RHYTHMICAL CONTRACTIONS IN MusoLE' 523 
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. 
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 
1“On the Similarity between the Absorption of Water by Muscles and by Soaps,” 
Part II, pp. 510. 
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