775 



the usual diffusible potassium in the circulating fluid. It is then 

 reduced to a standstill, from which it does not recover of itself, 

 and after which it can initial!}' resume its normal pulsations by 

 adding in excess an appropriate amount of one of the radio-active 

 salts, either of the one or of the other group. Now it appears that 

 in the like equilibria caesium must be classed under the light metals. 

 Potassium, rubidium, and caesium, therefore, constitute biologically 

 one group; uranium and thorium the other. 



In this connection it was expedient to ascertain whether the 

 caesium-heart responded to electric stimuli, as is the case with the 

 potassium- and the rubidium-heart. This proved to be the case. An 

 appropriate induction stimulus produces an extrasystole. When, 

 however, the ventricle was taken separately, the compensatory 

 pause was absent. 



Evidently the preparation acts merely upon ventricular automa- 

 ticity, originating on the distal side of the atrioventricular boundary- 

 line, and it consequently behaves like the sinus venosus as regards 

 the rhythm after extrasystole. 



It appears, then, that caesium resembles potassium also in its 

 relation to the extrasystoles, and must be contrasted with uranium, 

 which precludes extrasystoles ^). 



In a previous paper ^) I have pointed out that the potassium- (resp. 

 rubidium-)uranium-(resp. thorinm-)equilibria can be largely shifted 

 by the addition of fluorescein. This substance i-enders the heart 

 more susceptible to the influence of uranium or thorium than to 

 that of potassium or rubidium. This is why a heart, brought to a 

 standstill by a precise counterbalancing of light and heavy metal, 

 will resume its pulsations as. soon as 100 mgrms of fluorescein per 

 liter is added to the solution. The influence upon potassium (resp. 

 rubidium), therefore, is inconsiderable, that of uranium is great. In 

 this respect caesium is also to be classed with potassium. Fluorescein 

 has next to no influence upon a heart, beating in groups under 

 subminimal caesium-doses. 



From the above we conclude that, biologically, caesium is doubt- 

 lessly to be bracketed with potassium and rubidium and that it is 

 antagonistic to uranium and thorium, which was unknown here- 

 tofore. Suppose, therefore, that nature were to bring forth caesium- 

 animals as it brings forth potassium-animals everywhere, they 

 might be assumed to stand to radio-activity and electricity in the 



1) H. Zwaardemaker, Proceedings 1, May 26, 1917. Vol. 26. 



2) H. Zwaardemaker, Proceedings Sept. 1917 Vol. 26, p. 255. 



