CONDUCTIVITY CHANGES. 501 



instant of closure, is increased with the duration of the current which 

 causes it, for Tigerstedt found that the more prolonged make induced 

 currents gave the results far more easily than the more rapidly 

 developed currents caused by the complete break of the primary coil 

 circuit. 1 



The above are experimental demonstrations in motor nerve of the effect of 

 the anodic block, or cathodic after-block, upon the intensity of the excitatory 

 change which has to be propagated through the altered region. 



These alterations in conductivity, like those in excitability, have been 

 shown to be valid for excitatory states evoked by all varieties of local stimuli, 

 electrical, mechanical, and chemical, as well as for those due to discharge from 

 the central nervous system. They are present in afferent as well as efferent 

 nerves, and can be demonstrated in these by using the reflex response as the 

 index of the propagation. 



A further question upon which there is at present but little 

 information is, whether transmission through the cathodic region during 

 closure, or through the anodic on opening, augments the propagated 

 excitatory state. As a rule the blocking effects more than counter- 

 balance any such augmentation. 



In order to reduce these, the most effective method is that in which 

 tripolar leads are used t _> 



for the galvanic currents, ^s f , 



The arrangement allows 

 of three poles, for in- 

 stance, an anode I>, with 

 a cathode on each side, at 

 A and C, as in Fig. 261. 

 During closure the excita- 

 tion of a break induced 

 current at X, above the 



centripolar cathode A, evokes a far more pronounced muscle response 

 than can be obtained by the reversed arrangement, with the cathode 

 at B and the anodes at A and C. In both cases the same length of 

 nerve is subjected to current flow ; hence it may be inferred that the 

 excitatory change in the first tripolar arrangement gathers force during 

 its transmission through the second cathodic region. 2 A similar but 

 less marked augmentation has been observed by the present writer, 

 when, with the cathode in the middle and two anodes, one on each side, 

 the nerve is excited at X immediately after the galvanic current is 

 opened. In both instances the closure must be of short duration, and 

 the current intensity comparatively low. 



In addition to these changes in the magnitude of the transmitted 

 effect, the polar conditions undoubtedly affect the rate of propagation. 



The simplest proof of such alteration in rate is furnished by the 

 experiments of von Bezold, whose work forms a valuable extension of 

 Pfluger's. 3 The method was utilised by Eutherford, 4 and has been em- 

 ployed by the present writer. In its simplest form it consists in the 



1 Tigerstedt, Mitih. v. physiol. Lab. d. Carotin, med.-chir. Inst. in Stockholm, Bel. iii. ; 

 Tiegel, Arch. f. d. ges. Physiol., Bonn, Bd. xiii. 



a See Danilewsky, Centralbl.f. Physiol., Leipzig u. Wien, 1895, Bd. ix. S. 398. 



3 v. Bezold, "Elektr. Erreg. Nerven u. Muskeln," Leipzig, 1861. 



4 Rutherford, Journ. Anat. and Physiol., London, 1868, vol. ii. p. 87. 



