ence such seemingly forgotten ideas sometimes return as
though by a spontaneous movement of their own and by no discoverable
play of association.

It may be well to add that this immediate revival of impressions
previously received by the brain includes not only the actual
perceptions of waking life, but also the ideas derived from others, the
ideal fancies supplied by works of fiction, and even the images which
our unaided waking fancy is wont to shape for itself. Our daily
conjectures as to the future, the communications to us by others of
their thoughts, hopes, and fears,--these give rise to numberless vague
fugitive images, any one of which may become distinctly revived in
sleep.[92] This throws light on the curious fact that we often dream of
experiences and events quite unlike those of our individual life. Thus,
for example, the common construction by the dream-fancy of the
experience of flight in mid-air, and the creation of those weird forms
which the terror of a nightmare is wont to bring in its train, seem to
point to the past action of waking fancy. To imagine one's self flying
when looking at a bird is probably a common action with all persons, at
least in their earlier years, and images of preternaturally horrible
beings are apt to be supplied to most of us some time during life by
nurses or by books.

_Indirect Central Stimulation._

Besides these direct central stimulations, there are others which, in
contradistinction, may be called indirect, depending on some previous
excitation. These are, no doubt, the conditions of a very large number
of our dream-images. There must, of course, be some primary cerebral
excitation, whether that of a present peripheral stimulation, or that
which has been termed central and spontaneous; but when once this first
link of the imaginative chain is supplied, other links may be added in
large numbers through the operation of the forces of association. One
may, indeed, safely say that the large proportion of the contents of
every dream arise in this way.

The very simplest type of dream excited by a present sensation contains
these elements. To take an example, I once dreamt, as a consequence of
the loud barking of a dog, that a dog approached me when lying down, and
began to lick my face. Here the play of the associative forces was
apparent: a mere sensation of sound called up the appropriate visual
image, this again the representation of a characteristic action, and so