Papers from the Marine Biological Laboratory at Tortugas. 43 



By diluting the sea-water with 0.9 molecular dextrose, we maintain a 

 constant and normal osmotic pressure, but all the cations are reduced in 

 concentration, as they would have been had we added distilled water. 

 On the other hand, when we add 0.6 molecular NaCl we increase the abso- 

 lute concentration of the sodium, while the osmotic pressure of the solutions 

 as a whole remains constant, but the chief effect is to increase the relative 

 concentration of the sodium cation with respect to magnesium, calcium, 

 and potassium. 



1 .2 .3 .t .5 .6 .7 .8 .9 1. 



Relative Concentration of Mg, Ca, and K. 



Fig. 7. 



We may conclude that sodium is a powerful stimulant up to about 1.48 

 times its relative concentration with respect to the other cations in natural 

 sea-water augmenting the rate of nerve-conduction in proportion as its 

 relative concentration increases. In concentrations over and above 1.48 

 times that found in sea-water, it loses some of its stimulating effect, but the 

 rate of pulsation of the ring declines more slowly than it would in sea- 

 water diluted with a 0.9 molecular solution of dextrose, and thus the presence 

 of the sodium still acts as a constant stimulant. 



The experiment cited above is open to the serious objection that the 

 addition of 0.6 molecular NaCl increases the concentration of the Na ion 

 over and above that found in natural sea-water, and the stimulating effect 

 is evidently due in part at least to this increase, and not to the proportionate 

 decrease in concentration of magnesium, calcium, and potassium. In 

 order to avoid this error, we should add the NaCl in the form of a 0.487 

 molecular solution dissolved in a 0.075 molecular solution of dextrose, thus 

 maintaining the normal osmotic pressure and the normal concentration of 

 the sodium ion simply reducing the concentrations of the magnesium, 

 calcium, and potassium, and correspondingly increasing the relative concen- 

 tration of the sodium cation. 



Table 9 shows the results of this experiment upon 20 subumbrella rings 

 of Cassiopea, and here again it is evident that the sodium cation is a stimu- 

 lant to nerve-conduction. It maintains the rate of nerve-conduction at 

 about a normal rate until the magnesium, calcium, and potassium are 

 reduced to 80 per cent of the concentration found in natural sea-water. 



