stimulus that interrupts them, and the greater is the degree of surprise. I
have observed dogs, who have slept by the fire, and by their obscure
barking and struggling have appeared very intent on their prey, that shewed
great surprise for a few seconds after their awaking by looking eagerly
around them; which they did not do at other times of waking. And an
intelligent friend of mine has remarked, that his lady, who frequently
speaks much and articulately in her sleep, could never recollect her dreams
in the morning, when this happened to her: but that when she did not speak
in her sleep, she could always recollect them.

Hence, when our sensations act so strongly in sleep as to influence the
larger muscles, as in those, who talk or struggle in their dreams; or in
those, who are affected with complete reverie (as described in the next
Section), great surprise is produced, when they awake; and these as well as
those, who are completely drunk or delirious, totally forget afterwards
their imaginations at those times.

20. As the immediate cause of sleep consists in the suspension of volition,
it follows, that whatever diminishes the general quantity of sensorial
power, or derives it from the faculty of volition, will constitute a remote
cause of sleep; such as fatigue from muscular or mental exertion, which
diminishes the general quantity of sensorial power; or an increase of the
sensitive motions, as by attending to soft music, which diverts the
sensorial power from the faculty of volition; or lastly, by increase of the
irritative motions, as by wine, or food; or warmth; which not only by their
expenditure of sensorial power diminish the quantity of volition; but also
by their producing pleasureable sensations (which occasion other muscular
or sensual motions in consequence), doubly decrease the voluntary power,
and thus more forcibly produce sleep. See Sect. XXXIV. 1. 4.