662 



IRRITABILITY OF NERVE AND MUSCLE IN ELECTROTONUS. 



closure, (d) Ascending extrapolar katelectrotonus (B). The salt is placed at r. 

 In this case a distinction must be made in accordance with the strength of the 

 polarizing current: (i) If the current is extremely weak, and it can be appro- 

 priately regulated with the aid of the rheocord (Fig. 224), increase of the 

 contractions is observed after closure of the polarizing circuit. (2) If, however, 

 the current is stronger, the contractions become smaller, or they are even 

 wholly abolished. The reason for this latter apparently 

 abnormal relation lies in the fact that under the in- 

 fluence of stronger currents the conducting power at 

 the anode is diminished or even abolished. Although 

 in this case the salt acts upon an irritable segment of 

 nerve the effect does not appear in the muscle, as the 

 conduction of the stimulus to the latter is prevented. 



The laws of electrotonus can be demonstrated also 

 in an entirely isolated nerve. One extremity of the 

 nerve is applied to the electrodes of a galvanometer 

 for the production of a strong current. The polarizing 

 circuit is applied to the nerve at some distance. If, 

 now, the nerve with the circuit closed is irritated in 

 the anelectrotonic segment, as, for example, by induc- 

 tion-shocks, the negative current-variation is feebler 

 than if the polarizing circuit were open. Conversely, 

 the variation is stronger if the irritation be applied to 

 the katelectrotonic segment. Also the extrapolar cur- 

 rents appearing in electrotonus exhibit the negative va- 

 riation when the nerve is irritated. Katelectrotonus is 

 intensified by the action locally of elevation of temper- 

 ature, of acids and by tetanization, while anelectro- 

 tonus is diminished by the same influences. 

 The law of electrotonus has been demonstrated also in living human beings. If 

 it be desired to test electrotonus in living human beings the conditions of the 

 distribution of the current in the part of the body are especially to be considered. 

 If, for example, the two electrodes are placed in the course of the ulnar nerve 

 (Fig. 234), it will be seen that the currents appearing in the nerve at the anode 

 (+ ad) must diminish the irritability, although above and below the anode (at 

 c c) the positive current emerges in part from the nerve and naturally causes 

 katelectrotonus in these situations. In an analogous manner increased irritability 



FIG. 233. Testing the Irritability 

 in Electrotonus. 



Ul 



FIG. 234. Diagrammatic Representation of the Distribution of the Electric Current in the Arm on Galvanization 



of the Ulnar Nerve. 



prevails immediately at the point of application of the kathode (at c c} , but in 

 the portions of the nerve above and below, where (at a a) the positive current 

 (from + ) enters the nerve-path, the irritability is diminished anelectrotonus. If, 

 thus it be desired to apply irritation in the vicinity of an electrode, the application 

 would not be made to a portion of the nerve whose irritability is influenced by 

 that electrode. In order, therefore, to apply the irritation directly to the situation 

 occupied by the electrode, it is necessary at the same time to apply the irritation 

 igh the electrode itself, for example mechanically, or by electrical irritation, 



