378 ELECTRIC CURRENTS OF MUSCLES. 



place (he two electrodes so that the plugs touch each other, 

 or place a morsel of thread or paper, moistened with normal 

 saline solution, over the two plugs, and open the key. The 

 needle should remain at zero. If any deviation OCCUfSfit is 

 an indication of a current in the electrodes themselves. If 

 the deviation is slight and constant, its direction and amount 

 in degrees must be noted, and all subsequent observations 

 corrected by it. This may be done by shifting the scale a 

 little, so as to bring the spot of light to zero, or by bringing 

 tin- spot of light to zero by means of the adjusting magnet. If 

 it lie large, a fresh pair of electrodes must be prepared, which 

 shall give no such deviation. 



nim. III. The muscle may now be prepared. Take one of 

 the largo muscles of the thigh, c. g., the triceps (fig. 267 a); 

 with a sharp clean knife or scissors cut the tendon of inser- 

 tion clear away with a transverse cut; similarly make a 

 transverse cut at the origin. Place the muscle thus prepared 

 on a glass plate with the electrodes under a moist chamber. 

 The muscle will have a natural longitudinal surface and two 

 artificial transverse surfaces. Place one electrode on the 

 longitudinal surface at a point as near as possible midway 

 between the two ends, and the other as near as possible in the 

 centre of one of the transverse sections. Connect the elec- 

 trodes with the binding screws of the shunt, the plug of the 

 shunt being in place between the screws. Remove the plug. 

 A deviation of the needle will take place. Most probably the 

 spot of light will swing right out of sight beyond the limit of 

 the scale. If this is so, replace the plug; when the needle has 

 returned to zero, shunt by means of the second plug; for 

 instance, put the second plug in the hole marked 1-90 and 

 thus allow only l-100th of the muscle current to pass through 

 the galvanometer, and then remove the first plug. The deflec- 

 tion will be far less. Note its direction and amount (number 

 of degrees of scale). 



A current will be found passing through the galvanometer 

 from t ho mid-point of the longitudinal surface to the central 

 point of* the transverse section. Replace the plug, so as to 

 shut oil' all the current from the galvanometer. 



()/>*. IV. Keeping the one electrode still on the transverse 

 section, shift the other electrode from the mid-longitudinal 

 point to some point nearer that transverse section ; remove 

 the plug. The deflection of the needle will indicate a current 

 in the same direction as before, but of less strength. Replace 

 tin- plug. 



oh*. V. Place the electrodes in the following positions, 

 always replacing the plug (serving as key) between the binding 

 screws of the shunt after each observation, and always being 



