124 



PRACTICAL PHYSIOLOGY 



kathodic regions. These effects can be shown by the following 

 experiment. 



One Daniell battery is connected by two wires with a PohFs 

 reverser whereby the direction of the current can be changed ; from 

 the reverser the wires pass by means of a Du Bois key to a pair of 

 unpolarisable electrodes. This is the polarising circuit. The 

 stimulating circuit is set up separately for the production of single 

 induction-shocks (Fig. 124). A preparation of the sciatic nerve and 

 gastrocnemius muscle is carefully made from a recently pithed frog, 

 and is placed in a moist chamber ; a pin is fixed through the lower 

 extremity of the femur, and the tendo-Achillis is connected by a 

 thread with a lever. The sciatic nerve is placed across the kaolin 

 plugs of the unpolarisable electrodes. The drum can be moved by 

 hand. A minimal stimulus for the nerve is obtained, care being 



FIG. 124. Diagram of the experiment on the effects of a constant 

 electrical current upon the excitability and conductivity of nerve. 



taken to use only the break or make-shock. The minimal con- 

 traction is recorded on the stationary drum. 



The current from the polarising circuit is closed in an ascending 

 direction, so that the current enters the nerve on the side near the 

 muscle and immediately above the stimulating electrodes, which are 

 connected with the inductorium. The nerve around the point of 

 entry or anode of the polarising current is depressed in its excitability, 

 and the application of a minimal, or even stronger, stimulus is no 

 longer effective (Fig. 125). The polarising current is short-circuited 

 by the Du Bois key, and by means of the reverser is changed in its 

 direction, so that on opening the Du Bois key the current is descend- 

 ing, and the area of nerve near the stimulating electrodes passes 

 into a condition of katelectrotonus. The minimal stimuli now 



