the action of external powers, I shall call it abundant or
accumulated; but when it is found in a state not very capable of
receiving their action, I say it is deficient or exhausted. Let no
one however suppose that by these terms I mean to hint in the least
at the nature of the excitability. I do not mean by them that it is
really at one time increased in quantity or magnitude, and at another
time diminished: its abstract nature is by no means attempted to be
investigated. These or similar terms the poverty or imperfection of
language obliges us to use. We know nothing of the nature of the
excitability or vital principle, and by the terms here used I mean
only to say, that the excitability is sometimes easily acted on by
the external powers, and then I call it abundant or accumulated; at
other times the living body is with more difficulty excited, and then
I say the vital principle or excitability, whatever it may be, is
deficient or exhausted.

On examination we shall find the laws by which external powers act on
living bodies 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. Besides
the exciting powers which act on the body, which I mentioned; viz.
heat, food, and air, there are several others, such as light, sound,
odorous substances, &c. which will be examined in their proper
places. These powers, acting by a certain impulse, and producing a
vigorous action of the body, are called stimulants, and life we shall
find to be the effect of these and other stimulants acting on the
excitability.

The stimulus of light, though its influence in this respect is
feeble, when compared with some other external powers, yet has its
proportion of force. This stimulus acts upon the body through the
medium of the organ of vision. Its influence on the animal spirits
strongly demonstrates its connexion with animal life, and hence we
find a cheerful and depressed state of mind in many people, and more
especially in invalids, to be intimately connected with the presence
or absence of the sun. Indeed to be convinced of the effects of light
we have only to examine its influence on vegetables. Some of them
lose their colour when deprived o