278 PROTOPLASMIC ACTION AND NERVOUS ACTION 



be less than a certain critical time, or no summation 

 results. This ''summation time" is closely related to 

 the characteristic time-factor of the tissue, being brief 

 in tissue with brief chronaxie and vice versa/ Since the 

 stimuli normally acting in the intact organism are largely 

 repetitive or rhythmical, summation processes are of 

 special physiological interest. 



An exhaustive discussion of the effects of electricity 

 on living organisms is not possible within the limits of 

 space; but those features of electrical stimulation which 

 indicate its dependence on surface-alteration are of 

 fundamental theoretical significance and will be con- 

 sidered in some detail. 



Chief among these features are the time-relations of 

 electrical stimulation. A stimulating current of a given 

 intensity must flow uninterruptedly for more than a 

 certain time through the tissue or it produces no apparent 

 efl'ect. When the relations between the intensity of a 

 stimulating current and its minimal duration are investi- 

 gated, a highly characteristic relation appears, indicating 

 that the action of the current depends upon the transport 

 of ions to or from the semi-permeable surfaces of the 

 irritable tissue. The resulting change of electrical 

 surface-polarization forms the primary condition of 

 stimulation. This was first clearly shown by Nernst,^ 

 in a paper on the relation between the stimulating 

 action of alternating currents and the rate of alternation. 

 In a living tissue, wliich, considered from a simplified 

 physico-chemical point of view, represents an electrolyte 

 solution partitioned by membranes not readily permeable 



^ Cf. K. Lucas, Journal oj Physiology, XXXIX (1910), 461. 

 ^'Nernst, loc. cit. (1899). 



