380 ELECTRIC CURRENTS OF MUSCLES. 



that no changes have taken place in them during the observa- 

 tions, such as would give rise to a current. 



II. Negative Variation. Obs. XII. Get ready a nerve- 

 mufcle preparation, and make a transverse section through the 

 lower end of the muscle. Lay the muscle on a glass plate ; 

 connect the equator and transverse section of the muscle by 

 non-polarizable electrodes with the shunt and so with the gal- 

 vanometer ; lay the end of the nerve (as far away from the 

 muscle as possible) on another pair of electrodes. Connect 

 this second pair, or "exciting electrodes," as they may be 

 called, with an induction coil arranged for an interrupted cur- 

 rent. Let the induction coil be as far as possible away from 

 the galvanometer, and before commencing the observation 

 ascertain that the setting the induction machine in action does 

 not affect the needle. 



The spot of light being at zero, remove the plug of the shunt, 

 and when the spot has come to rest (using a shunt if the cur- 

 rent is too great for the scale), send a moderately strong inter- 

 mitted current through the exciting electrodes. The muscle 

 will become tetanized; at the same time the spot of light will 

 travel back a certain distance toward zero, i. e., the current 

 obtainable from the muscle at rest is diminished, or suffers a 

 negative variation during tetanus. Shut off the tetanizing cur- 

 rent ; the needle returns towards its former position. If the 

 muscle be laid flat on the glass plate, considerable tetanus may 

 be called forth without the electrodes at all shifting their posi- 

 tion in relation to the muscle, especialty if they be pressed 

 somewhat firmly on to the muscle to start with. 



06s. XIII. Having determined the negative variation as 

 above, tie a piece of wet silk or thread tightly round the nerve 

 between the muscle and the exciting electrodes, being very 

 careful to disturb nothing else. Now send the same inter- 

 rupted current as before through the exciting electrodes. 

 There will be no tetanus and no negative variation. The liga- 

 ture, having destroyed the vital continuity of the nerve, lias 

 prevented the passage of nervous impulses along the nerve to 

 the muscle. 



Should any influence on the galvanometer be observable, it 

 is an indication that an escape of current from the exciting 

 electrodes to the galvanometer electrodes has taken place. 

 The ligature of the nerve does not destroy the electrical con- 

 tinuity of the nerve, though it does its vital continuity. 



The exciting electrodes must be removed further from the 

 muscle, or a weaker current used, so as to prevent this escape 

 of current, and the observation then repeated. 



