RHYTHMICAL CONTRACTIONS IN MusoLE 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 ina 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 
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