OPENING AND CLOSING TETANUS. 511 



galvanic current, but only of the familiar alterations in excitability ; 

 and as regards the closing tetanus, it appears that in the medullated 

 nerve of the frog the phenomenon is generally of this nature. 



It is otherwise with the facts to be now detailed as to the opening 

 contraction, in which it will be noted that, although the alterations in 

 excitability are in all cases present, the effects cannot be accounted 

 for by " latent " excitation. 



It was shown by Mommsen l that weak alcoholic solutions of 

 NaCl (1 per cent.) slightly increase the excitability of motor nerves 

 which have been immersed in it. If such a nerve is subjected, on 

 removal from the solution, to stimulation by weak ascending galvanic 

 currents at periodic intervals, it is found that the closure evokes a large 

 single contraction, and that in a few minutes the opening of the current 

 evokes not merely the single twitch, but following this a prolonged 

 contraction. 



That the nerve itself is not in this instance continuously stimulated 

 by the solution, is shown by the fact that no spontaneous contraction is 

 evoked by its action alone ; the excitability increases to a certain point, 



FIG. 266. 



and then remains in this condition. There is little doubt that the part 

 played by the current in causing this continuous opening contraction is 

 not the mere production of a prolonged local rise of excitability, but 

 a true prolonged opening excitation, which is effectual because of the 

 greater general excitability of the nerve. 



The muscular response to the opening excitation shows two 

 peculiarities : it is delayed in its appearance so as to occur at a distinct 

 interval after the opening twitch, and it is dependent as to size and 

 form upon the duration of the current closure. 



In the last respect it differs from the opening tetanus, which 

 has been previously described as produced in a nerve which is con- 

 tinuously but inadequately excited by drying or by concentrated NaCl. 

 In these cases, when the opening of a weak current evokes the con- 

 tinuous effect, it is impossible to separate its start from that of the 

 opening twitch. In the nerve treated with the alcoholic solution, the pro- 

 longed opening effect is produced only on the cessation of moderately strong 

 currents, and varies directly with the length of the previous closure. 2 



1 Virchow's Archiv, Bd. Ixxxiii. S. 261. 



2 Biedermann, " Elektrophysiologie," 1895, S. 587. 



