TRANSMISSION OF EXCITEMENT DURING ELECTROTONUS. 141 



nerve, nn' (fig. 33) is traversed between c and d by an 

 ascending current, and that it is irritated between the 

 points e and/, above the portion traversed by the current. 

 The muscle is accordingly at 71', as in our previous ob- 

 servations. Irritation takes place on the side of the 

 kathode. An increase in the excitability should there- 

 fore occur. This may easily be shown when the cur- 

 rents used for effecting electrotonus are weak. Tf, 

 however, the current used for this purpose is somewhat 

 strengthened, no increase in the excitability is ob- 

 servable ; and, indeed, if the currents are sufficiently 

 strong, it becomes quite impossible to effect contrac- 

 tion in the muscle by irritation at ef. This may seem 

 to afford an exception to the law of the electrotonic 

 changes in the excitability. But from the previous 

 experiments it is evident that this must not be in- 

 ferred. Possibly the excitability is in reality increased 

 at e f in entire accordance with the law ; but in order 

 that the action of the excitement at this point should 

 become visible, the excitement must pass through the 

 portion under the influence of electrotonus, as well 

 as through the an electrotonic portion lying below the 

 latter, and it may be supposed that this propagation of 

 the excitement meets with an insuperable obstacle in 

 the condition of strong anelectrotonus which prevails 

 there. It can indeed be shown that this is the case. 

 If the current is reversed, so that it flows in a descend- 

 ing direction through the nerve, then irritation at 

 the portion a b will invariably show the existence of 

 heightened excitement, however strong the current 

 may be. But the portion a b is now under exactly the 

 same conditions as was the portion e f previously. It is 

 in itself very improbable that the nerve acts differently 



