treated of in Vol. II. Class I. 2. 1. 2. on the Diseases of the Voluntary
Power. Art. Somnus interruptus.

15. We come now to those motions which depend on irritation. The motions of
the arterial and glandular systems continue in our sleep, proceeding slower
indeed, but stronger and more uniformly, than in our waking hours, when
they are incommoded by external stimuli, or by the movements of volition;
the motions of the muscles subservient to respiration continue to be
stimulated into action, and the other internal senses of hunger, thirst,
and lust, are not only occasionally excited in our sleep, but their
irritative motions are succeeded by their usual sensations, and make a part
of the farrago of our dreams. These sensations of the want of air, of
hunger, thirst, and lust, in our dreams, contribute to prove, that the
nerves of the external senses are also alive and excitable in our sleep;
but as the stimuli of external objects are either excluded from them by the
darkness and silence of the night, or their access to them is prevented by
the suspension of volition, these nerves of sense fall more readily into
their connexions with sensation and with association; because much
sensorial power, which during the day was expended in moving the external
organs of sense in consequence of irritation from external stimuli, or in
consequence of volition, becomes now in some degree accumulated, and
renders the internal or immediate organs of sense more easily excitable by
the other sensorial powers. Thus in respect to the eye, the irritation from
external stimuli, and the power of volition during our waking hours,
elevate the eye-lids, adapt the aperture of the iris to the quantity of
light, the focus of the crystalline humour, and the angle of the optic
axises to the distance of the object, all which perpetual activity during
the day expends much sensorial power, which is saved during our sleep.

Hence it appears, that not only those parts of the system, which are always