the exciting powers, the excitability is
entirely exhausted, and death takes place.

We likewise see vegetables in the spring, while the exciting powers
have acted on them moderately, and for a short time, arrayed in their
verdant robes, and adorned with flowers of many mingling hues; but as
the exciting powers, which support their life, continue to be
applied, and some of them, for instance heat, as the summer advances,
become increased, they first lose their verdure, then grow brown, and
at the end of summer cease to live: because their excitability is
exhausted by the long continued action of the exciting powers: and
this does not happen merely in consequence of the heat of the summer
decreasing, for they grow brown, and die, even in a greater degree of
heat than that which in spring made them grow luxuriantly. In some of
the finest days of autumn, in which the sun acts with more power than
in the spring, the vegetable tribe droop, in consequence of this
exhausted state of their excitability, which renders them nearly
insensible of the action, even of a powerful stimulus.

These are examples of the final or irreparable exhaustion of the
excitability; but we find also that it may be exhausted for a time,
and accumulated again. Though the eye has been so dazzled by the
splendour of light, that it cannot see an object moderately
illuminated, yet if it be shut for some time, the excitability of the
optic nerve will accumulate again, and we shall again be capable of
seeing with an ordinary light.

We find also that we are not always equally capable of performing the
functions of life. When we have been engaged in any exertion, either
mental or corporeal, for some hours only, we find ourselves languid
and fatigued, and unfit to pursue our labours much longer.

If in this state several of the exciting powers are withdrawn,
particularly light and noise, and if we are laid in a posture which
does not require much muscular exertion, we soon fall into that state
which nature intended for the accumulation of the excitability, and
which we call sleep. In this state many of the exciting powers cannot
act upon us, unless applied with some violence, for we are insensible
to their moderate action. A moderate degree of light, or a moderate
noise, does not affect us, and the power of thinking, which very much
exhausts the excitability, is in a great measure suspended. When the
action of these powers has been suspended for six or e