THE PHENOMENON OF CONDUCTION. 



87 



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 with the skin each is 



t Fig. 33. 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. 



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 where the 

 muscle receives its motor branch. A diagram showing these motor 

 points for the arm is given in Fig. 34. In the same way the nerves 

 of the brachial plexus and other nerve trunks may be stimulated very 

 readily through the skin. When the induction current is used no 



