510 NERVE. 



thus produced may possess special characters of which the most prominent 

 is that of its duration. The evidence is founded upon the phenomena 

 known as Eitter's tetanus, in which excitation is prolonged both during 

 closure and after opening, but the conditions which favour the closing 

 do not necessarily favour the opening effects. 



Opening and closing tetanus. The passage of strong currents 

 through a motor nerve may cause not merely the third stage of 

 Pfliiger's law, but a prolonged excitation indicated by a continuous con- 

 traction of the muscle. This starts in the case of the ascending current 

 at the moment of cessation, the phenomenon being known as Eitter's 

 opening tetanus. In the case of the descending current it may occur 

 during closure as a closing tetanus. In both cases the term " continuous 

 contraction " expresses the condition far more precisely than the term 

 " tetanus," since there is no proof of repeated excitation. 



The opening tetanus with ascending currents is due to a state of 

 prolonged excitation at the anode. This is shown by the following 

 facts. If the effect is present, the severance of the nerve in the intra- 

 polar region above the anodic pole does not stop the continuous con- 

 traction. It ceases if the current is again closed through the intact 

 nerve, so as to make the anode the seat not of opening but of closing 

 changes. The effect, if present, is increased if a current is closed in 

 the reverse direction, thus making what was formerly the anode into 

 a cathode. A continuous opening contraction can be obtained after 

 the prolonged action of weaker currents, especially if the direction has 

 been reversed several times, and the circuit opened when the current 

 is ascending. This opening contraction is undoubtedly associated with 

 the production of extensive anelectrotonus, which, owing to its spread, 

 takes some time to disappear, and prolonged excitatory electromotive 

 changes are now found in the nerve in the region of the anode. 



Both the closing and the opening prolonged effects can be obtained 

 with comparatively weak currents, if the region of the exciting pole is 

 the seat of a continuous excitation too weak to evoke a muscular 

 response ; such a state is produced either by local drying, or by the 

 local application of concentrated NaCl solution. If the whole nerve 

 is treated with NaCl, then, since it is throughout in this condition of 

 slight inadequate excitation, the closure of a comparatively weak de- 

 scending current, or the opening of such an ascending one, now evokes 

 a prolonged contraction in the muscle. 



In these instances, of which Fig. 266 is an example, the current 

 effect acts by producing changes in excitability rather than true ex- 

 citation phenomena ; continuous excitation is produced by the chemical 

 or mechanical stimuli ; it is made effectual on closure at the cathode, 

 and on opening at the anode. 1 In other words, a " latent " (i.e. inade- 

 quate) nerve excitation is rendered adequate by the increased excit- 

 ability which accompanies the closure at the cathode and follows the 

 opening at the anode. That this condition may produce the results 

 was shown by Griinhagen. 2 A nerve is traversed near its central end 

 by an inadequate series of rapidly recurring induced currents; the 

 passage of a current may now cause either a closing or an opening 

 tetanus resembling in its time relations and form that just described. 

 There is thus no evidence, so far, of any prolonged excitation by the 



1 Engelmann, Arch. f. d. ges. PhysioL, Bonn, 1870, Bd. iii. S. 403. 



2 Ibid., 1871, Bd. iv. S. 548. 



