PROPAGATION OF ELECTROTONIC EFFECTS. 549 



current in the polarised branch spreading to the region where the two 

 branches are in close proximity, at which point it traverses and excites 

 the second nerve. The phenomenon was utilised by Helmholtz for 

 estimating the rate of the electrotonic propagation from the seat of 

 the galvanic current to the point of apposition of the nerve branches 

 (ab in Fig. 286). He found that the period of delay of the secondary 

 muscle was identical with that obtained by electrical excitation at a 

 corresponding distance (be in Fig. 286) along the secondary nerve itself, 

 and inferred that the electrotonic change was propagated at about the 

 same rate as the true excitatory process. 1 Hermann has pointed out 

 that the only conclusion to be drawn from this experiment is that 

 the electrotonic disturbance is developed to the same extent in both 

 branches of the nerve. 2 



The polar alterations in excitability have been utilised as an index, 

 and on the assumption that these are identical as regards origin and 

 time relations with the electromotive changes of electrotonus, the 

 determination of the rate of transmission of the former might throw 

 light on the propagation of the latter. The results have already been 

 referred to in connection with polar changes excitability; it will 

 be remembered that they exhibit considerable discrepancies. Thus, 

 according to Griinhagen, excitability alterations occur in all extrapolar 

 regions almost instan- 

 taneously 3 ; experi- 

 ments in Hermann's 

 laboratory show that 

 the anelectrotonic de- 

 crease in excitability is 

 propagated at least as 

 fast as 165 metres per FlG 2g6 



second, 4 whilst Asher 



found a rate of 30 metres per second ; 5 finally, Wundt found that the 

 cathodic increase was transmitted at the same rate as the excitatory 

 state, whilst the anodic decrease was transmitted at 1 metre per second. 6 

 In regard to these discrepancies, the most that can be said is that 

 both states are developed at the poles instantaneously, whilst the 

 diminution of the effects in portions far removed from the poles 

 renders all the slower times open to suspicion. 



In Hermann's model the rate of propagation of the electrical 

 changes has been estimated by rheotome observations as at least 60 

 metres per second. 7 Boruttau, working with polarising currents of 

 short duration and alternate direction, found a rate of cathodic spread 

 in the same model of over 100 metres per second. 8 In association with 

 Burch, the present writer found a rate of over 100 metres per second 

 for both anodic and cathodic states in a model of similar type, the 

 method used being that of the photographic record of the capillary 

 electrometer excursion produced by an extrapolar effect. Quite recently 



1 Helmholtz, Mmatsbl. Akad. d. Wissensch., Berlin, 1854, S. 329. 



2 Hermann, " Handbuch der Physiologic," 1879, Bd. ii. S. 162. 



3 Griinhagen, Arch. f. d. ges. PhysioL, Bonn, 1871, Bd. iv. S. 547. 



4 Baranowsky, ibid., 1880, Bd. xxi. S. 449 ; Hermann, ibid., 1880, Bd. xxi. S. 443. 



5 Asher, Ztschr. f. BioL, 1895, Bd. xxxii. S. 473. 



6 Wundt, "Untersuch. a. d. Nerven.," 1871, Abth. 1. 



7 Hermann and Samways, Arch. f. d. ges. PhysioL, Bonn, 1885, Bd. xxxv. S. 1, 



8 Boruttau, ibid., 1894, Bd. Iviii. S. 1. 



