RHYTHMICAL CONTRACTIONS IN MUSCLE 533 



it is only the application of simple laws to the muscle. We 

 now find, when we keep in mind what has been said above, 

 that the entrance of sodium ions (but also Cl, Br, and other 

 ions which have been mentioned above or are at present still 

 unknown) into certain muscle compounds is accompanied by 

 contractions. We observe, secondly, that the entrance of 

 Ca ions into the normal muscle inhibits these contractions. 

 Indeed, one might think that the process of substituting any 

 ions for Ca ions is adapted in a peculiar way to giving rise 

 to a contraction, while the reverse process, the substitution 

 of Ca ions for any ions, alters the muscle substance in an 

 opposite sense. In this way it might be explained why a 

 muscle does not contract in the NaCl solutions containing 

 Ca, while it begins to contract immediately when introduced 

 into a NaCl solution free from Ca. It also becomes intelli- 

 gible that the muscle which has been in a pure NaCl solution 

 for some time no longer contracts. It has no more Ca ions 

 to exchange. What has been said harmonizes also with the 

 previous observations of Ringer, that a muscle in a NaCl 

 solution remains irritable for a longer time when the solution 

 contains a trace of Ca and K salts. And it also harmonizes 

 with what has been said, that, according to Howell, a muscle 

 from which the Ca salts have been removed by washing with 

 oxalates loses its irritability. Our views and observations 

 do not, however, agree with Howell's claim that NaCl "is of 

 importance only in so far as it maintains the osmotic pressure 

 between the tissues and the surrounding fluid." This view 

 of Howell contradicts also the observations of Locke, who 

 showed that NaCl is not so indifferent a substance. I am 

 gradually coming to believe that, strictly speaking, there is 

 no solution which is merely of osmotic importance for living 

 tissues. Even an isotonic sugar solution (about 4.91 atmos- 

 pheres pressure) affects the irritability, and probably also 

 other properties, of the muscle to a high degree. We must 



