226 PHYSIOLOGY CHAP. 



excitability and conductivity of the nerve, so that the transmission 

 of excitation to the muscle is prevented. The muscles of the first 

 sciatic will then be thrown into tetanus which lasts for some 

 minutes and gradually dies away, while the muscles of the second 

 (polarised) sciatic remain absolutely quiet. In order to show that 

 the absence of tetanus in the first case is not due to fatigue or 

 exhaustion of the nerve, it is only necessary to break the polaris- 

 ing current which blocks the second nerve. The corresponding 

 muscles are at once thrown into tetanus of the same vigour and 

 duration as that of the other side, showing that the nerve had 

 preserved its excitability intact during the protracted stimulation. 



Schiff in 1858, by a method similar to that of Bernstein, arrived 

 at the same conclusion as to the great resistance of nerve to fatigue. 

 He applied the electrodes of a very weak battery, the circuit of 

 which was closed instantaneously every two seconds by the 

 pendulum of a clock, to the distal stump of the frog's sciatic, and 

 obtained a muscular twitch at each closure. If the electrodes of 

 a strong tetanising induction current were then applied to the 

 central end of the nerve the rhythmical contractions were replaced 

 by a tetanus that died out gradually, till finally it ceased altogether, 

 on which the muscle no longer reacted either to the intermittent 

 shocks of the battery or to the induced tetanising current. 

 Under these conditions it would seem as though the nerve were 

 exhausted, but proof to the contrary was shown in the fact that 

 directly the tetanising current was interrupted the rhythmical con- 

 tractions reappeared. To explain this fact Schiff' assumed that the 

 induced current produces a negative excitation, which was able to 

 neutralise the effect of the intermittent shocks. 



Wedensky (1884) improved on the methods of Bernstein and 

 Schiff, and confirmed and extended their researches. He tetanised 

 the sciatic with an induced current of given strength and frequency 

 till the phase of apparent exhaustion was reached. On then re- 

 ducing the intensity and frequency of the current the tetanus 

 reappeared, showing, according to Wedensky, that the nerve was 

 not exhausted, but acted as an inhibitory nerve. The experiment 

 can be repeated many times upon the same nerve, always with 

 the same result. 



This " paradoxical " phenomenon, viz. that a stronger or more 

 frequent stimulus produces less effect than a weaker or less frequent 

 stimulus, was satisfactorily interpreted by F. B. Hofmann, who in 

 1902-4 undertook a series of accurate investigations into muscular 

 tetanus from indirect stimulation. He refers it to fatigue of 

 the end-organs. The excitability of these is depressed after each 

 stimulation : recovery takes place after an interval which is longer 

 in proportion with the strength of the preceding excitation and 

 the degree of fatigue. If the stimuli are too strong, and follow 



O O' 



too rapidly, there is no recovery, and in excitability ensues ; if the 



