564.—The power which a person in perfect health possesses freely to exercise the different senses and to perform voluntary movements is called _wakefulness_ or the state of being awake; the absence of the power to make voluntary movements and to utilize the different senses, combined with the quietude of all of them, bears the name of _sleep_. 565.—In sleep the mind either stops thinking entirely of the things which have been stored up by the individual in his memory or which are well-known facts, or else it busies itself exclusively with certain ideas or with impressions that produce upon the mind, at the time, pictures almost as vivid as the actual things or occurrences which they represent would produce. The term “insomnia” is employed when it is desired to designate the latter condition of the mind, and the mental pictures thus presented produce the effect that—although voluntary motions are at the time all in abeyance, and although the mind is absolutely quiet in all other respects—there remain certain directions in which it continues to operate actively, thus producing an elevation of the spirits (_i.e._, a certain degree of excitement) and more or less wakefulness. Sometimes a certain number of voluntary movements are associated with these mental impressions, and this may occur in such a degree that the organs of speech and many of the joints—indeed at times all of them—are compelled to act in harmony with the mental impressions. When this degree of insomnia is reached the person so affected is called a “somnambulist.” 566.—But in sleep the distribution of the humors of the body goes on without let or hindrance; and, similarly, the circulation of the blood, the peristaltic action of the stomach, intestines and sphincters, and the respiratory movements continue their activity. This complex state of affairs—viz., the coexistence of quietude of certain parts of the body with continued motion in other parts—has made it difficult to ascertain the mechanical cause of sleep.[7] In his investigations into these subjects, von Haller placed his reliance mainly on vivisections and on experiments made upon animals. “A single experiment of this nature,” he said, “is often sufficient to disprove the deceptive conclusions or views that have prevailed through a period of years.” It is to John Hunter of England, however, says Puschmann, that the greatest credit is due for the introduction of the experimental method as a means of ascertaining the truth in questions of pathology; and von Haller was unquestionably one of the first German physicians to adopt the method. Von Haller died at Berne on December 12, 1777. * * * * *