A. J. CARLSON 563 



true for the heart of the crab. A solution of curare strong enough to 

 produce incomplete tetanus of the heart in this species abolishes the 

 rhythm permanently. 



The latent period of the stimulating action of curare varies with the 

 sensitiveness of the particular animal to the drug as well as with the 

 mode of application of the solution to the heart. In the molluscs 

 the alkaloid acts more quickly when poured into the cavity of the 

 heart than when applied to the surface of the heart. This is probably 

 due to a difference in permeabihty. When the solution is applied to 

 the surface of the heart it has to penetrate the epicardium in order 

 to act on the nervous and the muscular tissues, while there is no 

 endothelium lining the heart cavity. In the crustaceans the drug 

 acts equally quickly whether applied to the surface of the heart or 

 introduced into the heart cavity. When the curare solution is ap- 

 plied to the nerve cord on the dorsal side of the heart of Limtilus 

 the action is practically instantaneous. The crustacean heart is more 

 sensitive to curare than the molluscan heart (with the exception of 

 the heart of Loligo). Between closely related molluscs there may be a 

 great difference in sensitiveness. Thus the ventricle of Ariolimax 

 continues to beat with an accelerated rhythm for 25 to 30 minutes in 

 a 0.5 per cent solution of curare, a concentration producing cardiac 

 tetanus within a minute in the ventricles of Helix or Limax. 



The primary stimulating action of atropine and nicotine on the 

 heart is to all appearances very similar to that of curare. Nicotine 

 has to be used in dilutions of 1:1,000 to 1:10,000 to exhibit the true 

 stimulating action. In greater concentrations it usually stops the 

 arthropod heart at once without any primary stimulation. A strong 

 solution of curare has sometimes the same effect on the feebly pulsat- 

 ing crab heart. This difference is probably due to differences in 

 permeability. 



The strong stimulating action of curare in the invertebrate heart 

 (without exception) is in contrast to the relatively slight action of this 

 drug on the heart of vertebrates. Atropine and nicotine, on the other 

 hand, are strong stimulants also to the vertebrate heart. The stimu- 

 lating action of the curare solution may in part be due to the action of 

 potassium salts which are present as impurities in commercial curare. 

 I have found that a slight concentration of the potassium chloride in 



