^ THE CEREBRUM, AND ITS FUNCTIONS. 823 



jects at least, sleep maybe induced in a minute or less, by the positive assurance, 

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

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

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

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

 less effect in arousing the slumberer than those of a new and unaccustomed 

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

 which, even if not literally true, serves extremely well as an illustration 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 undisturbed 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 awake if that tread be 

 suspended. In this latter case, the influence of the simple cessation of the im- 

 pression will be augmented by the circumstance next to be alluded to, which 

 has received too little attention 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. This is, that 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 in a low tone, when it requires a much 

 louder sound of a different description to produce any manifestation of conscious- 

 ness. The same thing is seen in comatose states ; a patient being often capable 

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

 sound produces the least effect. The following circumstance, communicated 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 look- 

 out 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 in 

 repose, his sleep was so profound, that no noise of an ordinary kind, however 

 loud, would awake him ; and it used to be a favorite amusement with his com- 

 rades, 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 in the effect produced by the cessation, as in that of the occur- 

 rence, of sensory impressions. Thus in the case of the naval officer aroused by 



belief that a new process was being put in practice, will be found in the "Brit, and For. 

 Med. Rev.," vol. xix. p. 477. 



