II 



THROUGH the indefatigable labours of du Bois-Reymond, of 

 Pfliiger, and of several other observers, abundant material has been 

 accumulated, consisting of carefully-ascertained facts relative to 

 the exciting action of the electric current on nerves. These facts 

 may be summed up very briefly in the following propositions, 

 (i) It is not to the absolute value of the current density at any 

 moment that the motor nerve responds by contraction of its muscle, 

 but to the change of this value from one moment to the next, in 

 such sense that the excitation to movement following these changes 

 is the more considerable, the more rapidly they have proceeded 

 when of equal amount, or the greater their amount in a unit of 

 time (Law of du Bois-Reymond 1 ). (2) When the current is closed, 

 it excites at its negative pole ; when it is opened, the nerve is 

 stimulated at the positive pole 2 . (3) While the current is closed, 

 the excitability of the nerve is weakened in the neighbourhood of 

 the positive pole, strengthened in the neighbourhood of the nega- 

 tive pole 2 . (4) In the first moments after opening of the current, 

 the excitability of the nerve is strengthened at the positive pole, 

 and weakened at the negative pole 2 . The greater part of these 

 facts Pfliiger summed up in his well-known principle, 1 A given tract 

 of nerve is stimulated by the appearance of katelectrotonus and the 

 disappearance of anelectrotonus, not however by the disappearance of 

 katelectrotonus nor by the appearance ofanelectr otonus V He further 



1 Du Bois-Eeymond, Untersuchungen fiber thierische Elektricitat, Berlin, 1848, 

 pp. 258-259. (The original paper appeared in 1845.) 



2 Pfliiger, Untersuchungen fiber die Physiologic des Electrotonus, Berlin, 1859; 

 v. Bezold, Untersuchungen iiber die electrische Erregung der Nerven und Muskeln, 

 Leipzig, 1861. 



3 Pfliiger, loc. cit. p. 456. 



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