266 



ELECTRO-PHYSIOLOGY 



CHAP. 



of eliciting any considerable negative variation of the nerve 

 current by other than electrical stimulation. V. Uexkiill (29) 

 has recently obtained positive results from his mechanical tetano- 

 motor, and has, moreover, proved the justice of Bering's pre- 

 sumption, that the whole effect will fail, or die out, even under 

 electrical stimulation, if the long and transverse sections of the 

 plexus are brought into circuit by dipping them into physiological 

 saline before, or during, the appearance of the current of action. 



V. ELECTROMOTIVE CHANGES (ELECTROTONUS) 

 1. In Medullated Nerve 



It has already been stated that polar alterations of excita- 

 bility appear under the action -of a battery current, flowing 



FIG. 203. 



steadily at uniform density through any portion of a medullated 

 nerve. These changes are not (as in muscle) confined to the 

 points of contact with the electrodes, i.e. the visible entrance and 

 exit of the current, but extend beyond them, not merely into the 

 intrapolar tract, but to a greater or less extent over the extra- 

 polar region also. As early as 1843 du Bois-Eeymond showed 

 that there were corresponding changes of galvanic reaction which, 

 like those of excitability, must be diagnosed as one of the manifesta- 

 tions of electrotonus, representing in some degree two different 

 sides of one process. Let nn f (Fig. 203) be a nerve, A and K 

 the two electrodes through which a battery current is led in the 

 direction AK; A is therefore the anode, K the kathode, of the 

 current that produces electrotonus. On making this current, all 



