ADVANCED EXPERIMENTAL PHYSIOLOGY 333: 



The Galvanometer employed in these experiments is Kelvin's 

 reflecting galvanometer. It consists of a suspended system of magnets 

 so arranged as to make the system nearly " astatic " ; the magnets are 

 surrounded by coils of many turns of fine insulated wire. The resist- 

 ance is high, from 5000 to 20,000 ohms. The movements of the- 

 mirror attached to the magnets are indicated by a spot of light upon 

 the scale. 



The amount of current sent through the galvanometer is regulated 

 by means of a shunt, which is a resistance box whereby T Vth, yj^th, or 

 of the total current can be sent through the galvanometer 



FIG. 211. Scale and lamp for the reflecting galvanometer. 



The electric current from the muscle or nerve is led off by means of 

 unpolarisable electrodes, but before an experiment is performed the 

 electrodes are tested, for in most cases they are not perfectly iso- 

 electric. Any small deflection of the galvanometer due to this cause is 

 compensated by a graduated current from a standard battery sent 

 through the galvanometer in the opposite direction. 



Perfectly uninjured muscle and nerve are iso-electric, but they are 

 generally slightly damaged during the process of dissection and 

 preparation. The deflection due to this current of injury or demarca- 

 tion current (wrongly called the current of rest) is measured and is^ 

 then increased by a more pronounced injury caused by touching one 

 end of the muscle with a hot wire. The muscle is now stimulated by 

 a tetanising current applied to its uninjured end ; the deflection of the 

 galvanometer is in the reverse direction, due to the current of action 

 (formerly called the negative variation) which is produced when the 

 muscle contracts; 



The current of injury is, as Gotch pointed out, to^be considered as a 

 local current of action; around the injured portion the tissue is in a 

 condition of excitation. 



