articular molecular
rearrangement in the nerve; this constitutes the state of excitation and
is accompanied by local electrical changes as an ascertained physical
concomitant.'

'The excitatory state evoked by stimulus manifests itself in nerve
fibres by E.M. changes, and as far as our present knowledge goes by
these only. The conception of such an excitable living tissue as nerve
implies that of a molecular state which is in stable equilibrium. This
equilibrium can be readily upset by an external agency, the stimulus,
but the term "stable" expresses the fact that a change in any direction
must be succeeded by one of opposite character, this being the return of
the living structure to its previous state. Thus the electrical
manifestation of the excitatory state is one whose duration depends upon
the time during which the external agent is able to upset and retain in
a new poise the living equilibrium, and if this is extremely brief, then
the recoil of the tissue causes such manifestation to be itself of very
short duration.'--_Text-book of Physiology_, ed. by Schaefer, ii. 453.

[4] I shall here mention briefly one complication that might arise from
regarding the current of injury as the current of reference, and
designating the response current either positive or negative in relation
to it. If this current of injury remained always invariable in
direction--that is to say, from the injured to the uninjured--there
would be no source of uncertainty. But it is often found, for example in
the retina, that the current of injury undergoes a reversal, or is
reversed from the beginning. That is to say, the direction is now from
the uninjured to the injured, instead of the opposite. Confusion is thus
very apt to arise. No such misunderstanding can however occur if we call
the current of response towards the more excited _positive_, and towards
the less excited _negative_.

[5] 'The Electrical Sign of Life.... An isolated muscle gives sign of
life by contracting when stimulated.... An ordinary nerve, normally
connected with its terminal organs, gives sign of life by means of
muscle, which by direct or reflex path is set in motion when the nerve
trunk is stimulated. But such nerve separated from its natural termini,
isolated from the rest of the organism, gives no sign of life when
excited, either in the shape of chemical or of thermic changes, and it
is only by means of an electrical change that we can ascertain whether
or no