INORGANIC SALTS 153 



NaCl solution (0.7 per cent). No significant decline in 

 irritability was found until the salt-content fell below 

 0.15 per cent. At o.i per cent irritability was distinctly 

 less than normal, and, with further decrease in concentra- 

 tion, it declined rapidly, becoming zero at about 0.07 

 per cent. In mixtures containing less than 0.07 per 

 cent NaCl irritability was lost as rapidly as in pure 

 sugar solution.^ The time required for complete loss of 

 irritability thus represents the time required for diffusion 

 to reduce the NaCl of the intercellular spaces to this 

 concentration. 



Sugar-treated muscles rapidly regain irritability in 

 solutions of all Na salts. The nature of the anion 

 associated with the Na was found to be indifferent, 

 provided it was not too toxic; with most salts the 

 minimal concentration for the maintenance of irritability 

 was about the same as with NaCl. Apparently, there- 

 fore, it is the Na ion, and not the undissociated molecule 

 or the anion, which is responsible for the maintenance 

 of irritability. A definite function, that of preserving 

 the normal irritabihty of muscle and nerve, is thus to be 

 ascribed to the Na salts in blood plasma; their role is 

 not merely osmotic, as formerly supposed.^ This rela- 

 tion of Na ions to irritability and contractility is a 

 specific peculiarity of muscle cells; the conditions in 

 other contractile forms of protoplasm are often widely 

 different; thus the contraction of cilia, spermatozoa 



^ This effect apparently varies with oxygen tension. Pond has 

 recently shown that in solutions saturated with oxygen the NaCl content 

 may be reduced to less than .05 per cent without loss of irritability: 

 Jour. Gen. Physiol. ^ III (192 1), 807. 



2 Cf. also J. Loeb, "On the Production of Rhythmical Contractions 

 in Muscles by Ions," Festschrift fur Pick (1899), p. loi. 



