170 



ELECTRO-PHYSIOLOGY 



CHAP. 



first effect is invariably a considerable increase in height of the 

 closure twitches, without actual tetanus. Almost at the same 

 time (usually after 24 minutes) a twitch, which is usually 

 delayed, appears with the opening of the current also, provided 

 the duration of closure is not too brief (Fig. 190). 



The duration of closure required, under the given conditions, 

 to produce an excitation on opening the circuit, depends of course, 

 apart from its intensity, upon the degree of alcohol effect, i.e. 

 strength of solution and length of application. The rise of excita- 

 bility in the nerve usually occurs fairly soon, when it is treated 

 with not excessively weak alcoholic saline, and the break twitch 



-7" 



JL 



JL 



JL 



JL 



JL 



FIG. 190. Frog's nerve-muscle preparation. Excitation in the middle of the strip of nerve. 

 Ascending direction of current. After the nerve has been bathed for thirty seconds in alcohol- 

 ised saline (10 vols. %) the current discharges delayed opening twitches (II) along with the 

 closure twitch, preceded 'after long treatment with alcohol by the opening twitch (I), which 

 appears (according to duration of closure) alone, or partially or wholly fused with the opening 

 twitch (II). 



also appears quickly, even with weak currents and brief closure. 

 Here again it may be remarked that the opening of ascending- 

 currents excites, as a rule, a little sooner than descending currents 

 which is no doubt related to the appearance of the so-called 

 " negative modification " of katelectrotonus in the last case. 



The alcoholised nerve never sets up spontaneous tetanus, and 

 isolated twitches of the muscle only appear occasionally with 

 strongly alcoholised saline up to 20 vols. c / o (Mommsen) so that 

 the dependence of the opening excitation upon the state of excita- 

 bility of the nerve, as well as upon its special characteristics, 

 may be studied here as in no other case the raised excitability 

 remaining for a long time constant. 



Two characteristics of the break excitation as it appears 

 with artificially raised excitability of a nerve in consequence of 

 weak battery currents are, as we have noted (), the more or 



