prolonged and vigorous activity in media, 

 like isotonic KCl or MgCL which rapidly and completely 

 abolish irritability in muscle and nerve/ 



Since analysis shows little or no sodium in the 

 interior of the muscle cells, the conclusion follows that 

 the essential action of the Na ions is exerted upon the 

 semi-permeable surface layer or plasma membrane and 

 that penetration into the internal protoplasm is unneces- 

 sary. Overton, however, points out that it is not 

 necessary to assume impermeability to these ions under 

 all conditions; it is possible that under some conditions, 

 e.g., stimulation, there may be an exchange between 

 the Na ions in the medium and other cations (e.g., K) 

 present in the muscle cell. This suggests that impermea- 

 bility to Na ions is a characteristic of the muscle during 

 the resting state only, a view implying that changes of 

 permeability are an essential factor in stimulation. 

 Similar views were expressed by Bernstein and Briinings 

 at about the same time.^ 



Experiments on the substitution of other cations for 

 Na showed that lithium salts were the only ones capable 

 of maintaining irritability in the same manner as Na 

 salts. Pure isotonic solutions of LiCl (0.435 P^^ cent) 

 are injurious to muscle, but a mixture of this solution 

 with an equal volume of isotonic sugar solution has 

 indifferent properties like those of an NaCl solution. 

 The least concentration of LiCl required for maintaining 



^ The cilia of many marine animals, e.g., Arenicola and Mitykis, 

 remain active for hours in pure solutions of K and Mg salts. 



' Bernstein, Arck. ges. Physiol., XCII (1902), 521; Briinings, ihid.^ 

 XCVIII (1903), 241, and C, 367. 



