I 14 INSTRUMENTS. [CH. x. 



represents two common forms of key. A key is a piece of 

 apparatus by which the current can be allowed to pass or not 

 through the nerve or muscle laid on the electrodes. When the 

 key is open the current is broken, as in the next figure (fig. 135) ; 

 when it is closed the current is allowed to pass. The opening of 

 the key is called break ; the closing of the key is called make. A 

 contraction occurs only at make and break, not while the current 

 is quietly traversing the nerve or muscle. 



But it will be seen in the Du Bois Reyniond key (fig. 134) that 

 there are four binding screws. This key is used as a bridge or shwt 

 circuiting key, and for many reasons this is the best way to use 

 it. The next diagram (fig. 136) represents this diagrammatical!}*. 

 The two wires from the battery go one to each side of the key ; 

 the electrodes come off one from each side of the key. When the 



Fig. 136. 



key is open no current can get across it, and therefore all the 

 current has to go to the electrodes with the nerve resting on 

 them ; but when the key is closed, the current is cut off from the 

 nerve, as then practically all of it goes by the metal bridge, or 

 short cut, back to the battery. Theoretically a small amount of 

 current goes through the nerve ; but the resistance of animal 

 tissues to electrical currents is enormous as compared" to that of 

 metal, and the amount of electricity that flows through a conductor 

 is inversely proportional to the resistance ; the resistance in the 

 metal bridge is so small that for all practical purposes, all the 

 current passes through it. 



Another form of electrical stimulus is the induced current, pro- 

 duced in an induction coil. 



In a battery of which the metals are connected by a wire, 

 we have seen that the current in the wire travels from the 



