Such, then, are the principal characteristics of somnambulism--a state in which dreams are supplemented by more or less complete and appropriate action; ordinarily without subsequent recollection of either dream or action. The somnambulic dream usually occurs during or soon after the period of deepest sleep, when the influences of the external world are most completely suppressed. Released from the control of its sensory portion, the remainder of the brain awakens, and becomes aroused to a condition of relative exaltation. No longer distracted by the solicitations of external sense, the attention is concentrated upon the hallucinations which constitute the dream. In the simpler forms of noctambulism only the automatic locomotive apparatus is awakened, and the sleeper moves in accordance with the impressions derived from habit, aided by actual exaltation of the muscular and tactile senses. But, in some of the more complicated cases, a certain amount of special sensibility seems to exist. The patient is capable of exercising just that amount of perception which is necessary to accomplish his purpose, though blind and deaf and insensible to every other impression. The more complete the waking of the sense-organs, the closer the resemblance to the condition of ordinary life, or even to the condition of ecstasy, in which cerebral exaltation is the prominent feature, and in which the power of recollection generally persists. Accordingly, it sometimes happens that the somnambulist can recall the events of his paroxysm. In such cases the power of recollection is due to the same conditions that control the recollection of our ordinary dreams. But the phenomena of ordinary somnambulism are as completely as possible removed from all connection with the mental actions which arise directly from the operation of the senses. By reason of such isolation the ordinary association of ideas affords no help to the memory, and the dream remains in oblivion. Alfred Maury expresses the opinion[90] that the principal cause of forgetfulness of the events of somnambulism consists in the exhaustion of the cerebral elements through the intensity of the excitement to which they have been subjected during the paroxysm. Doubtless this, in certain cases, may contribute to the loss of memory, but it should be remembered that the excitement may be relative rather than absolute. Certain elements wake while others are asleep; and these waking cells may be aroused to a degree far in excess of what is usual during the sleep of the brain without attaining to the level of their diurnal activity. The mind, undisturbed by external impressions, gives its attention to the operation of these waking organs, and a dream with all its consequences, somnambulic or otherwise, is the result. In other words, the plane of consciousness, so to speak, is lowered in sleep to the level of these molecular vibrations. But when the entire brain has been completely reawakened, the residual vibrations of