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MANUAL OF PHYSIOLOGY. 



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 (y), 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 l = 

 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 corresponds 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 Pfliiger.) 



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 



