operate alike to release the lower levels of the mnemonic apparatus from the overshadowing influence of later impressions, so that, like an ancient palimpsest, it presents once more its long-forgotten characters for inspection by the mind. CHAPTER VI. SOMNAMBULISM. A great perturbation in nature! to receive at once the benefit of sleep, and do the effects of watching. --MACBETH. We have seen that in certain cases dreams manifest a tendency to pass into action. Thus, the dream with which I was on one occasion occupied became so amusing that I was awakened by a paroxysm of laughter that continued for some time after the termination of the dream. Sometimes the actions which are thus determined become more complicated, but do not suffice to arouse the sleeper. He continues to dream, and to act out his dream. This constitutes the ordinary form of somnambulism. It is a special affection of the nervous system encountered chiefly among persons of a decidedly neurotic constitution--especially among the victims of hysteria, epilepsy, and insanity. Sometimes occurring in cases apparently characterized by perfect health, it will usually be discovered by careful inquiry that the subject is nevertheless connected by near relationship with a neurotic stock. In a third class of patients the neuropathic diathesis is not congenital, but is the acquired result of particular injuries or diseases of the head. It is also observed sometimes as a consequence of transitory functional disturbances of the brain connected with the period of convalescence from diseases that profoundly affect the nutrition of the nervous system. In all cases, however, it is probable that a special nervous temperament exists as a predisposing cause of the phenomena, for the majority of people can sustain injuries of the head, or of the peripheral nervous system, and can pass through all kinds of illness without exhibiting any tendency to somnambulism. As the nervous temperament is the peculiar property of children and of the female sex, it is among them that the affection is usually observed. Before the age of puberty, however, the differential peculiarities of sex are not sufficient to produce any great preponderance in either direction; and small boys are, therefore, perhaps as frequently somnambulistic as their little sisters. Like other neuropathic disorders, this predisposition is frequently met with as an hereditary attribute which may be handed down from generation to generation. Occurring in the experience of young children, it frequently ceases when they arrive at years of maturity. Not always, however, thus ceasing as a consequence of improvement in the general health, but because of the substitution of some more serious disorder.