LABORATORY MANUAL OF PHYSIOLOGY. 



tact with the mercury. If a current be passed through the mercury 

 and sulphuric acid, the surface tensions of the fluids are so changed 

 that the mercury meniscus will move in the direction of the current. 

 The extent of the excursion of the mercury is in direct proportion 

 to the strength of the current. This may be measured by mercury 

 pressure as determined by the manometer. A fairly exact gradu- 

 ation of the instrument may be made by placing it in circuit with 

 currents of known strengths, and recording in terms of mercury 

 the amount of pressure needed to bring the meniscus back to its 

 original position. This instrument is exceedingly sensitive to very 

 small currents. 



5. Current of Injury of Muscle. Very carefully dissect out 

 the gracilis and semimembranosus muscles, avoiding crushing or 

 tearing as much as possible. Place the muscle on two non- 

 polarizable electrodes, connected by fine wire with the galva- 

 nometer or the capillary electrometer. Test the non-polarizable 

 electrodes first, by bringing them in contact with each other. 

 There should be no deflection of the galvanometer needle or of 

 the mercury meniscus of the capillary electrometer. 



(a) Interpose a key between the muscle and the current de- 

 tector. Close the key, so that the galvanometer is brought in cir- 

 cuit with the electrodes on the muscle. There should be but little 

 if any deflection of the needle. If the muscle were absolutely free 

 from injury and at rest, there should be no difference of potential 

 between any of its parts. 



(b) Cut across one end of the muscle with sharp scissors or scal- 

 pel. Place one electrode on the cut surface, the other on the smooth 

 surface. Bring the galvanometer in circuit again. There is now 

 a deflection of the needle. This is an indication of the current of 

 injury or the demarcation current of the muscle. 



(c) Cut off the nerve near the muscle and repeat (&), using the 

 nerve instead of the muscle. 



6. Action Current of Muscle. Make a careful sciatic-gastroc- 

 nemius preparation. Place the muscle on the non-polarizable elec- 

 trodes, connected with the galvanometer. Place the nerve on elec- 



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