EFFECTS OF CAIvCIUM. 



45 



The disk of Cassiopea does not pulsate in sea-water, because tlie 

 sea-water as a whole does not stimulate it. Disks of Aurelia and 

 Dactylometra behave in sea -water as if they were weakly stimulated . 



Howell (1901, pp. 200, 204) concludes as a result of Ms own work 

 and a review of the labors of others that potassium acts somewhat as 

 an inhibitory agent upon the rhythmical pulsation of the heart muscle 

 of the ventricle of the terrapin, for it lengthens the period of diastole, 

 causing the rate to become slower,* but at the same time the heart 

 muscle pulsates longer when potassium is present than it does when 

 only sodium and calcium are present. A small excess of potassium in 

 physiological doses is not toxic in its effects, yet it inhibits the pulsa- 

 tion of the heart muscle; but the muscle will beat again in solutions 

 containing less potassium or more calcium. Other physiologists con- 

 clude that small amounts of potassium stimulate "the vertebrate 

 heart." (See Carlson, 1906, p. 397.) 



It is interesting to observe that Macallum (1903) finds that the 

 bodies of Cyanea and Aurelia contain considerably more potassium 

 than does sea-water. He found the various elements to exist in the 

 following proportions : 



GENBRAI^ INFLUENCE OF CALCIUM UPON PULSATION. 



Calcium is essential for pulsation on account of its power to coun- 

 teract the inhibiting influence of magnesium. Its importance in con- 

 nection with sodium and potassium in maintaining pulsation has been 

 known since Ringer's important experiments in 1883. 



If we place perfect Medusse of Cassiopea, with marginal sense- 

 organs intact, in a solution resembling sea-water but merely lacking 

 calcium, t the Medusae pulsate more and more weakly, and all move- 

 ment ceases in less than 6 minutes. The Medusse are not poisoned, 

 however, for if, after remaining motionless for fully an hour we add 

 calcium to the solution, or restore the Medusse to sea-water, pulsation 

 is resumed almost at once, beginning feebly at first but rapidly regain- 

 ing its normal vigor in a few minutes. 



* I find that in the embryo loggerhead turtle, 14 days old, the heart pulsates faster 

 in NaCl + KCl than it does in pure NaCl. 



t96 o.c. CHzO + 2.7 grams NaCl + 03.7 gram MgCla + 0.16 gramMgSOi + 0.085 

 gram K2SO1, or 100 NaCl + 2.2 KCl + 7.8 MgClj + 3.8 MgSoi, all of f|n concentra- 

 tion. 



