538 STUDIES IN GENERAL PHYSIOLOGY 
the irritability of the muscle suffers less and is maintained 
for a longer time than in the pure physiological salt solu- 
tion in which these rhythmical contractions take place. It 
is rather to be believed that the entrance of Ca (and K) 
into definite compounds in the muscle renders difficult or 
impossible the rhythmical contractions. 
c) Hydroxyl and hydrogen ions accelerate the beginning 
of rhythmical contractions when added in a sufficient dilu- 
tion to the solutions of the ions mentioned under a). If, 
however, they are added to solutions of non-electrolytes or 
ions which inhibit contractions, they do not have these 
effects. They have, therefore, a catalytic effect in the start- 
ing of rhythmical contractions by other substances with- 
out, however, being able to call forth these rhythmical 
contractions directly. 
d) According to our present state of knowledge, which is 
still limited, we dare say, that only ions, and not non-electro- 
lytes, are able to call forth rhythmical contractions in skeletal 
muscle. 
e) The laws governing the periodic contractions of the 
ventricle of the heart seem to be the same as those which 
have shown themselves to govern voluntary muscle. 
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