POLAR DEVELOPMENT OF ELECTROTONLC EFFECTS. 547 



sign as the demarcation current, increases this, whilst the catelectrotonic 

 current, heing opposed, diminishes it. 



The amount of the electrotonic effects. — The amount of these electro- 

 tonic currents varies under the following conditions : — 



(a) As the intensity of the polarising current is increased the 

 electrotonic effects are augmented ; a maximum can be reached, but this 

 may be as much as twenty times the nerve demarcation current. 



(b) The effects vary directly with the length of nerve subjected to 

 the polarising current, provided that the alteration of electrical resist- 

 ance due to the longer nerve is allowed for or eliminated. In other 

 words, the electrotonic effects increase in proportion as the poles are 

 separated. 



(c) The condition of the nerve affects the production of electrotonus. 

 Excised nerves continue to show the changes as long as the structural con- 

 tinuity is maintained, but in diminishing amount. Degenerative changes, 

 which interfere with such continuity, diminish the electrotonic effects. 



(d) The polarisability of the nerve. All circumstances which 

 diminish the polarisation constant of the nerve, and thus decrease the 

 susceptibility to electrolytic change, diminish the electrotonic currents. 

 One circumstance of special interest in this respect is the change pro- 

 duced by a rapid series of excitations ; the results of such stimulation 

 appear to show that excited nerve is less polarisable, and therefore less 

 susceptible to the production of electrotonic effects. 



(e) The effects diminish in amount in proportion as the extrapolar 

 region under investigation is removed from the neighbourhood of the 

 polarising current. 



The above conditions affect the changes, not only in medullated 

 nerve, but also in the artificial model ; according to Boruttau, even 

 the excitatory changes of a nerve can be reproduced on the model 

 if one end is subjected to a series of alternating induced currents, 

 such as are used to excite the end of the nerve. 



One of the most important features of the electrotonic currents is 

 that of their time relations. These present two aspects for investigation, 

 the rate of development at the seat of the change, the pole, and the 

 time of appearance at various distances outside the pole, i.e. the rate 

 of extrapolar propagation. 



The -polar development of electrotonic effects. — This has been investi- 

 gated by Hermann, Bernstein, and others, by means of the rheotome, 

 which is arranged so that at each revolution both the pola.rising circuit 

 and extrapolar galvanometer circuit are closed for a short period of 

 definite duration ; the time interval between the two closures is, as in 

 all experiments of this nature, susceptible of alteration. 



The experiment shows that both in nerve and in the artificial 

 model, extrapolar electrotonus can be appreciated in the neighbour- 

 hood of either pole within '001 second after the polarising current 

 has commenced to flow. 1 At the actual poles the electrolytic dis- 

 sociation probably commences at the moment of closure. 2 



It will be remembered that in nerve the polar changes in excita- 

 bility also begin at the moment of closure. 3 Pfluger has shown that 



1 Bernstein, Untcrsuch. a. d. physiol. List. d. Univ. Halle, 1888, S. 75 ; Naturwiss. 

 Eundsch., 1887, S. 9. 



2 Hermann, Arch. f. d. ges. Physiol., Bonn, 1888, Bd. lxii. S. 1 to 83. 



3 Griinhagen, lor., cit. 



