power, in the latter a want of irritability.

Hence also those constitutions which are deficient in quantity of
irritability, and which possess too great sensibility, as during the pain
of hunger, of hysteric spasms, or nervous headachs, are generally supposed
to have too much irritability; and opium, which in its due dose is a most
powerful stimulant, is erroneously called a sedative; because by increasing
the irritative motions it decreases the pains arising from defect of them.

Why the pulse should become quicker both from an increase of irritation, as
in the synocha irritativa, or irritative fever with strong pulse; and from
the decrease of it, as in the typhus irritativus, or irritative fever with
weak pulse; seems paradoxical. The former circumstance needs no
illustration; since if the stimulus of the blood, or the irritability of
the sanguiferous system be increased, and the strength of the patient not
diminished, it is plain that the motions must be performed quicker and
stronger.

In the latter circumstance the weakness of the muscular power of the heart
is soon over-balanced by the elasticity of the coats of the arteries, which
they possess besides a muscular power of contraction; and hence the
arteries are distended to less than their usual diameters. The heart being
thus stopped, when it is but half emptied, begins sooner to dilate again;
and the arteries being dilated to less than their usual diameters, begin so
much sooner to contract themselves; insomuch, that in the last stages of
fevers with weakness the frequency of pulsation of the heart and arteries
becomes doubled; which, however, is never the case in fevers with strength,
in which they seldom exceed 118 or 120 pulsations in a minute. It must be
added, that in these cases, while the pulse is very small and very quick,
the heart often feels large, and labouring to one's hand; which coincides
with the above explanation, shewing that it does not completely empty
itself.

3. In cases however of debility from paucity of blood, as in animals which
are bleeding to death in the slaughter-house, the quick pulsations of the
heart and arteries may be owing to their not being distended to more than
half their usual diastole; and in consequence they must contract sooner, or
more frequently, in a given time. As weak people are liable to a deficient
quantity of blood, this cause may occasionally contribute to quicken the
pulse in fevers with debility, which ma