CHAP, ii.] 



THE CONTRACTILE TISSUES. 



61 



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 

 clu 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. 2, the brass bridge 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 

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

 electrode or kathode Kat. and thence back to the negative pole 

 (end of the positive (zinc) element) in the battery; on raising the 



FIG. 2. DIAGRAM OF Du BOIS-BEYHOND KEY USED, A, FOB MAKING AND BREAKING, 



B, FOR SHORT CIRCUITING. 



brass bridge the circuit is opened, the current 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 arrangement 



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 key ; hence this is called ' short 

 circuiting.' When the bridge is raised the current passes through the 

 nerve on the electrodes. Thus 'putting down' and 'raising' or 'opening' 



