it will be necessary to use, to avoid circumlocution, and at the
same time to give us more distinct ideas on the subject.

When the excitability is in such a state as to be very susceptible of
the action of external powers, I shall call it _abundant_, or
_accumulated_; but when it is found not very capable of receiving
their action, I shall say, it is _deficient_, or _exhausted_. I
would not wish however, to have it thought, that by these terms I
mean in the least to hint at the _nature_ of excitability, nor that
it is _really_ one while increased, and at another diminished in
quantity, for the abstract question is in no shape considered; we
know not whether the excitability, or the vital principle, depends
on a particular arrangement of matter, or from whatever cause it may
originate; by the terms here used, I mean only to say, that the
excitability is easily acted on when I call it abundant, or
accumulated; at other times the living body is with more difficulty
excited, and then I say, the vital principle is deficient, or
exhausted.

The laws by which external powers act on living bodies, will, on a
careful examination, be found to be the following--

First, when the powerful action of the exciting powers ceases for
some time, the excitability accumulates, or becomes more capable of
receiving their action, and is more powerfully affected by them.

If we examine separately the different exciting powers, which act on
the body, we shall find abundant confirmation of this law. Let us
first consider Light; if a person be kept in darkness for some time,
and be then brought into a room in which there is only an ordinary