IRRITABILITY. 



445 



the circuit is also closed and a current passes. The closing of the 

 key is make; its opening, break; these being abbreviated expres- 

 sions to imply that the circuit is closed or made, and open or 

 broken. 



Du Bois-Reymond Key (Fig. 247). By this key the circuit 



FIG. 248. Electric circuiting. 



may be either closed or the current short-circuited. In Fig. 248 

 these two methods of the use of the key are shown. At a the 

 current is passing through the nerve because the key is closed, 

 and at 6 it is not so passing, because the key is open. When used 



FIG. 249. Schema of induction apparatus. 



in the manner shown at c and d the battery is at all times connected 

 with the electrodes which are in connection with the nerve, so that 

 the current is at all times taking this path when the key is open at 

 d ; but when the key is closed, as at c, the key offering less resist- 



