THE PHENOMENON OF CONDUCTION. 



91 



tense, will stimulate the nerve. The arrangement is represented in 

 the accompanying schema (Fig. 34), 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 conductor of the electrical current, and to reduce the 

 resistance at the points at which the electrodes come in contact 



_ Fig. 34. Schema to show the unipolar method of stimulation in man. The anode, 

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

 , is the indifferent pole. 



with the skin each is covered with cotton or chamois skin kept 

 moistened with a dilute saline solution. 



Motor Points. By means of the unipolar method nearly every 

 voluntary muscle of the body may be stimulated separately. All 

 that is necessary, when the induced current is used, is to bring the 

 active electrode as nearly as possible over the spot at which the 

 muscle receives its motor branch. A diagram showing these motor 

 points for the arm is given in Fig. 35. In the same way the 



