THE PHENOMENON OF CONDUCTION. 



87 



unipolar method consists in placing one electrode, the active or 

 stimulating electrode, over the nerve at the point which it is desired 

 to stimulate, while the other electrode, the inactive or indifferent 

 electrode, is applied to the skin at some more or less remote part, 

 usually at the back of the neck. The indifferent electrode is made 

 large enough to cover several square centimeters of the skin and 

 one may conceive the threads of current passing from it into the 

 moist tissues of the body and thence to the active electrode. As the 

 threads of current condense to this electrode they pass through 

 the motor nerve which lies under it, and if sufficiently intense will 



Fig. 33. Schema to show the unipolar method of stimulation in man. The anode, 

 -K is represented as the stimulating pole, applied over the median nerve. The cathode, 

 , is the indifferent pole. 



stimulate the nerve. The arrangement is represented in the accom- 

 panying schema (Fig. 33), showing the disposition of the electrodes 

 for stimulating the median nerve. At the indifferent electrode the 

 sensory nerves of the skin are of course stimulated, but no motor 

 response is obtained, as no motor nerve lies immediately under the 

 skin. Moreover the large size of this electrode tends to diffuse the 

 current and thus reduce its effectiveness in stimulating. The active 

 or stimulating electrode is small in size, particularly when induction 

 currents are employed, so that the current may be condensed and 

 thus gain in effectiveness. The dry surface of the skin is a poor 



