STIMULATION AND TRANSMISSION 277 



activity (at the same time depressing irritability) at 

 the anode. When a current already flowing through 

 the tissue is broken, stimulation also results, but the 

 polar relations are reversed; i.e., stimulation is then at 

 the anode, inhibition at the cathode. 'J^his summarized 

 statement is the usual form of the ''law of polar stimula- 

 tion." It is important to note, however, that a polar 

 action is seen in many other physiological processes 

 occurring under the influence of the current; e.g., 

 electrotonus, polar disintegration of cells, galvanotropic 

 growth, and galvanotaxis. All of these phenomena show 

 that the direction of the current, relatively to the cell 

 surface, determines the nature of its physiological action. 

 The parallel to electrolysis, at the surface of any electrode, 

 is especially clear in phenomena of this class; it is well 

 known that where the positive stream passes from the 

 metallic electrode to the solution (at the anode) it 

 produces chemical effects (in general of an oxidative 

 kind) of the reverse nature to those produced where it 

 passes from solution to electrode (cathode); here the 

 general chemical action is reducing. 



5. That a variation in the electrical state of the 

 irritable elements, sufficient in degree and rate, is the 

 determining factor in the physiological action of the 

 current is seen in the fact that a change in either direction, 

 i.e., make or break, increase or decrease, may stimulate 

 or produce other characteristic physiological elTects. 



6. Finally, summation efl'ects are highly character- 

 istic; i.e., two or more electric stimuli (induction shocks) 

 which, acting singly, are ineffective, may cause stimula- 

 tion if sent in sufficiently rapid succession into the tissue. 

 The inter\^al between the successive single stimuli must 



