524 NERVE. 



previous excitation. These modifications will be referred to in detail 

 in connection with the electromotive phenomena of excited nerve. It 

 will be sufficient, at this stage, to mention that if a nerve is subjected 

 to short periods of excitation occurring at regular intervals, and the 

 amount of the resting difference is noted between these intervals, the 

 difference declines more rapidly than it does if the nerve is left un- 

 stimulated. Similarly, if a nerve is subjected to prolonged rapid 

 stimulation, the difference after such excitation is generally found to 

 be more diminished in amount than can be accounted for by the general 

 decline. On the other hand, it occasionally happens that with the fresh 

 nerves of cooled or winter frogs, 1 and with nerves excised and left in a 

 bath of physiological saline, 2 the after effect of excitation is to cause 

 an increase in the difference. If the diminution is considered as, to 

 a large extent, due to an impairment in the functional capacity of the 

 uninjured surface, then the augmentation is explicable, in cooled frogs, 

 by the greater functional capacity, which is a prominent feature of 

 such nerves. 



The whole matter may be complicated by the presence of prolonged excita- 

 tory electromotive changes, and the possibility that these are similar or 

 opposed to the difference; if present, they must sum algebraically with the 

 persistent resting difference, thus the decline may, in some cases, be acceler- 

 ated, in others arrested or converted into temporary rise. 



It is clear that there is a most intimate connection between the electromotive 

 changes in resting and in excited nerve. Such a relationship is implied in all 

 theories which have been put forward to explain the electromotive phenomena 

 of nerve, whether the molecular theories of du Bois-Keymond and of Bernstein, 

 or the alteration theory of Hermann, and its modification by Hering. The 

 further consideration of the so-called after-effects of excitation may be thus 

 profitably deferred until the actual excitatory changes have themselves been 

 discussed. 



EXCITATORY ELECTROMOTIVE CHANGES OF NERVE. 



The excitatory "negative variation." The discovery of the resting 

 nerve current by du Bois-Keymond was immediately succeeded by that 

 of electromotive changes evoked by excitation. These show them- 

 selves when an excised nerve is connected with a high resistance galvano- 

 meter by longitudinal and cross sectional contacts, and the distal portion 

 remote from these contacts is stimulated by a rapid series of excita- 

 tions. If the nerve is the sciatic of the frog, and the excitation con- 

 sists of a series of alternating induced currents, a fall in the resting 

 difference is observed, which, on the cessation of the stimulation, passes 

 away more or less completely. Such a decline was termed by du Bois- 

 Keymond 3 the excitatory " negative variation/' the term " negative " 

 being used in its algebraic sense. The decline was rightly considered 

 by its discoverer as due to electromotive changes set up in the tissues 

 which respond to the excitation ; these are such that the portion under 

 the surface contact becomes less positive to the cross section than it 

 was in the unexcited state, and currents, termed " action currents," thus 

 flow through the nerve fibres, from the portion under surface contact 



1 Head, Arch. f. d. ges. PhysioL, Bonn, 1887, Bd. xl. S. 207. 

 Waller, Phil. Trans., London, 1897, B, p. 47. 

 3 E. du Bois-Keymond, "Untersuchungen," loc. cif. 



