EFFECTS OF SODIUM. 47 



It will not pulsate, however, in NaCl+ MgSO^ or MgClj or both, 

 and it is evident that the magnesium salts contained in sea-water 

 counteract the stimulating effect of the sodium. Disks that have 

 ceased to pulsate in §4n NaCl will revive a few pulsations if supplied 

 with calcium, or with a strong «;c«w of potassium, or both, but no 

 revival results when magnesium is added to the NaCl solution. 

 Indeed, it may be said of the sea-water that the chief stimulant, 

 owing to its large amount, is sodium chloride, and the chief inhibitor 

 of pulsation is the magnesium. As is well known, however, pure 

 sodium chloride solutions cannot sustain pulsation, for in all known 

 cases of rhythmical movement from that of Medusae to that of the 

 vertebrate heart, calcium and potassium must be associated with the 

 sodium, and I find that magnesium must also be present to restrain the 

 highly stimulating influence of the combination of sodium, calcium, 

 and potassium. Indeed, in order to pulsate rhythmically an organ 

 must be in that delicately balanced state known to physiologists as 

 being upon the threshold of stimulation. When in this condition a 

 constantly accumulating internal stimulus, which is reduced at each 

 contraction, will maintain rhythmical pulsation. 



Normal Medusae of Cassiopea with marginal sense-organs intact will 

 pulsate for a short time with abnormal rapidity in a pure Y%n NaCl 

 solution, but their rate quickly declines so as to become abnormally 

 slow, and in about 10 minutes they begin to pulsate only at intervals 

 with longer and longer periods of rest between periods of pulsation. 

 Practically all movement ceases at the end of about 30 minutes. I^ittle 

 or no toxic effect is produced, however, for recovery is almost instan- 

 taneous in sea-water, and pulsation can be revived, even after several 

 hours, by the addition of any calcium salt to the NaCl solution. 



Pulsation of normal Cassiopea ceases in i to 6 minutes in a solution 

 containing the amounts of NaCl and MgSO^ + MgClj found in sea- 

 water, but it can sometimes be revived temporarily by adding potas- 

 sium, or always by the amount of calcium found in sea-water. 



Normal Medusae of Cassiopea are but little affected by an excess of 

 NaCl in the sea-water, and will pulsate for more than 18 hours in sea- 

 water -I- 1 per cent excess of NaCl . Their pulsation , however, becomes 

 somewhat irregular, although of practically normal average rate, but 

 the mouth-arms are strongly and abnormally contracted. Recovery in 

 sea-water is, however, very rapid and no apparent toxic effects are 

 produced. A Medusa in sea-water -1- 1.55 per cent excess of NaCl 

 pulsates with abnormal rapidity for half an hour, and although shriv- 

 eled recovers quickly on being replaced in normal sea-water. 



