m through
the sympathetic nervous system.[86]

It is hardly necessary, perhaps, to give many illustrations of the
effect of such organic sensations on our dreams. Among the most common
provocatives of dreams are sensations connected with a difficulty in
breathing, due to the closeness of the air or to the pressure of the
bed-clothes on the mouth. J. Boerner investigated the influence of these
circumstances by covering with the bed-clothes the mouth and a part of
the nostrils of persons who were sound asleep. This was followed by a
protraction of the act of breathing, a reddening of the face, efforts to
throw off the clothes, etc. On being roused, the sleeper testified that
he had experienced a nightmare, in which a horrid animal seemed to be
weighing him down.[87] Irregularity of the heart's action is also a
frequent cause of dreams. It is not improbable that the familiar
dream-experience of flying arises from disturbances of the respiratory
and circulatory movements.

Again, the effects of indigestion, and more particularly stomachic
derangement, on dreams are too well known to require illustration. It
may be enough to allude to the famous dream which Hood traces to an
excessive indulgence at supper. It is known that the varying condition
of the organs of secretion influences our dream-fancy in a number of
ways.

Finally, it is to be observed that an injury done to any part of the
organism is apt to give rise to appropriate dream-images. In this way,
very slight disturbances which would hardly affect waking consciousness
may make themselves felt during sleep. Thus, for example, an incipient
toothache has been known to suggest that the teeth are being
extracted.[88]

It is worth observing that the interpretation of these various orders of
sensations by the imagination of the dreamer takes very different forms
according to the person's character, previous experience, ruling
emotions, and so on. This is what is meant by saying that during sleep
every man has a world of his own, whereas, when awake, he shares in the
common world of perception.

_Dream-Exaggeration._

It is to be noticed, further, that this interpretation of sensation
during sleep is uniformly a process of exaggeration.[89] The exciting
causes of the feeling of discomfort, for example, are always absurdly
magnified. The reason of this seems to be that, owing to the condition
of the mind during sleep, the nature of the sensation is not clearly
rec