hat when bodies are possessed of
it, they become capable of being acted upon by external powers, and
thus the living functions are produced; we shall therefore call this
property _excitability_, and in using this term it is necessary to
mention, that I mean only to express a fact, without the least
intention of pointing out the nature of that property which
distinguishes living from dead matter, and in this we have the
example of the great Newton, who called the property which causes
bodies in certain situations to approach each other, _gravitation_,
without in the least hinting at its nature; yet, though he knew not
what gravitation was, he investigated the laws by which bodies were
acted on by it, in the same manner, though we are ignorant of
excitability, or the nature of that property which distinguishes
living from dead matter, we can investigate the laws by which dead
matter acts on living bodies through this medium. We know not what
magnetic attraction is, and yet we can investigate its laws; the
same holds good with regard to electricity; if we ever should attain
a knowledge of the nature of this property, it would make no
alteration in the laws which we had before discovered.

I shall now proceed to the investigation of the laws by which the
excitability is acted on; but I must first define some terms which
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
c