means always the case. In a NaF solution equimolecular 

 with a 0.7 per cent. NaCl solution contractions ceased after 

 twenty-five minutes. After forty-five minutes I found that 

 the muscle could still be stimulated at a distance of the 

 secondary coil of 370 mm. a degree of irritability which 

 was not far below the normal. At the same time, however, 

 I found that a muscle which had been in a 0.7 per cent. 

 NaCl solution for twenty-four hours still contracted, even 

 though its threshold of stimulation, as determined by the 

 same induction coil, had fallen to 300 mm. We have, there- 

 fore, a different threshold of stimulation for different ions. 

 One might think that the periodic contractions in the differ- 

 ent solutions are brought about in that the ions which 

 penetrate the muscle substance enter into definite combina- 

 tions with it. That such must exist I have proved in another 

 paper by showing that Na, K, and Ca ions entering the 

 muscle produce specific alterations in its osmotic behavior. 1 

 Under this assumption it might be thought that when a 

 certain number of F ions have entered the muscle, the further 

 entrance of Na and F ions no longer brings about contrac- 

 tions, while at the same time an induction current is still 

 able to bring about marked contractions. I need scarcely 

 emphasize the fact that a muscle loses its electrical irrita- 

 bility more rapidly in a NaF solution than in a NaCl 

 solution isosmotic with it. 



6. If in these experiments the rhythmical contractions are 

 brought about through ions, it was to be expected that no 

 periodic contractions should occur in non-electrolytes. I 

 never saw rhythmical contractions begin in chemically pure 

 distilled water. 



I also prepared solutions of glycerin, dextrose, cane 

 sugar, and milk sugar . isosmotic with a 0.7 per cent. NaCl 



i "On the Similarity between the Absorption of Water by Muscles and by Soaps," 

 Part II, pp. 510. 



