GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 45 



it is to place it upon the electrodes and close the connection between the zinc and copper at 

 some other part of the circuit ; this may be done by what is called a key. Any mechanism 

 which can be used to complete the circuit could 

 receive this name, and there are a number 

 of convenient forms. The one most used by 

 physiologists is that devised by Du Bois-Rey- 

 mond, and which bears his name (see Fig. 7). 

 This has the advantage of being capable of 

 being used in two different ways one simply 

 as a means to close the circuit, and the other 

 to short-circuit the current. These two meth- 

 ods are shown in Figure 8. 



By the former method the key supplies a 

 movable piece of metal by which contact be- 

 tween the two ends of the wires may be made 

 as in a (Fig. 8), or broken as in 6, and the 

 current be sent through the nerve, or prevented 

 from entering it. By the latter method the 

 battery is all the time connected with the 

 electrodes, and the key acts as a movable 

 bridge between the wires, and when closed gives a path of slight resistance by which 

 the current can return to the battery without passing through the nerve. The current 

 always takes the path of least resistance, and so, if the key be closed as in c, all the cur- 

 rent will pass through the key and none will go to the nerve, which has a high resistance, 

 whereas if the key be opened as in d, the bridge being removed, all the current will go 

 through the nerve. It is often better to let the cell or battery work a short time and to 

 get its full strength before letting the current enter the nerve, and the short-circuiting key 

 permits of this. Moreover, there are times when a nerve may be stimulated if connected 



FIG. 7. Electric key. 



FIG. 8. Electric circuiting. 



with the source of electricity by only one wire, the circuit being completed through the 

 earth ; when the nerve is so excited, it is called unipolar stimulation ; this may be pre- 

 vented by the short-circuiting key. 



As has been said, a nerve is irritated if it be connected with a battery and 

 an electric current suddenly passes through it. Unless the current be very 

 strong the irritation is transient, however; the muscle connected with the 



O ' ? 



