17-2 



ELECTRO-PHYSIOLOGY 



CHAP. 



water, or treatment with NaCl, correspond almost exactly. Yet 

 the main features of the curve of contraction (Fig. 192, II) differ 

 albeit inessentially. In the drying nerve the discharge of 

 simple opening twitches (at a certain stage which immediately 

 precedes the appearance of spontaneous excitatory effects) fails 

 even with very weak currents, so that there is only tetanic 

 contraction of the muscle (Hitter's opening tetanus') as appears in 

 an even higher degree in nerves treated with NaCI ; the alcohol- 

 ised nerve, on the other hand, requires a tolerably protracted 

 passage of current, along with moderate intensity of stimulation, 

 to produce a definite opening tetanus. At most there will only 

 be extended twitches even with prolonged closure, which 



FIG. 192. Frog's nerve-muscle preparation. Ascending direction of current, otherwise the same 

 experimental conditions as in the previous Figs. Effect of commencing desiccation on the 

 result of breaking a battery current of medium strength. The opening twitch (I) appears 

 as the initial phase of the delayed opening tetanus (II). 



must be regarded as transitional forms between the simple opening- 

 twitch and persistent tetanic shortening of the muscle. 



This agrees with the fact that the appearance of closure 

 tetanus in weak electrical excitation of alcoholised nerve must 

 be regarded as exceptional, although the curves of both closing 

 and opening twitches are distinguished by their rounded tops 

 from such as are obtained on exciting normal nerve by 

 instantaneous stimuli (single induction shocks), or by the closure 

 of a battery current. 



These experiments show conclusively that while in normal, 

 uninjured nerve it is never possible to obtain an opening 

 excitation from weak currents, this may result when excitability 

 is artificially raised : thus seeming to justify the view that the 

 appearance of the break twitch on applying a cross-section to the 

 nerve is due to the consequent rise of excitability. 



Yet (in addition to the objections already cited) the bare 

 comparison of the opening effects of excitation in the two cases 



