532 NEEVOUS SYSTEM. 



During the passage of a feeble constant current through a nerve, what- 

 ever be its direction, there are 110 convulsive movements and no evidences of 

 pain. This fact has long been recognized by physiologists, who at first lim- 

 ited the effects of electricity upon the nerves to two periods, one at the clos- 

 ing of the circuit and the other at its opening. It will be seen, however, that 

 the passage of electricity through a portion of a nervous trunk produces a 

 peculiar condition in the nerve, which has been described under the name 

 of electrotonus ; but the fact remains that neither motion nor sensation is 

 excited in a mixed nerve during the actual passage of a feeble constant cur- 

 rent. 



Law of Contraction. All who have experimented upon the action of 

 galvanism upon the nerves have noted the fact alluded to above, that con- 

 traction occurs only on closing or on opening the circuit. Take, for exam- 

 ple, a frog's leg prepared with the nerve attached : Place one pole of a gal- 

 vanic apparatus on the nerve and then make the connection, including a 

 portion of the nerve in the circuit. With the feeblest current, contraction oc- 

 curs only on closing the circuit. With what is called the " weak " current 

 (Pfluger), contraction occurs only on closing the circuit, for currents in 

 either direction. With the " moderate " current, contraction occurs both on 

 closing and on opening the circuit, for currents in either direction. With 

 the " strong " current, contraction occurs only on closing the circuit, with 

 the descending current, and only on opening the circuit, with the ascending 

 current. The above phenomena constitute what is called Pfliiger's ie law of 

 contraction." The explanations of this law are the following : 



The stimulus which gives rise to the closing contraction occurs at the 

 cathode, when the electrotonus produced by the passage of the current be- 

 gins. The stimulus which produces the opening contraction occurs at the 

 anode, when the electrotonus disappears. The impulse is always stronger 

 when the electrotonus begins than when it disappears. Therefore, when the 

 current is so feeble that but one contraction is produced, this contraction 

 occurs only on closing the circuit, for both ascending and descending currents. 



With the " moderate " current, the strength of the opening impulse is 

 sufficient to produce a contraction ; and contractions therefore occur both on 

 opening and closing the circuit, for both ascending and descending currents. 



Strong currents produce closing contraction with the descending current, 

 for the reason that the current destroys the conducting power of that portion 

 of the nerve included between the poles of the battery, and, the stimulus 

 occurring only at the cathode (see above), and the cathode being applied to 

 that portion of the nerve nearest the muscle, the closing impulse only is 

 conveyed to the muscle. The opening impulse (at the anode) is cut off from 

 the muscle by the loss of conducting power in the intrapolar portion of the 

 nerve. With the ascending current, the opening impulse, occurring at the 

 anode, which is nearest the muscle, produces an opening contraction, and 

 the closing impulse, which is at the cathode, is not conducted to the muscle. 



While the constant current does not usually excite contractions during 

 the time of its passage through a nerve, with a certain strength of current, 



