558 STUDIES IN GENERAL PHYSIOLOGY 
4. The reason for all these peculiar effects of the Na, 
Ca, and K ions is that these (and other) ions form combina- 
tions with the proteids of the protoplasm. The various 
metal-proteids show various physical qualities. Muscles are 
only contractile as long as they contain all three classes of 
ions in a certain proportion, which, however, may vary within 
certain limits. In a pure solution of NaCl Na ions will 
gradually take the place of the Ca and K ions in the ion 
proteids of the tissues, and this leads to a loss of contractility 
or irritability. This is the reason why a pure NaCl solution 
is poisonous. For the same reason pure equimolecular solu- 
tions of other chlorides are also poisonous. 
5. The conditions for the ciliary motion of the larve of 
the sea-urchin are in various respects different from those 
mentioned above. The ciliary motion of these organisms 
can continue for several days in a solution of MgCl, and 
CaCl, which is free from Na ions. 
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