RHYTHMICAL CONTRACTIONS IN MUSCLE 521 



contractions are due to a specific effect of sodium salts. 

 Without entering into many details, I will only emphasize 

 the fact that lithium, caesium, and rubidium also bring about 

 such periodic contractions. So far as the anions are con- 

 cerned, rhythmical contractions occur not only in the chlo- 

 rides of the metals mentioned above, but also in the Br, I, 

 and F salts. In the last-named varieties of salts contrac- 

 tions begin even earlier than in the corresponding chlorides. 

 In a 0.7 per cent. NaCl solution it usually takes more than an 

 hour before the regular rhythmical contractions begin. 

 These show themselves at first in a slight tremor at the 

 femoral end of the gastrocnemius, become stronger, and 

 finally affect the whole muscle so that the tendon of Achilles 

 executes pendulum-like movements. These periodic move- 

 ments can still be observed on the following day even at 

 room temperature. In a NaF solution, equimolecular with 

 a 0.7 per cent. NaCl solution, the most vigorous contractions 

 set in immediately, but last only about half an hour and then 

 stop. In LiBr or NaBr the contractions are stronger, and 

 occur earlier than in NaCl. But whether this can be 

 attributed to the fact that Br ions are more effective than Cl 

 ions is questionable. As the muscle is always surrounded 

 by a NaCl solution and contains Cl ions, it is clear that F 

 or Br or I ions must at first pass in greater number from 

 the solution into the muscle than the Cl ions. I cannot 

 say definitely that I ions are more effective than Br ions. 

 The Rb and Cs ions have a poisonous effect like the F ions; 

 that is to say, the rhythmical contractions which are pro- 

 duced at first soon cease. In NaBr, LiBr, Nal, Lil, and 

 LiCl solutions isosmotic with a 0.7 per cent. NaCl solution 

 the contractions may continue at room temperature for one 

 or two days, in which, of course, periods of rest are often 

 noticed. 



4. If the number of ions entering the muscle determines 



