ELECTROTONUS 89 



Experiment 8. — Repeat the preceding experiment, but tetanise the 

 nerve at the spot to be tested instead of giving it a single stimulus. The 

 shocks used should be just above minimal stimuli. Record the results obtained 

 on a slowly moving drum. It will be found that if the spot stimulated be in 

 anelectrotonus that the contraction on tetanisation is either abolished or 

 diminished ; if, on the other hand, it be in katelectrotonus, the contraction is 

 increased. 



These experiments show that a piece of nerve in anelectrotonus is 

 less excitable than normally, for it does not give so large a con- 

 traction, and, on the other hand, the state of katelectrotonus is charac- 

 terised by an increase of excitability. These same results have also 

 been confirmed by other modes of stimulation, both mechanical and 

 chemical. The latter can very easily be shown by the following 

 experiment : — 



Experiment 9. — Instead of using the secondary coil for stimulating, 

 paint the nerve with a 10 per cent, solution of NaCl at a spot between the 

 lower electrode and the muscle. In a short time the muscle is set twitching 

 by the stimulus. Arrange the polarising circuit to give an ascending current 

 and close the key K r The twitching ceases. Reverse the key, so that the 

 kathode is now nearest the muscle. The twitching becomes more marked. 

 Record these results on a slowly moving drum. 



By varying the course of experiments of this kind differences have 

 been made out in regard to the degree of change of excitability 

 and the time it lasts. The katelectrotonic increase of excitability 

 reaches its maximum height directly after the closure of the current, 

 and then gradually decreases whilst the anelectrotonic decrease 

 develops and extends much more slowly, its maximum being reached 

 some time after closure. The amount of change of excitability, as 

 measured by the amount of contraction, produced by a stimulus of 

 fixed strength is found to vary considerably with the strength of the 

 polarising current. As the excitability is increased around the kathode 

 and decreased around the anode, it follows that there must be one 

 spot between anode and kathode at which excitability remains 

 unchanged. This is called the indifferent point. The changes of 

 excitability are conveniently represented diagrammatically, as in fig. 76. 

 In this figure the abscissa line g i is taken to represent the nerve, and 

 the polarising current is supposed to enter at A and leave at K. An 

 increase of excitability is represented by an ordinate above the 

 abscissa, a decrease by one below. With a weak current the changes 

 in excitability are represented by the curve anbrc, which means 

 that at the point m of the nerve the excitability is decreased to an 

 amount represented by the vertical m n. At the point p it is increased 

 by an amount represented by j) r. Similarly for all other points 

 between a and c. The indifferent point is at b, which is seen to be 

 nearer the anode A than the kathode k. The changes in excitability 



