502 COMPARATIVE ELECTRO-PHYSIOLOGY 



the ' Avalanche Theory/ namely, that during the passage of 

 excitation down the nerve it actually gathers strength. But it 

 is clear that this cannot be true, since we have seen that, other 

 things being equal, excitation is always greater the nearer the 

 point of stimulation to the responding region, and on this fact 

 have depended all those experiments already described, which 

 involved a delicate balance of equal excitations. It follows 

 that the observed enhancement of excitability of a point on the 

 nerve which is distal from the muscle, and in the neighbour- 

 hood of a section, must be ascribed to some other cause. In 

 reference to this Heidenhain, indeed, explained the greater 

 excitability of higher tracts of divided nerve by the proximity 

 of the artificial section. For the lower end of the nerve at 

 once exhibits the same marked activity as the upper end if 

 a section be made lower down. Excitability is, in fact, 

 raised near the section, wherever the section may be. The 

 distance travelled by the excitation could not, therefore, be 

 the determining factor in the magnitude of effect. For so far 

 from increasing it, this, as a matter of fact, causes diminution. 

 It is to be remembered that though the excitability is 

 increased near the point of section, yet at the section itself 

 it is almost abolished, otherwise there could not have been 

 any response by so-called negative variation. The question 

 now arises, Why should the excitability be raised near- the 

 point of section ? 



It has been supposed that this was due to certain electri- 

 cal changes induced by section, which in turn gave rise to 

 electro-tonic variations of excitability. We shall see, in 

 Chapter XL, that the passage of an electrical current through 

 a living tissue induces changes of excitability. And this 

 phenomenon is known as the electro-tonic effect/ Now any 

 ' injury,' such as a mechanical or thermal section, is known 

 to induce galvanometric negativity, or anodic change, at or 

 near the point of section. But it is the kathode-effect which 

 is excitatory. And the observed greater excitability of the 

 nerve near a point of section is supposed to be due to kat- 

 electrotonus, produced within a certain tract from the cross- 



