real sensations, more or less
metamorphosed, may arise (a) from the states of the internal organs, (b)
from muscular states, (c) from subjective sensations due to the
circulation, &c., or (d) from the ordinary cause of the action of
external stimuli on the organs of sense.

(a) The state of the stomach, heart, &c., has long been recognized as
important in the causation of dreams (see below, Classical Views). The
common sensation of flying seems to be due in many cases to the
disturbance of these organs setting up sensations resembling those felt
in rapidly ascending or descending, as in a swing or a lift. Indigestion
is a frequent cause of nightmare--the term given to oppressive and
horrible dreams--and bodily discomfort is sometimes translated into the
moral region, giving rise to the dream that a murder has been committed.
(b) Dreams of flying, &c., have also been attributed to the condition of
the muscles during sleep; W. Wundt remarks that the movements of the
body, such as breathing, extensions of the limbs and so on, must give
rise to dream fancies; the awkward position of the limbs may also excite
images. (c) Especially important, probably, for the dreams of the early
part of the night are the retinal conditions to which are due the
_illusions hypnagogiques_ of the preliminary drowsy stage; but probably
Ladd goes too far in maintaining that entoptic stimuli, either intra- or
extra-organic in origin, condition all dreams. _Illusions
hypnagogiques_, termed popularly "faces in the dark," of which Maury has
given a full account, are the not uncommon sensations experienced,
usually visual and seen with both open and closed eyes, in the interval
between retiring to rest and actually falling asleep; they are
comparable to the crystal-gazing visions of waking moments; though
mainly visual they may also affect other senses. Besides the eye the ear
may supply material for dreams, when the circulation of the blood
suggests rushing waters or similar ideas. (d) It is a matter of common
observation that the temperature of the surface of the body determines
in many cases the character of the dreams, the real circumstances, as
might be expected from the general character of the dream state, being
exaggerated. In the same way the pressure of bed-clothes, obstruction of
the supply of air, &c., may serve as the starting-point of dreams. The
common dream of being unclothed may perhaps be due to this cause, the
sensations associated