This definition, though unnecessarily long and by no means perfect, will nevertheless suffice for a general description of the chief phenomena of the affection. It accords with the generally received theory. My own views of the nature of somnambulism will appear in the course of the following remarks. In the introduction to his classical work on the subject under consideration, Bertrand[110] classifies the different kinds of somnambulism according to their causes. He recognizes-- 1. A particular nervous temperament which predisposes individuals otherwise in good health to paroxysms of somnambulism during their ordinary sleep. 2. It is sometimes produced in the course of certain diseases, of which it may be considered a symptom or a crisis. 3. It is often seen in the course of the proceeding necessary to bring on the condition known as animal magnetism. 4. It may result as the consequence of a high degree of mental exaltation. It is in this case contagious by imitation to such persons as are submitted to the same influence. From these four divisions of causes, Bertrand makes four kinds of somnambulism--the essential, the symptomatic, the artificial, and the ecstatic. As he wrote nearly twenty years before the publication of Mr. Braid’s remarkable researches, he was of course unacquainted with that form of artificial somnambulism now known as hypnotism, and which may properly be included in his third class. I shall simplify his arrangement by dividing the several kinds of somnambulism into two classes--the natural and artificial. _Natural somnambulism_ may occur in persons who exhibit no marked deviations from the standard of health, and in whom there is no very evident nervous excitability. It is usually, though not always, manifested during ordinary sleep, and it is common for authors to speak of it as being necessarily connected with a dream. Thus, Macario[111] says it is a sleep in which the nervo-motor system and all the other organs are put in action under the influence of a dream. A few cases cited from other authors, and from my own experience, will tend to the more complete elucidation of the symptoms of this curious affection. Bertrand[112] quotes the following instance from the _Encyclopædia_: