ix 



ELECTRICAL EXCITATION OF NERVE 



reversing the current the closure produces a vigorous tetanus. 

 It is necessary either to strengthen the ascending current, or to 

 prolong the closure in the same proportion, in order to obtain 

 effective (usually tetanic) opening excitation. Neither of these 

 is required at a later stage of local salt treatment. The muscle 

 then becomes disturbed very quickly, and goes into the familiar 

 salt tetanus, which makes further experiment impossible, unless 

 the affected part of the nerve is washed with 0'5 / Q NaCl (after 

 taking away the pad), when it returns to the state in which it 

 shows increased excitability without discharging any spontaneous 

 excitatory phenomena. 



In other respects the character of the opening effects after 



n 



J\ 



FIG. 189. Frog's nerve-muscle preparation. Stimulation in the middle of the .-strip of nerve. 

 Ascending current. After applying a pad of cotton wool soaked in concentrated Nad to the 

 anode for three minutes, opening of a weak battery current produces tetanus after a brief 

 closure, which makes a delayed entrance (II). After single closure of a stronger current, the 

 opening twitch (I) appears between the. moment of opening a current of equal intensity with 

 the former, and the commencement of tetanus. 



treatment with NaCl corresponds almost completely with the 

 analogous phenomena described above for drying nerve ; the de- 

 layed entrance of the break contraction, and its dependence upon 

 the duration of closure, being in most cases very obvious (Fig. 

 189). It is therefore unnecessary to go into further details in 

 respect of these curves, and we may pass on to the interesting 

 effects of treatment with very dilute alcohol. 



Mommsen (supra) showed that the excitability of motor 

 nerves was considerably increased by the application of a weakly 

 alcoholised (12 vols. '/) NaCl solution, the augmentation only 

 giving way after prolonged treatment to diminution, and subse- 

 quent inexcitability. Even then it is possible to restore excita- 

 bility by washing with 0'6 ' NaOl. 



When the sciatic of a frog's nerve -muscle preparation is 

 treated in its entire length with alcoholic saline, and excited every 

 minute with a weak ascending or descending battery current, the 



