406 



GENERAL PHYSIOLOGY 



flows through the preparation (Fig. 190, /). But, in order to be 

 able at any moment conveniently to interrupt the circuit and 

 again to close it and thus to control arbitrarily the influence of 

 the stimulus upon the preparation, there is inserted into one wire 

 a so-called galvanic key, which consists of a cup set into a plate of 

 insulating hard rubber and containing mercury into which one 

 end of the wire dips, while the other is in metallic connection 

 with a small lever ; at any moment the lever can be dipped into 

 the mercury or withdrawn, so that the metallic conduction can be 

 established and again interrupted, or, in other words, the current 

 can be made and broken (Fig. 190, //). 



When currents are allowed to act on the preparation for a 

 considerable time the metallic wires themselves should not be 



\j 

 S 



FIG. 190. 7, Circuit between the element E and the nerve N of a nerve-muscle preparation ; in 

 the circuit is the key S. //, Mercury key. 



laid as electrodes on the nerve, the muscle or other tissue, since 

 at the place of contact of the metal with the preparation, which 

 latter is a moist conductor, opportunity would be given for the 

 development of polarisation-currents, which would themselves 

 stimulate the preparation and thus disturb the experiment. In 

 order to avoid this, so-called non-polarisable 'electrodes have been 

 constructed, which allow no polarisation-current to develop at the 

 place of contact with the preparation. 1 These non-polarisable 

 electrodes consist in their most convenient form of a short glass 

 tube, closed below by a stopper of plastic clay, into which a short 

 soft camel's-hair brush projects ; the lumen of the tube is filled 

 with a concentrated solution of zinc sulphate, into which dips a 

 zinc rod connected with the conducting wire (Fig. 191). The 

 electrodes are held in adjustable stands and can be handled with 

 extreme ease, the pointed brushes being laid upon the preparation. 

 After having become acquainted with a reliable source of 



1 Cf. p. 268. 



