e soon ensues; and the muscles cease to act owing to the
temporary exhaustion of the spirit of animation; as soon as this is again
accumulated in the muscles, they are ready to contract again by the efforts
of volition.

Those violent muscular actions induced by pain become in the same manner
intermitted and recurrent; as in labour-pains, vomiting, tenesmus,
strangury; owing likewise to the temporary exhaustion of the spirit of
animation, as above mentioned.

When any stimulus continues long to act with unnatural violence, so as to
produce too energetic action of any of our moving organs, those motions
soon cease, though the stimulus continues to act; as in looking long on a
bright object, as on an inch-square of red silk laid on white paper in the
sunshine. See Plate I. in Sect. III. 1.

On the contrary, where less of the stimulus of volition, sensation, or
irritation, have been applied to a muscle than usual; there appears to be
an accumulation of the spirit of animation in the moving organ; by which it
is liable to act with greater energy from less quantity of stimulus, than
was previously necessary to excite it into so great action; as after having
been immersed in snow the cutaneous vessels of our hands are excited into
stronger action by the stimulus of a less degree of heat, than would
previously have produced that effect.

From hence the periods of some fever-fits may take their origin, either
simply, or by their accidental coincidence with lunar and solar periods, or
with the diurnal periods of heat and cold, to be treated of below; for
during the cold fit at the commencement of a fever, from whatever cause
that cold fit may have been induced, it follows, 1. That the spirit of
animation must become accumulated in the parts, which exert during this
cold fit less than their natural quantity of action. 2. If the cause
producing the cold fit does not increase, or becomes diminished; the parts
before benumbed or inactive become now excitable by smaller stimulus, and
are thence thrown into more violent action than is natural; that is a hot
fit succeeds the cold one. 3. By the energetic action of the system during
the hot fit, if it continues long, an exhaustion of the spirit of animation
takes place; and another cold fit is liable to succeed, from the moving
system not being excitable into action from its usual stimulus. This
inirritability of the system from a too great previous stimulus, and
consequent exhaustio