INFLUENCE OF ELECTRICAL CURRENTS ON MOTOR NERVES. 589 



tive pole is next the muscle. Lastly, Fig. 202 shows the condition of excita- 

 bility in the nerve in centripetal extrapolar an electro ton us. Pfliiger found 

 that in all these cases the strength of the polarizing current was of great im- 

 portance, and that within certain limits the increase of excitability near the 

 cathode, and the decrease near the anode, augmented ; but that, if these 



FIG. 202. 



Centripetal Extrapolar Aneleetrotonus. Here, as is indicated by the curve, 6, e, f, tbe excitability 

 of the nerve falls as the positive pole or anode of the polarizing current is approximated. 



limits be passed, and currents of much greater strength than usual were 

 employed, the phenomena were less marked, and at length failed altogether 

 to occur. From an examination of the curved lines in the preceding figures, 

 it will also be seen that the excitability of the nerve varies with the distance 

 of the part examined from the electrode, the increase or diminution being 

 always greatest at the cathode and anode respectively, whilst the effect di- 

 minishes as these are receded from. There is one other circumstance that 

 affects the excitability of the nerve, namely, the length of the intrapolar por- 

 tion ; and it would appear the longer this is, the greater is the extent and de- 

 gree of tMe excitability of the nerve around the cathode. As regards time, 

 the state of increased excitability always attains its full intensity with great 

 rapidity. Thus much for the variations of excitability in a nerve rendered 

 electrotonic. 



475. The increase of electro-motive power that occurs in the vicinity of 

 the positive pole, and the depression in that of the negative, has already 

 been noticed. Another alteration effected in the nerve by the induction of 

 the electrotonic state, is a variation in the velocity with which it will con- 

 duct impressions ; this, according to V. Bezold, being invariably diminished. 

 Dr. Rutherford, however, 1 finds that if currents of medium strength instead 

 of the strong ones employed by V. Bezold be used, the negative pole quickens 

 the rate at which the nervous influence is transmitted, whilst the positive 

 pole retards it. As the general result of these researches, it appears then 

 that the conditions present in a nerve, through a portion of which a closed 

 continuous current is passing, and which is, therefore, in a state of electro- 

 tonus, are such that at the anode or positive pole of the current there is an 

 exaltation of the intensity of the ordinary nerve-current (positive phase), 

 and depression of the excitability (auelectrotouus), and of the conductivity 

 or velocity of transmission of impressions. On the contrary, at the cathode 

 there is constantly a depression of the intensity of the ordinary nerve-current 

 (negative phase), and coincident exaltation of the excitability (cathelectro- 

 tonus) and of the conductivity. 2 



476. From the foregoing observations it will be intelligible that the action 



1 Humphry and Turner's Journal of Anat. and Phys., vol. ii, 1868, p. 95. 



2 A very different explanation of the phenomena of electrotonus, founded on chem- 

 ical considerations, has been suggested by Hermann (see his Untersuchungen, pub- 

 lished at Berlin in 1867) ; but his views have been vigorously ass-ailed in an able paper 

 by Du Bois-Keyrnond, in the Berichte der Berlin Akadem., 1867, p. 597. 



