It is well known that somnambulism, in the ordinary sense of the word, designates the state of persons who suffer from an affection which disturbs their sleep and causes them to perform strange or ordinary actions, as it may happen, in a state in which they are apparently half awake and half asleep. This disease is already mentioned in the most ancient authors, and its symptoms are correctly reported in Aristotle. (_De Gener. Anim._) He states that the sufferers rise in their sleep, walk about and converse, that they distinguish objects as if they were awake, ascend trees, pursue enemies, perform tasks, and then quietly return to bed. The state of somnambulism seems to be intermediate between ordinary dreaming and magnetic clairvoyance, and is probably the effect of a serious disturbance in our physical life, which causes the brain to act in an unusual and abnormal manner. It has always been observed at night only, and most frequently at full moon, since the moon seems to affect somnambulists not merely by her light, but in each of the different phases in a peculiar manner. The immediate causes of night-walking are often most trivial; as Muratori, for instance, tells us of a priest who became a somnambulist whenever he neglected for more than two months to have his hair cut! Richard (_Théorie des Songes_, p. 288) mentions an analogous case of an old woman whom he knew to be subject to the same penalty. While nightmares oppress us and make apparently all motion impossible, somnambulism, on the contrary, produces a peculiar facility of locomotion and an irresistible impulse to mount eminences, favored either by an actual diminution of specific gravity, or by an increase of power. This tendency lies again half-way between the sensation of flying, which is quite common in dreams, and the actual elevation from the ground and suspension in the air, which occur in extreme cases of ecstasy. The senses remain during night-walking in a state of semi-activity; the somnambulist may appear as if fast asleep, seeing and hearing nothing, so that the loudest noises and even violent shaking do not rouse him; or he may, like a dreamer, be partly under the influence of outward impressions. One will rise at night, go to the stable, saddle his horse and ride into the woods, while another mounts the window-ledge and performs all the motions of a man on horseback. Many move with unfailing certainty on perilous paths, and find their way in deepest darkness; others make blunders and fall, as Professor J. Feller did, who mistook an open window for a door. By what means they perceive the nature of their surroundings, is still unexplained; it may be the action of the ordinary senses, although these seem to be closed, or they may possess those exceptional faculties which constitute the magic phenomena connected with somnambulism. Thus Forbes (_Brit. and For. Med. Rev._, 1846) ascribes their power to an increased sensitiveness of the retina, and mentions the case of Dr.