190 



EXPERIMENTAL PHYSIOLOGY. 



current is short-circuited. When working with muscle, 

 keep a plug in the hole opposite - on the shunt. Arrange 

 the lamp and scale so as to have a good image of the mirror 

 on the zero of the scale ; adjusting, if necessary, by means of 

 the magnet moved by the milled head on the top of the 

 glass shade (Fig. 82, m). 



Fig. 82. Arrangement of Apparatus for the Demarcation Current of 

 Muscle. M, Muscle on a glass plate, P ; S, shunt ; G, galvanometer ; 

 Mg, its magnet moved by the milled head, m ; L and Sc, lamp and 

 scale. 



(c.) Test the electrodes, either by bringing them together, 

 or by joining them with a piece of silk thread covered with 

 china-clay paste. After removing all the plugs from the 

 shunt, there ought to be no deflection of the spot of light. 

 If there is none, there is no polarity, and the electrodes are 

 perfect 



(d.) Ascertain the Direction of Current in Galvanometer. 

 Make a small Smee's battery with a two-ounce bottle. Place 

 in the bottle dilute sulphuric acid (1 : 20) and two wires of 

 zinc ( - ) and copper ( + ), with wires soldered to them. Con- 

 nect them with the galvanometer. Arrange the shunt so 

 that j^ or y^-jj- part of the current thus generated goes 

 through the galvanometer. Note the deflection and its 

 direction. Arrange the N.P. electrodes in the same 



