The phenomena of somnambulism are of apparently spontaneous origin, during ordinary sleep. But from the remotest antiquity it has been known that certain persons may be thrown into an artificial sleep which closely resembles the condition of the somnambulist. Such a degree of susceptibility is not common to all persons. Heidenhain, experimenting upon his class of medical students, found only one in twelve who was thus susceptible. My own experiments lead me to think that American medical students are less easily influenced in this direction. Charcot, whose field of observation covers the inmates of the Salpêtrière Hospital, finds the best examples of the hypnotic state among the hystero-epileptic females in that asylum. To the experiments of Heidenhain, in Germany, of Braid, in England, and of Charcot, in France, we are indebted for the most thoroughly scientific observation and interpretation of the phenomena of hypnotism. The antecedent physical condition most favorable to the development of the hypnotic state is a highly unstable constitution of the nervous system. For this reason the larger number of qualified subjects is furnished by the female sex--especially by those who possess the hysterical temperament. Frequent repetition of hypnotic exercises renders the subject still more susceptible. Heidenhain was, at first, inclined to the belief that such experiences were not prejudicial to the health of the subject, but the observations of Harting, in the University of Utrecht, and of Milne-Edwards, in Paris,[91] have demonstrated the fact of danger to the health of animals subjected to similar experiments. Hysterical patients have often exhibited considerable exhaustion after hypnotic exhibition in the hospitals of Paris (Charcot and Richer), consequently, it cannot be admitted that the practice is devoid of risk to the health of the individual.