MEMBRANE CHANGES DURING STIMULATION 367 



is irreversible and is associated with a marked increase 

 of permeability and rapid death. Phenomena of a 

 similar kind are seen in vertebrate skeletal muscle. 

 The permanent or irreversible shortening or '^ contrac- 

 ture" of frogs' muscle in solutions containing cytolytic 

 substances, e.g., saturated solution of chloroform in 

 Ringer's solution, is well known; this contraction is 

 associated with a large production of lactic acid, and 

 apart from its irreversible or ^' rigor" character bears 

 many resemblances to normal contraction. A similar 

 contraction accompanies the onset of heat-rigor and other 

 forms of death-rigor, and in all such cases the structure 

 of the cells is profoundly altered, the permeability 

 undergoing marked increase while the fibrils lose their 

 tensile strength and elasticity. These changes have a 

 close general resemblance to those already described as 

 accompanying the accelerated rhythmical activity—^ 

 indicating excessive stimulation — -of the ctenophore 

 swimming plate in pure Na salt solutions. 



SENSITIZATION AND RELATED EFFECTS 



The foregoing contraction-producing action of cytoly- 

 tic substances on frog's muscle may be made to resemble 

 more closely the phenomena of normal stimulation by 

 first ''sensitizing" the muscle by irnmersing it for a few 

 minutes in a pure isotonic solution of a neutral Na salt 

 (NaCl, NaBr, Nal, NaN03, etc.). The fresh isolated 

 gastrocnemius (normal or curarized), immersed in 

 Ringer's solution and arranged so as to write upon a 

 smoked drum, is transferred for four or five minutes to 

 the pure solution of the Na salt, from which it is brought 

 directly into the solution containing the cytolytic 



