STIMULATION OF MUSCLE AND NERVE 131 



of the secondary coil is the cathode when the 

 primary current is broken (page 158). To this 

 pole connect the small (stimulating) 'electrode ; 

 to the other pole connect the large (indifferent) 

 electrode. Place the indifferent electrode on 

 the arm or neck. With the small electrode 

 make out the motor points indicated in Figs. 35 

 and 36. 



Polar Stimulation of Human Nerves. In the 

 hands of the earlier observers the stimulation 

 of nerves within the body gave results often 

 contrary to the law of polar stimulation so easily 

 demonstrated in extirpated nerves. The ex- 

 planation of these inconstant results lay in the 

 failure to comprehend the distinction between 

 the stimulating positive and negative electrodes 

 and the physiological anode and cathode (compare 

 page 108). Even when the monopolar method 

 is employed, and a small electrode is brought as 

 near as possible to the nerve to be stimulated, 

 while a large indifferent electrode is placed on 

 some other part of the body, it is impossible to 

 secure true monopolar stimulation. The current 

 entering at the anode does not remain in the 

 nerve, but very soon passes out into the sur- 

 rounding tissues (Fig. 37). Hence there are 

 physiological cathodes on both sides of the posi- 

 tive electrode, and for the like reason physiologi- 



