36 



Papers from the Marine Biological Laboratory at Tortugas. 



that in the pulsation of Cassiopea the calcium of the sea-water assists the 

 NaCl to counteract the inhibiting influence of magnesium, and later Meltzer 

 and Auer, in an important series of studies upon vertebrates, found that 

 solutions containing both calcium and sodium counteracted the inhibiting 

 tendencies of magnesium. Experiments upon Cassiopea show that cal- 

 cium alone can not produce tetanus and does not offset the effects of 

 magnesium if sodium be absent, and thus it appears that the so-called 

 "calcium tetanus" is actually a result of the association of sodium with 

 calcium. Mines (191 1 a) believes that calcium enters into combination with 

 cardiac tissue, but magnesium does not, and our results support this view. 

 Fortunately for physiological experimentation, however, Cassiopea is 

 but little affected by considerable changes in osmotic pressure, and table 3 

 is valid for all practical purposes. 



Table 3. — Changes in osmotic pressure and in concentration of dissolved salts in diluted and 

 in concentrated sea-water from Tortugas, Florida. 



RELATION BETWEEN THE CONCENTRATION OF THE CATIONS OF SEA- 

 WATER AND THE RATE OF NERVE-CONDUCTION. 



If we dilute sea-water by mixing it with distilled water, or concentrate 

 it by slow evaporation at ordinary temperatures, we not only decrease or 

 increase the concentrations of the electrolytes, but we change the osmotic 

 pressure in a nearly equal ratio. We may, however, avoid lowered osmotic 

 changes by diluting the sea-water with a 0.9 molecular solution of puie 

 dextrose,^ which is practically isotonic with sea-water, and in this manner 

 we may reduce the concentrations of the electrolytes and at the same time 

 maintain the normal osmotic pressure. Thus we may determine the effect of 

 changes in concentration of the electrolytes independent of osmotic pressure. 



The results of such experiments are shown in tables 4, 5, and 6, while 

 table 7 shows the arithmetical averages derived from the results recorded 



1 The dextrose used in these experiments was the best that could be obtained from Merck and from 

 Kahlbaum. The accepted samples were tested for sodium and free acids and found to contain inappreciable 

 quantities of these impurities. The dextrose solutions were made up afresh each day. 



