he excitability, from
a variety of stimulants, and other circumstances, which are not
entirely under our direction, is sometimes more, and sometimes less
abundant, than this middle degree. There is, however, a considerable
latitude, on each side the point of health, within which the
excitement may vary, and yet no disease, nor any disturbance of the
functions may take place: but this has its limits, beyond which if
the excitement be brought, on either side, it is evident that an
uneasy or unpleasant exercise of the functions must take place. There
is not, however, any precise line or boundary between this state, and
that in which the functions begin to be disturbed; on the contrary,
the law of continuity and gradation seems to extend throughout every
part of nature. This departure from the healthy state, and approach
to disease, in which what has been called the nervous state consists,
is gradual and scarcely perceptible; but is apt to be produced by any
circumstances, which lead the excitement beyond its proper limits.

Nervous complaints may therefore be divided, like all other diseases,
into two classes. First, those in which the excitement is increased,
or in which it verges to, or has actually reached, the point of
predisposition to sthenic disease; Secondly, those in which the
excitement is diminished, or in which it verges towards asthenic
disease. This last class, as has been done before, may be subdivided
into two orders. The first will comprise those diseases in which the
excitability is sufficiently abundant, or even accumulated, but where
the excitement is deficient from a want of energy in the exciting
powers. In the second, there has been no deficiency in the action of
the exciting powers; but on the contrary, probably for a considerable
time, some of the diffusible stimuli not natural to life have been
applied; in this case, the excitability has become exhausted, and a
proper degree of excitement cannot be produced by the action of the
common exciting powers.

No diseases show so clearly the fallacy of trusting to symptoms, as
those of the former class. I have met with innumerable cases of this
kind, in which, if you were to trust to the patients own description,
they laboured under considerable debility; and had it not been for
the particular attention I paid to my own case, I should not probably
have suspected that a directly opposite state of the system may
produce these symptoms.

From inheriting