take a
greater quantity of food than usual, and adds a quantity of wine, all
the functions will at first be increased in vigour, but at last they
will be irregularly performed, and inflammation, with other symptoms
of too great excitement, will be the consequence. This state is
called sthenic diathesis or disease. But if the stimulant power be
pushed still further, the excitability will become gradually
exhausted, till at last there will be too little to produce the
healthy actions, even though there may be plenty of stimulus. This
state of asthenic diathesis is called indirect debility, because it
is not produced by directly subtracting the powers which support
life, but indirectly, by over stimulating. An instance of this latter
state is afforded by that debility which is the consequence of
intoxication.

There is a state however between perfect health and disease, which is
called predisposition; and in which, though the functions are
undisturbed, the slightest cause will bring on disease. Strictly
speaking, there is perhaps only one point, or one degree of
excitement, at which the health is perfect: the first alterations
from this point, on either side, are scarcely perceptible, but if the
morbid causes be continued, the functions will become gradually more
and more disturbed, till at last they become so uneasy or painful
that they are termed disease.

In order to render what has been said still more plain, it may be
proper to make use of an illustration by means of numbers: we must
recollect however that it is merely for the sake of illustration, for
we have not data to enable us to reduce either the excitability, or
excitement, or stimulus, to numerical calculation; if we could do
this, the science of medicine would be perfect, and we could cure
diseases as easily as we could perform any chemical or philosophical
experiment. A very principal object however is to understand the
nature of predisposition, and the kind of diathesis, whether sthenic
or asthenic, to which it inclines: this not only throws light on the
nature of the disease, but affords us the only means of preventing
it. When a slight uneasiness or predisposition is felt, it is almost
impossible to say from our feelings whether it leads to a sthenic or
an asthenic state: here we must be guided chiefly by the exciting
powers. If we find that these have acted too powerfully; that is, if
we have lived freely, been exposed to heat, and perhaps indulged i