284 PROTOPLASMIC ACTION AND NERVOUS ACTION 



to render the external layer of the solution in contact with 

 the cell surface less positive than before, i.e., a depolari- 

 zation, is the critical or initiatory event in stimulation/ 

 But a current of too weak intensity, or one rising to 

 its maximum too slowly, will not stimulate, whatever 

 its duration. These discrepancies from the simple 

 polarization theory must be referred to the special 

 properties which the irritable tissue possesses by virtue 

 of being a living structure. Apparently the irritable 

 element is able to compensate slight or gradual changes 

 of polarization as a part of its general regulatory capacity. 

 Thus, if a current be led gradually into a nerve or muscle, 

 a considerable intensity may be reached without stimula- 

 tion. But if then the current be suddenly broken, 

 stimulation results. This behavior seems to imply 

 that while the current is gradually increasing, the cell by 

 some regulatory process maintains its normal or resting 

 physiological polarization essentially unaltered. During 

 the flow of the external current, part of the polarization 

 at the cell surface must depend on the presence of this 

 current, which steadily conveys ions to (or from) the 

 surface. When this influence is suddenly withdrawn, 

 by breaking the current, the effect is to alter the polariza- 

 tion more rapidly than can be compensated by the 

 activity of the cell, and stimulation results. The fact 

 that the rate of change to which the irritable element 

 can thus adjust itself without undergoing stimulation is 

 rapid for rapidly reacting tissues (i.e., those with brief 

 chronaxie) and gradual for ''slow" tissues indicates that 



* Cf. Briinings, Arch. ges. Physiol., C (1903), 367. Hermann also 

 recognized that the polarization change of stimulation is in the direction 

 of rendering the external surface of the irritable element less positive 

 than before {loc. cit.). 



