CH. XV.] 



CHANGES IN EXCITABILITY 



173 



may be recorded on a stationary blackened cylinder. The cylinder is 

 moved on a short distance, and this is repeated. The height of the 

 lines drawn may be taken as a measure of the excitability of the nerve. 

 The polarising current is then thrown in, in a descending direction 

 (i.e. towards the muscle) ; the kathode is thus the non-polarisable 

 electrode near to the exciting electrodes. While the polarising current 

 is flowing, take some more tracings by breaking the exciting current. 

 The increase in the excitability of the nerve is shown by the much 

 larger contractions of the muscle; probably a contraction will be 

 obtained now at both make and break of the exciting current. After 

 removing the polarising current, the contractions obtained by excit- 

 ing the nerve will be for a short time smaller than the normal, but 

 soon return to their original size. 



Exactly the reverse occurs when the polarising current is ascend- 

 ing, i.e. from the muscle towards the spinal cord. The non-polarisable 

 electrode near the exciting electrodes is now the anode. While the 

 polarising current is passing, the excitability of the nerve is diminished 

 so that induction shocks which previously produced contractions of a 

 certain size, now produce smaller contractions, or none at all. On 

 removing the polarising current, the after-effect is increase of excit- 

 ability. 



The following figure is a reproduction of a tracing from an actual 

 experiment. The after-effects 

 are not shown. N represents 

 a series of contractions ob- 

 tained when the nerve is 

 normal, K when it is kat- 

 electrotonic, A when it is 

 anelectrotonic. 



Exactly similar results are 

 obtained if one uses mechani- 

 cal stimuli instead of in- 

 duction shocks. The best 

 mechanical form of stimulus 

 is to allow drops of mercury 

 to fall on the nerve. 



The same is true for 

 chemical stimuli. If the ex- 

 citing electrodes are removed, 

 and salt sprinkled on the 

 nerve near the muscle, the latter soon begins to quiver ; its con- 

 tractions are increased by throwing in a descending and diminished 

 by an ascending polarising current. 



The increase in irritability is called katelectrotonus, and the 

 decrease is called anelectrotonus. The accompanying diagram (fig. 



FIG. 170. Electrotonus. M, make. B, break. 



