APPLICATIONS OF THE GRAPHIC METHOD. 197 
It is found that when the constant (polarizing) current is pass- 
ing from above downward—that is, when the cathode (nega- 
tive pole) is on the side toward the muscle—the irritability of 
the nerve is increased, and the reverse when the opposite con- 
ditions prevail. 
This altered condition is known as electrotonus. Unfor- 
tunately this term is used somewhat loosely, sometimes being 
employed in the sense now explained; sometimes to denote 
a change of electro-motive force that accompanies the altera- 
tion of irritability ; and again to cover all the conditions implied 
in the experiment. It is a fact that during the passage of a 
constant current the natural nerve-current is affected, being 
increased or diminished according to the direction of the polar- 
izing current. There is, however, so much difference of opinion 
in regard to this subject that it is very doubtful whether it 
should be more than noticed in passing. 
But to return to electrotonus, which is both interesting and 
important, it has been found as a result of many experiments 
that profound modifications of the irritability of a nerve do 
take place during the passage of a constant current. These are 
diagrammatically represented in Fig. 191. 
Fig. 191.—Diagrammatic representation of variations in electrotonus according to strength of 
current employed (after Pfliiger). » n’, asection of nerve ; a, anode (+ po le) ; k, kathode 
(— pole). Curves above the horizontal denote katelectrotonus ; below, the opposite. 
Briefly stated, they are these: 1. The nature of the change 
depends on the direction of the polarizing (constant) current; 
hence, if the current is descending, there is an increase of irri- 
tability (catelectrotonus) in the portion of the nerve nearest the 
muscle, and vice versa. 2. The extent of the change of irrita- 
bility is dependent on the strength of the polarizing current. 
3. This change is most marked close to the electrodes, spreads 
to a considerable extent beyond this point without the elec- 
trodes (extra-polar regions), and also exists within the region 
of contact of the electrodes (intra-polar regions). 4. It follows 
