1 62 PROTOPLASMIC ACTION AND NERVOUS ACTION 



molluscan heart (Pecteny and in the heart of elasmo- 

 branch fishes. 



Much further evidence could be cited indicating that 

 changes in the plasma membranes of the living cells, 

 produced by salts or other compounds which show no 

 evidence of penetrating into the cell interior, may 

 profoundly affect the properties of irritable tissues or 

 organisms. An interesting example is seen in the 

 changes produced by isotonic salt solutions in the 

 sensitivity of frog's muscle to various forms of chemical 

 stimuH and to physical agents like heat or contact. 

 A curarized frog's gastrocnemius, placed for a few 

 minutes in a pure isotonic solution of a sodium salt 

 (NaCl, NaBr, NaN03, Nal, NaC103, etc.) and then 

 dipped into a solution containing a stimulating compound 

 (K salt) or a cytolytic agent (chloroform in saturated 

 solution), contracts much more vigorously than a 

 normal muscle which is dipped into the same solution 

 directly from Ringer's solution. The pure Na salt 

 solution increases the responsiveness to the stimulating 

 agent; i.e., induces a sensitization; this effect is readily 

 antagonized by CaCla. The reverse effect (desensitiza- 

 tion) is produced by exposure to isotonic CaCla, SrCla, 

 or MgCL; after a brief stay in these solutions the muscle 

 fails to respond to the stimulating solution or responds 

 subnormally. In all such cases normal irritabiUty 

 returns in Ringer's solution.^ Various facts indicate 

 that Ca compounds in the surface layer of the cell are 



' G. R. Mines, Journal of Physiology, XLIII (1912), 467. 



* R. S. Lillie, Proceedings of the Society of Experimental Biology and 

 Medicine, VI (19 10), 170; American Journal of Physiology, XXVIII 

 (1911), 197; cf. p. 215. 



