ons of sight and hearing. A
close observation will show that in nearly every dream we imagine
ourselves either moving among the objects we perceive or striving to
move when some weighty obstacle obstructs us. All of us are familiar
with the common forms of nightmare, in which we strive hopelessly to
flee from some menacing evil, and this dream-experience, it may be
presumed, frequently comes from a feeling of strain in the muscles, due
to an awkward disposition of the limbs during sleep. The common
dream-illusion of falling down a vast abyss is plausibly referred by
Wundt to an involuntary extension of the foot of the sleeper.

_Action of Internal Stimuli._

Let us now pass from the action of stimuli lying outside the organism,
to that of stimuli lying within the peripheral regions of the
sense-organs. I have already spoken of the influence of subjective
sensations of sight, hearing, etc., on the illusions of waking life, and
it is now to be added that these sensations play an important part in
our dream-life. Johannes Mueller lays great prominence on the part taken
by ocular spectra in the production of dreams. As he observes, the
apparent rays of light, light-patches, mists of light, and so on, due to
changes of blood-pressure in the retina, only manifest themselves
clearly when the eyes are closed and the more powerful effect of the
external stimulus cut off. These subjective spectra come into prominence
in the sleepy condition, giving rise to what M. Maury calls
"hallucinations hypnagogiques," and which he regards (after Gruithuisen)
as the chaos out of which the dream-cosmos is evolved.[83] They are
pretty certainly the starting-point in those picturesque dreams in which
figure a number of bright objects, such as beautiful birds, butterflies,
flowers, or angels.

That the visual images of our sleep do often involve the peripheral
regions of the organ of sight, seems to be proved by the singular fact
that they sometimes persist after waking. Spinoza and Jean Paul Richter
both experienced this survival of dream-images. Still more pertinent is
the fact that the effects of retinal fatigue are producible by
dream-images. The physiologist Gruithuisen had a dream, in which the
principal feature was a violet flame, and which left behind it, _after
waking_, for an appreciable duration, a complementary image of a yellow
spot.[84]

Subjective auditory sensations appear to be much less frequent causes of
dream-illusions