94 ELECTRO-PHYSIOLOGY 



the same strength of excitation at all points of the nerve, 

 while the excitation itself increases with the distance from the 

 motor organ (muscle). Pfl tiger decided for the last alternative, and 

 concluded from the above facts that the excitation gathers strength 

 in its passage down the nerve (" avalanche theory "), although the 

 contrary would a priori be expected. The avalanche-like increment 

 would, as Hermann pointed out (Handbuch, ii. 1, p. 113), justify 

 the important conclusion " that conduction depends not simply 

 upon an undulatory movement propagating itself from particle to 

 particle, but upon the independent discharge of potential energy in 

 the nerve, from which it follows that the transformed energy in 

 each consecutive element of the nerve is slightly greater than in 

 the preceding element." The wide bearing of this conclusion 

 makes it essential that the underlying facts should be thoroughly 

 sifted. Heidenhain (44) referred the cause of the greater activity 

 of the higher tracts of divided nerves to the proximity of the 

 artificial section. " The lower end of the nerve at once exhibits 

 the same marked activity as the upper end, if a section is made 

 lower down ; excitability is raised near the section. The distance 

 from the divided end is not, therefore, the determining factor in the 

 magnitude of effect," and this applies to each stage of the survival 

 of medullated nerve. The cause of this marked action from the 

 cross-section will be discussed below, in considering the effect of 

 electrical stimulation upon the excitability of the nerve. 



Given that the unequal excitability of different points of the 

 divided nerve is conditioned by the cross-section, there is on the 

 other hand evidence to show that regular, local differences of 

 excitability exist in the undivided nerve as well, the points at 

 which branches are given off being in particular characterised 

 by irregular excitability. According to Heidenhain (Lc.), excit- 

 ability in the frog's sciatic is, as a rule, higher in the upper 

 two -thirds than in the lower third. The curve shows a 

 turning-point somewhat above the bifurcation of the sciatic into 

 peroneal and tibial branches. It is lowest at this point, highest near 

 the point of departure of the branches to the thigh (Fig. 164). 

 Still more complicated, according to Hermann and v. Fleischl (45), 

 is the curve of excitability of uninjured nerve, since the direction 

 of the exciting currents is also of extreme importance to the 

 magnitude of the effect. Descending induction currents are 

 particularly effective at upper points of the sciatic, ascending 



