486 NERVOUS SYSTEM 



who at first limited the effects of electricity on the nerves to two periods, 

 one at the closing 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, described 

 under the name of electrotonus ; but the fact remains that neither mo- 

 tion nor sensation is excited in a mixed nerve during the actual passage 

 of a feeble constant current. 



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

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

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

 example, a frog's leg prepared with the nerve attached : Place one pole 

 of a galvanic apparatus on the nerve and then make the connection, in- 

 cluding a portion of the nerve in the circuit. With the feeblest current, 

 contraction occurs only on closing the circuit. With what is called the 

 " weak " current (Pfliiger), contraction occurs only on closing the circuit, 

 for currents in either direction. With the " moderate " current, contrac- 

 tion 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 " law of contraction." The explanations of this 

 law are the following : 



The stimulus that gives rise to a closing contraction occurs at the 

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

 begins. The stimulus which produces an opening contraction occurs at 

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

 when the electrotonus begins than when it ceases. 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 as well 

 as of the closing impulse is sufficient to produce a contraction ; and con- 

 tractions 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 conductivity 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 nearer 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 conductivity in the in- 

 trapolar portion of the nerve. With the ascending current, the opening 



