OF SLEEP AND SOMNAMBULISM. 713 



which the mind of the individual becomes possessed, that it will and must 

 supervene. 



578. The influence of previous mental states is yet more remarkable in 

 determining the effects produced upon the sleeper by different sensory im- 

 pressions. The general rule is, that habitual impressions of any kind have 

 much less effect iu arousing the slumberer than those of a new or unaccus- 

 tomed character. An amusing instance of this kind has been related to the 

 Author, which, even if not literally true, serves extremely well as an illus- 

 tration of what is unquestionably the ordinary fact. A gentleman who had 

 taken his passage on board a ship of war, was aroused on the first morning 

 by the report of the morning gun, which chanced to be fired just above his 

 berth ; the shock was so violent as to cause him to jump out of bed. On 

 the second morning he was again awoke, but this time he merely started 

 and sat up in bed ; on the third morning the report had simply the effect of 

 causing him to open his eyes for a moment and turn in his bed ; on the fourth 

 morning it ceased to affect him at all ; and his slumbers continued to be un- 

 disturbed by the report so long as he remained on board. It often happens 

 that sleep is terminated by the cessation, of an accustomed sound, especially 

 if this be one whose monotony or continuous repetition had been the original 

 inducement to repose. Thus, a person who has been read or preached to 

 sleep, will awake, if his slumber be not very profound, on the cessation of 

 the voice; and a naval officer, sleeping beneath the measured tread of the 

 watch on deck, will a\vake if that tread be suspended. In this latter case 

 the influence of the simple cessation of the impression will be augmented 

 by the circumstances next to be alluded to, which has received too little at- 

 tention from writers on this subject, but which is of peculiar interest both in 

 a physiological and psychological point of view, and is practically familiar 

 to almost every one. 



579. The awakening power of sensory impressions is greatly modified by 

 our habitual state of mind in regard to them. Thus, if we are accustomed 

 to attend to these impressions, and our perception of them is thus increased 

 in acuteness, we are much more easily aroused by them than we are by others, 

 which are in themselves much stronger, but which we have been accustomed 

 to disregard. Thus, most sleepers are aroused by the sound of their own 

 names uttered iu a low tone, when it requires a much louder sound of a 

 different description to produce any manifestation of consciousness. The 

 same thing is seen in comatose states; a patient being often found capable 

 of being momentarily aroused by shouting his name into his ear, when no 

 other sound produces the least effect. The following circumstance, commu- 

 nicated to the Author by the late Sir Edward Codrington, is a most apposite 

 illustration of this principle. When a young man, he was serving as signal 

 lieutenant under Lord Hood, at the time when the French fleet was confined 

 in Toulon harbor; and being desirous of obtaining the favorable notice of 

 his commander, he devoted himself to his duty (that of watching for signals 

 made by the lookout frigates) with the greatest energy and perseverance, 

 often remaining on deck nineteen hours out of the twenty-four, with his 

 attention constantly directed towards this one object. During the few hours 

 which he spent iu repose his sleep was so profound that no noise of an ordi- 

 nary kind, however loud, would awake him ; and it used to be a favorite 

 amusement with his comrades to try various experiments devised to test the 

 soundness of his sleep. But if the word "signal" was even whispered in 

 his ear, he was instantly aroused, and fit for immediate duty. The influence 

 of habitual attention is shown as much iu the effect produced by the cessa- 

 tion, as in that of the occurrence, of sensory impressions. Thus, in the case 

 of the naval officer aroused by the suspension of the measured tread of the 



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