546 STUDIES IN GENERAL PHYSIOLOGY 
occasionally, namely, rhythmical contractions of the muscles 
of the skeleton.’ I found that such rhythmical contractions 
occur only in solutions of electrolytes, 2. e., in compounds 
which are capable of ionization. In solutions of non- 
conductors (urea, various sugars, and glycerin) these 
rhythmical contractions are impossible. This is an indica- 
tion that they are a function of the ion-proteids. But only 
in certain ion solutions are such rhythmical contractions 
possible. All the solutions of Na salts are able to produce 
them, but in a 0.7 per cent. NaCl solution contractions be- 
gin later and are less powerful than in an equimolecular 
NaBr solution. This indicates that not only the metal ion 
influences the physical qualities of the ion-proteids, but that 
the anion does soas well. This might be understood from 
the assumption that anions as well as kations may combine 
with proteids. This forces us to raise the question whether 
both ions may not combine with the same proteid molecule, 
only with the difference that the two different ions be added 
at different places in the molecule. My colleague, Professor 
Stieglitz, with whom I discussed this question, called my 
attention to the behavior of amido-acetic acid, which indeed 
acts in a similar way. From all we know concerning the 
constitution of proteids it seems justifiable to assume that 
some of them may very well share certain peculiarities of 
the amido acids.” 
The experiments on the rhythmical contractions of the 
muscles of the skeleton, however, led to some other data 
concerning the ion proteids. Solutions of Na ions produce 
rhythmical contractions only if the muscle cells contain Ca 
ions in sufficient numbers. As soon as there is a lack of Ca 
ions in the tissues the Na ions are no longer able to cause 
rhythmical contractions. On the other hand, if we add Ca 
1 Part II, p. 518. 
2 Sprro, Zettschrife fiir physiologische Chemie, Vol. XXVIII (1899), p. 174. 
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