_Representative Dreams._--Centrally initiated dreams may be due to a kind of automatic excitation of the cerebral regions, especially in the case of those clearly arising from the occupations or sensations of the day or the hours immediately preceding the dream. To the same cause we may attribute the recalling of images apparently long since forgotten. Some of these revivals of memory may be due to the fact that links of association which are insufficient to restore an idea to consciousness in the waking state may suffice to do so in sleep. Just as a good visualizer in his waking moments may call up an object never clearly seen and yet distinguish the parts, so in sleep, as L. F. A. Maury (1817-1892) and others have shown, an image may be more distinct in a dream than it was when originally presented (see also below, Memory). _Presentative Dreams._--The dreams due to 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