INFLUENCE OF ELECTRICAL CURRENTS ON MOTOR NERVES. 591 



From this table it is apparent that in the highest grade of excitability of 

 the nerve, the centripetal current induces contraction only on closure of the 

 current, none occurring on opening it. Contraction on opening the current 

 first happens in the second stage i.e., when the excitability of the nerve is 

 somewhat diminished, and then only weakly. In the third stage of excita- 

 bility of Hitter, the first of Nobili, the vigor of the opening and closing 

 contraction is about equal. In the fourth grade, the closing contraction is 

 weak, the opening strong. In the fifth stage, there is no contraction on 

 closure, but it occurs on opening the current; whilst in the sixth and last 

 stage no contraction follows either the closure or the opening of the current. 

 For th? descending or centrifugal current, the phenomena present themselves 

 in an inverse order, in the first or highest stage of excitability, there is 

 contraction only on opening the current then weak contraction on closure, 

 as well as contraction on opening then contraction of equal strength at 

 both periods then contraction at both periods, though weaker on opening 

 the current then contraction only on closure; and finally, in which respect 

 it differs slightly from the effects of the centripetal current, weak contrac- 

 tion on closure of the current. From the preceding table, the statement 

 of Nobili, who only experimented during the last four stages of irritability, 

 becomes intelligible, that there is essentially only one strong contraction for 

 each direction of the current; an opening contraction being the most marked 

 with the centripetal current, and a closing contraction with the centrifugal 

 current. 



477. Heidenhain 1 and Pflugeiy in following up this train of research, 

 now showed that in freshly-prepared nerves, whose excitability ivas there- 

 fore of the highest grade, the law of contraction was dependent upon the 

 strength of the current. ; and the phenomena have been thus formularized 

 by Pfliiger : 



Thus beginning with currents so feeble that no contraction was induced either 

 on making or on breaking contact in very excitable nerves, Pfliiger found 

 that on passing a slightly stronger, but still weak current, in a centripetal 

 direction, contraction first occurred on closure, but none on opening. With 

 currents of moderate strength, contraction occurs both on closing and open- 

 ing the current ; and lastly, if the current exceeds a certain strength, no 

 contraction occurs on closing, though it is well marked on opening the cur- 

 rent. If the same experiments are repeated with the centrifugal current, 

 we obtain with the weakest current contraction on closure alone (exception- 

 ally also on opening); with moderately strong currents, contraction both on 



1 Heiclenhain, in Archiv f. Phys. Heilk., 1857, p. 442. 



2 Pfluger, Archiv f. Path. Anat., Bd. xii ; and Untersuchungen iiber d. Physiol- 

 ogic des Electrotonus, Berlin, 1859. 



