INFLUENCE OF PHYSIOLOGICAL ALTERATIONS. 551 



the galvanometric circuit (Fig. 287). If the nerve is excited under 

 these conditions, a perceptible diminution in this anodic, extrapolar 

 current is produced. The reversal of the polarising current causes a 

 cathodic extrapolar effect, opposed in direction to the previous anodic 

 one. On exciting, this is also diminished. The excitatory diminution 

 has been explained on the assumption that during excitation the 

 susceptibility of the nerve to polarisation is decreased, and thus there 

 will be a corresponding diminution in the amount of the electrotonic 

 currents. 



The same decrease in the electrotonic currents during excitation may 

 be demonstrated when the galvanometer contacts include the nerve 

 cross section (Fig. 288). 



V 



FIG. 288. 



Iii this case the direction of the demarcation current in the nerve 

 may be represented by the arrow 2 of Fig. 288, and the excitatory negative 

 variation by the arrow 3. If the galvanometric contacts lie in the 

 anodic extrapolar region, an electrotonic current during closure 

 represented in the figure by the arrow 1, is produced. This sums with 

 the resting demarcation current, and on excitation a much larger 

 negative variation occurs, since both resting and anelectrotonic currents 

 are diminished, represented by arrows 3 and 4. If, on the other hand, 

 the contacts lie in the cathodic extrapolar region, the catelectrotonic 

 current diminishes that of the resting difference. On excitation, a very 

 small excitatory negative variation occurs, as both the demarcation and 

 the catelectrotonic currents diminish, and, being opposed, their diminu- 

 tions (3 and 4 in the figure) are counter to one another. 



What is the rationale of the apparent excitatory decrease in the 

 polarisation ? The excitatory electrical responses, starting from the seat 

 of stimulation, are propagated through the polarised region ; they will 

 cause the apparent decrease in polarisation if they should become more 

 pronounced as they approach the anode, less pronounced as they 

 approach the cathode. The result of such alterations would be that 

 the anodic portion of the nerve would tend to become galvanometrically 

 negative to the cathodic portion, and axial currents would thus tend 

 to now through the nerve, directed from the anode to the cathode, i.e. 

 counter to polarisation. There is evidence, by rheotomic observations, 

 that such an alteration in the electrical response is actually present near 

 the poles, and that the polarisation currents, being counter to this, are 

 thus decreased. 1 But, further, the electrical response, when propagated 

 through the extrapolar regions, is profoundly affected. Every point 

 nearer the anode will, owing to the electrolytic change present in its 

 neighbourhood, be the seat of a more intense electrical response than 

 one further off (Fig. 289). The whole anodic extrapolar region will 

 thus become the seat of brief excitatory .currents, directed so that the 



1 Hermann, " Handbuch," 1879, "Allg. Nervenphysiologie," Bd. ii. Th. 1, S. 166-167 ; 

 Fleischl von Marxow, Sitzungsb. d. k. Akad. d. Wissensch., Wien, 1878, Bd. Ixxvii. Abth. 3. 



