POLAR EFFECTS OF GALVANIC CURRENT 59 



key in connection with the primary coil ; the secondary coil is finished 

 as usual with a short-circuit key, with which a pair of ordinary 

 metallic electrodes are connected ; these electrodes are brought in 

 contact with the nerve of a muscle-nerve preparation near the muscle. 

 Be careful to keep the nerve moist. The non-polarisable electrodes, 

 which may be of the boot pattern, are slightly raised above the myo- 

 graph cork : they must not be allowed to be short-circuited by the 

 Ringer solution used to keep the preparation moist ; the upper part of 

 the nerve is laid upon them (Fig. 50). The record of the muscular 

 contractions obtained is made on a stationary drum. 



Place the secondary coil at such a distance from the primary coil 

 that faradisation (Helmholtz modification) just produces a small con- 

 traction. Now put in the polarising current (1) in an ascending and 

 (2) in a descending direction, and determine the effect of its poles in 

 diminishing or increasing the excitability of the nerve as tested by the 

 height of the ordinates described by the myograph lever. 



This experiment can be performed without taking a graphic record 

 by noting at what distance the secondary coil must be placed in order 

 just to produce a contraction. In this way the varying conditions of 

 excitability produced by the polarising current may be recorded 

 numerically. 



