STIMULATION OF MUSCLES AND OF NERVES 425 



While the current is flowing the excitability is increased on both sides of the 

 cathode; on both sides of the anode it is decreased. These alterations appear 

 immediately (within 0.00007 second at most) after closing the current. In 

 the intrapolar region there is found an indifferent point where the excita- 

 bility of the nerve is not changed; and as the constant current increases in 

 strength this point moves toward the cathode. At the same time the extra- 

 polar alterations of excitability spread over greater lengths of the nerve. 



Likewise during the first few moments after the current is opened altera- 

 tions in the excitability appear, but they are just the reverse of those which 

 occur while the current is closed i. e., reduced at the cathode and increased 

 at the anode. 



These alterations may be studied in the following manner. A nerve is 

 stimulated rhythmically, say once a second, with a current of constant strength, 

 and the resulting contractions are recorded in the usual manner. If now while 

 the stimulation is going on at the regular rhythm, a constant current be led into 

 the nerve, and the stimuli fall in the neighborhood of the cathode of this cur- 

 rent, the contractions at once become stronger; if they fall in the neighborhood 

 of the anode, the contractions decrease in size and disappear altogether (see 

 Figs. 161 and 162). When the current being led through the nerve is broken, 

 contractions from stimuli applied at the cathode become smaller, those from 

 stimuli at the anode become larger. 



The increase of excitability at the cathode while the current is closed soon 

 fails and passes over into a depressed condition, which, as Biedermann observes, 

 is probably the expression of a local fatigue of the nerve. 



The phenomena comprehended under the law of contraction may then be 

 explained through the law of polar excitation as follows: Weak currents give a 

 closing contraction because when the current is closed, the sudden rise in irri- 

 tability of the nerve at the cathode is great enough to constitute a stimulus of 

 itself. The stimulus is effective whether the current be ascending or descend- 

 ing, for in the one case the cathode is toward the muscle, and in the other the 



FIG. 162. Anelectro+onus. The tracing to be read from right to left. A series of stimuli just 

 strong enough to produce slight contractions were applied in the neighborhood of the anode 

 for the polarizing current. When the polarizing current was turned on the stimuli became 

 ineffective. When it was again turned off the stimuli again became effective. 



resistance at the anode, due to the decrease in irritability, is not great enough 

 to block the stimulus. The sudden increase in excitability produced in the nerve 

 at the anode when the current is broken is not yet sufficient to constitute a 

 stimulus. With the medium current the increase in excitability at the cathode 

 on closing and at the anode on opening are both sufficient to produce a stimulus, 

 and in neither case is the opposite pole strong enough to block it. The strong 

 current is distinguished from the medium by the circumstance that while the 

 current is closed the resistance at the anode is stronger than the excitation at 

 the cathode and vice versa when the current is opened. Consequently with the 

 ascending current the excitation started at the cathode cannot break through 



