THE PHENOMENA OF MUSCLE AND NERVE. 



59 



trode, and the negative pole the negative electrode. The positive electrode is often 

 spoken of as the anode (ana, up), and the negative electrode as the kathode (kata, 

 down). 



A piece of nerve of ordinary length, though not a good conductor, is still a 

 conductor, and when placed on the electrodes completes the circuit, permitting 

 the current to pass through it ; in order to remove the nerve from the influence 

 of the current it must be lifted off from the electrodes. This is obviously incon- 

 venient ; and hence it is usual to arrange a means of opening or closing the cir- 

 cuit at some point along one of the two wires. This may be done in various ways 

 by fastening one part of the wire into a cup of mercury, and so by dipping the 

 other part of the wire into the cup to close the circuit and make the current, and 

 by lifting it out of the mercury to open the circuit and break the current ; or by 

 arranging between the two parts of the wires a movable bridge of good con- 

 ducting material, such as brass, which can be put down to close the circuit or 

 raised up to open the circuit ; or in other ways. Such a means of closing and 

 opening a circuit, and so of making or breaking a current, is called a key. 



A key which is frequently used by physiologists goes by the name of Du Bois- 

 Reymond's key. Though undesirable in many respects, it has the advantage that it 

 can be used in two different ways ; when arranged as in A, Fig. 12, the brass bridge 



FIG. 12. 



Kat 



Diagram of Du Bois-Reymond Key used, A, for making and breaking ; B, for short-circuiting. 



of K, the key, being down, and forming a means of good conduction between the 

 brass plates to which the wires are screwed, the circuit is closed and the current 

 passes from the positive pole (end of the negative (copper) element) to the posi- 

 tive electrode, or anode, An., through the nerve, to the negative electrode, or 

 kathode, Kat., and thenc^ back to the negative pole (end of the positive (zinc) 

 element) in the battery ; on raising the brass bridge, the circuit is opened, the 

 current is broken, and no current passes through the electrodes. When arranged as 

 in B, if the brass bridge be " down," the resistance offered by it is so small, compared 

 with the resistance offered by the nerve between the electrodes, that the whole 

 current from the battery passes through the bridge back to the battery, and none, 

 or only an infinitesimal portion, passes into the nerve. When, on the other hand, 

 the bridge is raised, and so the conduction between the two sides suspended, the 

 current is not able to pass directly from one side to the other, but can and does 

 pass along the wire through the nerve back to the battery. Hence, in arrange- 

 ment A, "putting down the key," as it is called, makes a current in the nerve, 

 and "raising" or "opening the key" breaks the current. In arrangement B. 

 however, putting down the key diverts the current from the nerve by sending it 

 through the bridge, and so back to the battery ; the current, instead of making 

 the longer circuit through the electrodes, makes the shorter circuit through the 



