356 PROTOPLASMIC ACTION AND NERVOUS ACTION 



and other cells, exhibit under appropriate conditions a 

 similar activating influence.^ 



INCREASE OF PERMEABILITY DURING NORMAL STIMU- 

 LATION, ACTIVATION, AND CELL-DIVISION 



Direct proof of the increased permeability of rapidly 

 responding tissues like vertebrate muscle or nerve 

 during normal stimulation is difficult to obtain. In the 

 case of muscle the results gained with the method of 

 electrical conductivity^ are of uncertain value, since the 

 change in the form of the tissue, the production of 

 electrolytes (e.g., lactic acid) in the process itself, and 

 the change in the distribution or quantity of the inter- 

 cellular fluids (lymph), all affect the total conductivity; 

 hence the observed alterations may depend on other 

 factors than the changing permeability of the plasma 

 membranes. Direct observation of the penetration of 

 easily detectable compounds into the muscle cell has 

 given better results; recently Mitchell and his associates^ 

 have shown that rubidium and caesium chlorides do not 

 enter the muscle cells when the resting muscle is perfused 

 with Ringer's solution containing these salts, but 

 penetrate readily when the tissue is thrown into contrac- 

 tion by stimulating the nerve. In this case there seems 

 to be an unequivocal demonstration of increased permea- 

 biHty to inorganic salts during stimulation. Embden 



^ For the activation of starfish eggs by heat cf. Delage, Arch. zool. 

 exper. et gen., IX (1901), Series III, 285; R. S. Lillie, Journal of Experi- 

 mental Zoology, V (1908), 375; Biological Bulletin, XXVIII (1915), 260. 

 For the action of ultra-violet rays cf. J. Loeb, Science, XL (1914), 680; 

 R. S. Lillie, American Journal of Physiology, LX (1922), 57, 272. 



2 Cf . McClendon, American Journal oj Physiology, XXIX (19 12), 

 302. 



3 Mitchell, Wilson, and Stanton, Jour. Gen. Physiol., IV (192 1), 141. 



