STIMULATION AND TRANSMISSION 271 



reversible variation of potential accompanies the passage 

 of the excitation-wave. The resemblance of the trans- 

 mission phenomenon in passive iron to the protoplasmic 

 type of transmission is in fact so detailed as to confirm 

 strongly the hypothesis that in the latter case also the 

 essential feature of the transmission-process is the 

 breakdown and reconstruction of the thin protoplasmic 

 surface-film under the influence of the local bioelectric 

 circuits. 



If the processes in the living system and in the simple 

 inorganic model are in fact similar in their dependence 

 on surface-changes of this kind, it becomes evident at 

 once why protoplasm is so readily excited by conditions 

 acting upon its surface layer. A mechanical agent, 

 by interrupting the continuity of the surface-film, or by 

 otherwise altering it so as to give rise to a local circuit, 

 may initiate a wave of electromotor variation and 

 disintegration which travels automatically over, the whole 

 surface, and in so doing alters the physiological activity 

 of the whole system. Any other sufficient local altera- 

 tion (chemical, thermal, etc.) may produce the same 

 effect. The critical factor in the local process of excita- 

 tion thus appears to be the alteration of the protoplasmic 

 surface-film in such a manner as to change locally the 

 potential difference between the protoplasm and the 

 external medium, to a sufficient degree and at a sufficient 

 rate. The local circuit arising between this altered 

 region and the as yet unaltered regions adjoining forms 

 the next link in the chain of events; and if this local 

 current has sufficient intensity and local density to 

 break down electrolytically the film over a certain area 

 of the adjoining region, an indefinite wave of propagation 



