6 



PEACTICAL PHYSIOLOGY 



The Pohl's Reverser consists of six mercury cups hollowed out in 

 a block of vulcanite, each cup being connected to a binding-screw 

 (Fig. SA). The four corner cups are connected diagonally by stout 

 copper wires which do not touch each other. The two side cups are 



joined by stout 

 copper wires to 

 a non-conducting 

 cross-piece, which 

 acts as a handle. 

 Each end of the 

 handle also carries 

 a semicircle of 

 copper wire which 

 is connected to the 

 wire going into 

 the side cup, and 



is of such a length 

 that it will dip 



into the cup at either end by turning the handle over towards that end. 

 If the handle is in such a position that a current, entering the reverser 

 by one of the side cups, emerges by an end cup of the same side, then, 

 by turning the handle over, the cross-wires come into use, and the 

 current will now emerge by the end cup of the opposite side. 



The instrument may also be used to send a current into either of 

 two circuits. The cross-wires are removed, the wires from the battery 

 are connected to the two side binding-screws, and to each pair of end 

 cups the wires of the two alternative circuits (Fig. 9). Then by turning 

 the handle over the current may be sent into either of these two circuits. 



A much more efficient instrument is the universal key (Fig. SB), 

 which has recently been introduced by Gotch. It can be used as 

 a double break-key, a reverser and a shunt. 



The term Electrodes is applied to the free ends of the two wires 

 which conduct the current to the tissue to be stimulated. They consist 



FIG. 9. Plan of the arrangement of the two alternative circuits. 



FIGS. 10 AND 11. Two forms of electrodes. 



of two insulated wires, the ends of which are clean and free from 

 insulating material, carried in some form of holder ; this is generally 

 made by running the wires through a piece of vulcanite, cork, or model- 



