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 larvae 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 2 and 

 CaCl 2 which is free from Na ions. 



