METHODS OF ELECTRICAL STIMULATION 69 



key to the nerve no longer produces contraction. 

 Now let this key remain closed and make and 

 break the primary circuit. . 



The muscle will contract both on opening and 

 closure. The induction currents developed in 

 the primary coil when the primary current is 

 made and broken stimulate the nerve, al- 

 though the galvanic current itself is powerless 

 to do so. 



Tetanizing Currents. Connect a dry cell to 

 posts 2 and 3 of the primary coil. The vibrat- 

 ing hammer will automatically make and break 

 the current. Place the electrodes against the 

 nerve or muscle. 



The muscle will contract once for each induc- 

 tion current, but the contractions are so rapid 

 that they fuse into a prolonged shortening termed 

 tetanus. 



Induction in Nerves. Faraday discovered that 

 currents can be induced in electrolytes as well 

 as in metallic conductors. Induced currents may 

 therefore appear in nerves lying sufficiently near 

 a primary circuit. 



Lay the well-moistened nerve of a nerve-muscle 

 preparation around the primary coil protected by 

 a piece of paraffin paper in such a way that the 

 free end of the nerve touches the nerve near the 

 muscle or touches the muscle itself, so as to form 



