MEMBRANE CHANGES DURING STIMULATION 369 



is correspondingly increased. The relative insuscepti- 

 bility of the normal muscle depends on the presence of 

 Ca salts in the external medium; if, instead of a pure 

 solution of the sensitizing salt, one containing CaClz 

 (i mol CaCla to 20 Na salt) is used, no such effects are 

 obtained.' The sensitizing action is thus subject to 

 typical salt-antagonism, like so many other biological 

 processes, especially those involving alteration of the 

 protoplasmic surface layers. Muscles which have been 

 rendered hypersensitive by exposure to the pure salt 

 solution, rapidly recover their normal properties on 

 return to Ringer's solution. The inverse type of effect, 

 decrease of susceptibiHty to chemical stimulation, may 

 be induced by a similar exposure to isotonic solutions 

 of CaClz, MgClz or similar salts. Such desensitizing 

 effects are closely related to those classed under narcosis, 

 depression, or anaesthesia, and are also reversible in 

 Ringer's solution. 



A related type of salt sensitization, produced by 

 isotonic solutions of Na salts whose anions precipitate 

 calcium (or remove Ca ions from solution) , was described 

 by Loeb in 1901.^ Muscles dipped for a few minutes in 

 solutions of Na sulphate, tartrate, citrate, or similar 

 salt, and then brought into the air (or other foreign 

 medium, e.g., oil), exhibit vigorous tetanic contractions, 

 which cease or are diminished on return to the salt 

 solution. Apparently this reaction depends on an 

 altered contact-sensibility, due to some modification of 

 the cell surface. It illustrates a type of effect which 

 appears to be widely prevalent in irritable elements. 



1 Unpublished observations in the Biological Laboratory of Clark 

 University. 



2 J. Loeb, Afnerican Journal of Physiology, V (1901), 362. 



