24 RALPH S. LILLIE 



There are indications that something more is involved here 

 than a mere formal resemblance or analogy. In stimulation a 

 temporary change in the electrical polarization of the limiting 

 membrane of the irritable element is the essential or critical 

 event. 1 This change — so far as the present evidence extends — ■ 

 appears to be conditioned by a temporary variation in the prop- 

 erties of the limiting membrane. Apparently the latter under- 

 goes a rapid and automatically reversible increase of permeabil- 

 ity; hence the electrical polarization of the membrane, which in 

 the resting cell is a function of the normal semi-permeability, - 

 is temporarily diminished; the characteristic electrical variation 

 of stimulation is an expression of this change. In other words, 

 the membrane temporarily loses its semi-permeability during 

 stimulation. The evidence of this is largely indirect, in the 

 nature of the case, but fortunately one irritable tissue is known 

 in which there is clear and unequivocal indication that the mem- 

 branes lose their semi-permeability during excitation; this is the 

 osmotic motor mechanism of sensitive plants, where movement 

 results from a sudden loss of turgor. Since turgor is dependent 

 on semi-permeability, its sudden loss can only mean loss of semi- 

 permeability. Evidence that similar conditions exist in irrit- 

 able animal tissues is seen in the increase of electrical conductiv- 

 ity of muscle during excitation,^ and in the temporary loss of 

 irritability (the so-called refractory period) which accompanies 

 the rising phase of the electrical variation: it is clear that if the 

 membranes of the irritable tissue lose their semi-permeability 

 during stimulation, any repetition of stimulation will be impos- 

 sible until the semi-permeability necessary, to this process is 

 regained, since relative impermeability to ions is a necessary 

 condition of the polarizing action of the electrical current, and 

 it is this action — as Nernst has shown — which is the essential 

 in electrical stimulation. I have also adduced evidence in favor 



1 This is the necessary inference from the studies of Nernst and his successors 

 on electrical stimulation. 



2 Increase of permeability, however induced, decreases or abolishes the de- 

 marcation-current potential. 



3 Cf. McClcndon, American Journal of Physiology, 1912, vol. 29, p. 302; Gale- 

 otti, Zentralblatt flir Physiologie, 1912, vol. 26, p. 536. 



