84 THE PHYSIOLOGY OF MUSCLE AND NERVE. 



of a nerve impulse through it. The changes on the cathodal side 

 are not so constant nor so distinct. Later observers* have shown, 

 in fact, that if the polarizing current is continued for some time 

 the heightened irritability at the cathode soon diminishes and 

 sinks below normal, so that in fact at the cathode as well as at 

 the anode the irritability may be lost entirely. If the polarizing 

 current is very strong this depressed irritability at the cathode 

 comes on practically at once. Moreover, when a strong current 

 that has been passing through a nerve is broken the condition of 

 depressed irritability at the cathode persists for some time after 

 the opening of the current. 



Pfliiger's Law of Stimulation. It was said above that when 

 a galvanic current is passed into a nerve there is a stimulus (catho- 

 dal) at the making of the current and another stimulus (anodal) 



Fig. 31. Electrotonic alterations of irritability caused by weak, medium, and strong 

 battery currents: A and B indicate the points of application of the electrodes to the nerve, A 

 being the anode, B the cathode. The horizontal line represents the nerve at normal irri- 

 tability; the curved lines illustrate how the irritability is altered at different parts of the 

 nerve with currents of different strengths. Curve y l shows the effect of a weak current, the 

 part below the line indicating decreased, and that above the line increased irritability; at x l 

 the curve crosses the line, this being the indifferent point at which the catelectrotonic effects 

 are compensated for by anelectrotonic effects; y- gives the effect of a stronger current, and 

 y 3 , of a still stronger current. As the strength of the current is increased the effect becomes 

 greater and extends farther into the extrapolar regions. In the intrapolar region the in- 

 different point is seen to advance, with increasing strengths of current, from the anode 

 toward the cathode. (Lombard.) 



at the breaking of the current. This statement is true, however, 

 only for a certain range of currents. Of the two stimuli, the making 

 or cathodal stimulus is the stronger, and it follows, therefore, 

 that when the strength of the current is diminished there will come 

 a certain point at which the anodal stimulus will drop out. With 

 weak currents there is then a stimulus only at the make. On the 

 other hand, when very strong currents are used the stimuli that act 

 at the two poles set up nerve impulses whose passage to the muscle 

 may be blocked by the depressed conductivity caused by the electro- 

 tonic changes. Whether or not the stimulus will be effective in 



* Werigo, " Pfliiger's Archiv," 84, 547, 1901. See Biedermann, " Elec- 

 trophysiology," translated by Welby, vol. ii, p. 140. 



