384 PROTOPLASMIC ACTION AND NERVOUS ACTION 



cathode of a pair of stimulating electrodes. A current 

 traversing the protoplasmic surface in this direction arises 

 as soon as the local area of stimulation, S, is altered 

 (e.g., by mechanical or chemical action) sufficiently to 

 render it negative relatively to the areas adjoining. 

 The appearance of this current, which acts at a distance 

 from the directly altered area, is apparently the primary 

 effect in stimulation. It initiates the propagated wave- 

 like disturbance, because each secondarily stimulated 



B 



Fig. 4. — In A the arrows represent the direction of the current in the active-inactive 

 circuit on one side of the stimulated region 5. In B the course of an external stimulating 

 current from a battery is represented. Stimulation originates, on make, at the cathodal 

 region, where the current has the same direction, relatively to the membrane, as at R. 



area, e.g., at region R, on itself becoming negative, 

 produces automatically the same effect on regions 

 beyond; and this effect is repeated at each boundary 

 (or region of transition) between an active area and the 

 resting area in immediate advance of it. 



There is an analogy of a general kind between the 

 spread of excitation over an irritable protoplasmic 

 element and the spread of combustion along a fuse, 

 with the difference that in the fuse the chemically 

 active area initiates action in the adjoining area through 

 the heat generated in the local reaction, while in the 



