510 



MANUAL OF PHYSIOLOGY. 



toward the middle point, between the cathode and the anode, so 

 that here we find an unaffected part, which has been called the 

 indifferent point. 



It is a remarkable fact that this indifferent point is not always 

 midway between the two poles, but decreases its distance from 

 the cathode in proportion as the polarizing current is made 

 stronger. That is to say (^), with strong polarizing currents the 

 indifferent point is near the cathode (B) ; with weak currents it 

 lies near the anode (A) (Fig. 203). 



Besides becoming less irritable in proportion as the polarizing 

 current becomes more powerful (<5), the anelectrotonic region of 



FIG. 203. 



Diagram illustrating the variations of irritability of different parts of a nerve during 

 the passage of polarizing currents of varying strength through a portion of it. A = 

 Anode ; B = Cathode ; AB = Intrapolar district ; y^ = Effect of weak current ; y 2 = Effect 

 of medium current ; y 3 = Effect of strong current. The degree of effect is shown by the 

 distance of the curves from the straight line. The part of curve below the line corre- 

 sponds to decrease, that above to increase of irritability. Where the curves cross the line 

 is called the indifferent point. With strong currents this approaches the cathode. 

 (From Foster after Pflttger.) 



the nerve loses its ability to conduct impulses, and may, finally, 

 with a very strong current, even when applied for a short time, 

 become quite incapable of conducting an impulse. 



If the polarizing current be now opened, so as to stop its pass- 

 age through the nerve, and remove the anelectrotonic and the 

 catelectrotonic states (e), a kind of rebound occurs in the condi- 

 tion of both the altered regions, and the part which has just 

 ceased to be catelectrotonic, and was, therefore, over-irritable, 

 becomes, by a kind of negative modification, very much lowered 

 in its irritability ; while, on the other hand, the anelectrotonic 



