p. H. MITCHELL, J. W. WILSON, AND R. E. STANTON 143 



our observations we would conclude that it shows the former to be 

 true even though the effect upon irritability may be at the cell surface. 

 So far as we have been able to read Zwaardemaker's reports in the 

 original we have found no evidence on this aspect of the question and 

 in this connection our results seem of some significance. 



EXPERIMENTAL. 



We perfused frog muscles with a Ringer solution modified by 

 the replacement of potassium chloride with an aequimolar concen- 

 tration of rubidium chloride. While both legs were perfused the 

 muscles of one were made to contract by stimulation of the lumbar 

 plexus with maximal tetanizing induction shocks lasting 1 second at 

 30 second intervals during one-half hour periods with alternating 

 one-half hour periods of complete rest. In one experiment this pro- 

 cedure was continued during 5 hours and was followed by perfusion 

 with an isotonic cane sugar solution during 1| hours. All the muscles 

 of both legs showed irritability at the end of the experiment. Samples 

 of the gastrocnemius and sartorius of each leg were then decomposed 

 in a mixture of nitric and sulphuric acids. The resulting solutions 

 enabled us to detect rubidium spectroscopically in the muscles of 

 the stimulated leg but not in those of the other. In another similar 

 experiment the muscles of one leg were given nine half-hour work 

 periods, that is, 540 contractions of 1 second each, and were then 

 while resting, perfused during 2 hours with a potassium-free Ringer 

 solution. The muscles of both legs showed good irritability at the 

 end of the experiment. The wet-ashed muscle samples, taken from 

 the gastrocnemius and vastus muscles of each leg, were made up to 

 10 cc. and examined spectroscopically by Gooch's method for quanti- 

 tative estimation of rubidium. No trace of rubidium could be de- 

 tected in the muscles perfused without stimulation. The samples 

 used were 1.27 gm. from the gastrocnemius and 2.37 gm. from the 

 vastus. In the muscles of the stimulated legs, however, we found 

 approximately 0.011 per cent of rubidium. The standard solution 

 for spectroscopic comparison with the muscle material contained 3 

 per cent of H2SO4, 0.075 per cent of KCl, 0.025 per cent of XaCl, and 

 0.0025 per cent of rubidium added in the form of the carbonate. 

 It was designed to imitate, approximately, the acid, potassium and 



