nt to
sensation, as in the impotency of age; others to irritation, as in
schirrous viscera; and others to association, as in impediment of speech;
yet though all these may become inexcitable, or dead, in respect to that
kind of stimulus, which has previously exhausted them, whether of volition,
or sensation, or irritation, or association, they may still in many cases
be excited by the others.

SPECIES.

1. _Lassitudo._ Fatigue or weariness after much voluntary exertion. From
the too great expenditure of sensorial power the muscles are with
difficulty brought again into voluntary contraction; and seem to require a
greater quantity or energy of volition for this purpose. At the same time
they still remain obedient to the stimulus of agreeable sensation, as
appears in tired dancers finding a renovation of their aptitude to motion
on the acquisition of an agreeable partner; or from a tired child riding on
a gold-headed cane, as in Sect. XXXIV. 2. 6. These muscles are likewise
still obedient to the sensorial power of association, because the motions,
when thus excited, are performed in their designed directions, and are not
broken into variety of gesticulation, as in St. Vitus's dance.

A lassitude likewise frequently occurs with yawning at the beginning of
ague-fits; where the production of sensorial power in the brain is less
than its expenditure. For in this case the torpor may either originate in
the brain, or the torpor of some distant parts of the system may by
sympathy affect the brain, though in a less proportionate degree than the
parts primarily affected.

2. _Vacillatio senilis._ Some elderly people acquire a see-saw motion of
their bodies from one side to the other, as they sit, like the oscillation
of a pendulum. By these motions the muscles, which preserve the
perpendicularity of the body, are alternately quiescent, and exerted; and
are thus less liable to fatigue or exhaustion. This therefore resembles the
tremors of old people above mentioned, and not those spasmodic movements of
the face or limbs, which are called tricks, described in Class IV. 1. 3. 2.
which originate from excess of sensorial power, or from efforts to relieve
disagreeable sensation, and are afterwards continued by habit.

3. _Tremor senilis._ Tremor of old age consists of a perpetual trembling of
the hands, or of the head, or of other muscles, when they are exerted; and
is erroneously called paralytic; and seems owing to the small quant