556 



NERVE. 



on either view the resultant phenomena in a living nerve are physiological. 

 Probably the qualifying expression " unknown " is the characteristic 

 which Biedermann desires to accentuate. Those who maintain that the 

 resultant phenomena are in themselves the electrical expression of 

 electrolytic change, are in his opinion open to the criticism that the 

 "vital" character of nerve change is ignored; but surely electrolytic 

 polarisation in nerve must depend upon the chemical and mechanical 

 conditions of the structure, and it is the peculiarity of the living 

 state that these conditions are eminently favourable for its production. 

 Thus the whole group of phenomena remain physiological, and even 

 the use of the term " physical " for the current escape along the nerve 

 is only justified in so far as such escape is independent of electrolytic 

 changes produced in the tissue, in consequence of its living molecular 

 state. 



Excitatory after effects Secondary electromotive phenomena. 

 The electromotive changes which follow the passage of galvanic 

 currents through tissues have been termed "secondary electromotive 

 phenomena." These after effects, like those in muscle, are partly the 

 more obvious polarisation changes previously described, and are partly 

 localised excitatory effects produced in the tissue by such changes. 



The discovery by du Bois- 



n/"t^\ Eeymond that the cessation of 



) the passage of a galvanic cur- 



A:- I rent of sufficient intensity and 



"'j*""c"~ *"V""*tf"" > of short duration was acconi- 



FiG.290.-In the left-hand portion of the figure P a * ied ty Sp-called "positive 

 the ordinary polarisation after effects, intra- polarisation, has been already 

 polar and extrapolar, are shown ; the flow alluded to ill the case of 

 of the polarising current through the nerve ^^pip l nphp qnrnp rh en fi- 

 lms been from A to C, and has just ceased ; mU 



the right-hand portion shows the anodic menon is readily produced in 



excitatory after effects, the direction of the nerve. The causation is ill 



currents thus produced being indicated by b ^ instances of the Same 



the dark arrows ; the polarisation currents _ , . 



are indicated by the dotted lines. character. It is a prolonged 



opening excitation localised 



at the anode, and of such character that the nerve at this point is 

 galvanometrically negative. Hence currents flow along the nerve 

 from more distant points to the anode, and these may be so intense 

 as to overpower the proper polarisation effects. The results of this 

 anodic excitatory after effect upon current distribution are shown in 

 Fig. 290. 



The proof that the effect is due to a localised anodic effect is 

 due to Hermann. 2 With a long nerve, and thus a considerable 

 separation of the two polar contacts, galvanometric investigation of 

 the anodic extrapolar region shows that excitatory effects are present, 

 when the polarising current ceases, in sufficient amount to overpower 

 the anodic polarisation change, whilst in the cathodic extrapolar 

 region the polarisation current alone manifests itself. Any localised 

 injury of the anodic region abolishes or largely diminishes the effect. 

 The anodic opening effect is thus explained on the supposition that 

 this region, on opening a current, is the basis of a prolonged localised 



1 E. du Bois-Reymond, Sitzungsb. L Preuss Akad. d. Wissenscli., Berlin, 1888, 

 S. 404. 



- Hermann, Arch. f. d. ges. PliysioL. Bonn, 1884, Bd. xxxiii. S. 103. 



