As a proof that the image physiologically assumes the form of a real appearance, I may mention the experience of myself and others.  When suddenly awakened from a vivid dream I have sometimes, even when I was fully awake, seen for an instant the figures of my dream still moving, and projected on the wall.  This fact shows that even the images of our waking state have, in the physiological conditions of the brain, a tendency to take real forms, so that they may be termed normal, or more properly, inchoate hallucinations, corrected by the conscious efforts of our waking state and external consciousness.  So that it might be said that dreams are at first the transformation of our waking thoughts into normal images and hallucinations, and afterwards into those of dreams, properly so called.

If the hypnagogic phase actually affects the cerebral cellules in connection with the various senses of which they are the organs, the phases of sleep and dreams, strictly so called, have more general conditions.  The idea, converted into an image presented to the senses, may thus be said to have three stages:  that of the waking state, which depends as we have said on the intensity and vividness with which it is reproduced, aided by a momentary detachment from the real environment; secondly, the hypnagogic phase, in which there is the physiological action of the nervous centres, which produce the image, though still with the implicit consciousness of the waking state; and finally, the actual dream, in which this implicit consciousness is almost always wanting, and the psychical exercise of thought is completely transformed into visions and figures which are believed to be real.  This in its turn depends upon the other two causes, and on the physiological relaxation of the body, which is to a great extent isolated, so that the effectual impulses of external nature are greatly attenuated.

In the waking state, the whole body and all its organs of relation and movement are in tension.  The cerebro-spinal axis virtually excites the whole muscular and peripheral system in such a way that relaxation or relative repose becomes impossible.  But the brain, with all its dependencies and appendices, is not only the organ of thought, but it stimulates and directs our whole system, as numerous experiments have shown.  In the waking state both these functions are exercised equally, as far as the impulses and functions of the body are concerned, and as long as the psychical and organic characteristics of the waking state continue.  But in sleep the exciting influence of the brain is diminished, and the brain transmits much less of the normal excitement and normal tension to the spinal axis with its ramifications in the afferent and efferent nerves; in the waking state an external impression is promptly conveyed to the centres, whence it returns in corresponding movements with the usual connection and rapidity, whether reflex or deliberate.  Since in sleep the relative condition is flaccid and torpid, this action no longer takes place.  For if the brain be affected by strong impressions, and these are followed by corresponding movements due to reflex action, as is often the case, even in sleep, the dreamer is only obscurely conscious of them, and they almost wholly depend on the spinal axis, and the peripheral ganglia.