There can be no doubt that somnambulism is hereditary. Horstius mentions three brothers who were affected with it at the same period; and Dr. Willis knew a whole family subject to it. It is considered by all medical men as a peculiar form of disease. It seldom manifests itself before the age of six, and scarcely ever continues beyond the sixtieth year. It depends, physically, upon the susceptibility or delicacy of the nervous system; and on this account females are more liable to it than males; and in youth it manifests itself more frequently than in mature age. It is caused mentally by any violent and profound emotion; as well as by excessive study, and over-fatiguing the intellectual faculties. Some persons walk periodically in their sleep; the fit returns at stated intervals--perhaps two or three times only in the month. It has been also observed--although we by no means vouch for the fact--by an eminent German physician, that some persons walk at the full, others at the new moon, but especially at its changes. One German authority--Burdach--goes the comical length of asserting that the propensity of somnambulists to walk on the roofs of houses is owing to the attraction of the moon, and that they have a peculiar pleasure in contemplating the moon, even in the day time. Whatever may be the cause of the affection, somnambulism undoubtedly assumes different degrees of intensity. The first degree evinces itself by the movements we have referred to and by sleep-talking. This stage is said to be marked by an impossibility of opening the eyes, which are as if glued together. There are many curious circumstances to be observed concerning sleep-talking. The intonation of the voice differs from the waking state, and persons for the most part express themselves with unusual facility.