SLEEP 619 



have slept soundly in the saddle ; persons will sometimes sleep during 

 the din of battle ; and other instances illustrating the imperative demand 

 for sleep after prolonged vigilance might be cited. It is remarkable, 

 also, how noises to which one has become accustomed may fail to dis- 

 turb natural rest. Those who have been long habituated to the noise 

 of a crowded city frequently find difficulty in sleeping in the stillness of 

 the country. Prolonged exposure to intense cold induces excessive 

 somnolence ; and if this is not resisted, the sleep passes into stupor, the 

 power of resistance to cold rapidly diminishes and death is the result. 

 Intense heat often produces drowsiness, but, as is well known, is not 

 favorable to natural sleep. 



Sleep is preceded by a feeling of drowsiness, an indisposition to 

 mental or physical exertion, and a general relaxation of the muscular 

 system. It then requires a decided effort to keep awake. In sleep the 

 voluntary muscles are inactive, the lids are closed, the ordinary impres- 

 sions of sound are not appreciated, and sometimes there is a dreamless 

 condition, in which the consciousness of existence is lost. 



Dreams. There may be, during sleep, mental operations of which 

 there is no consciousness or recollection, unconscious cerebration, as it 

 has been called. It is well known that dreams are vividly remembered 

 immediately on awakening, but that the recollection of them rapidly 

 fades away, unless they are brought to mind by an effort to recall and 

 relate them. Whatever may be the condition of the mind in sleep, if 

 the sleep is normal, there is repose of the cerebro-spinal system and an 

 absence of voluntary effort, which restore the capacity for mental and 

 physical exertion. 



The impressionability and the activity of the human mind are so 

 great, most of the animal functions are so far subordinate to its 

 influence and the organism is so subject to unusual mental conditions, 

 that it is difficult to determine with exactness the phenomena of sleep 

 that are physiological and to separate those that are slightly abnor- 

 mal. It can not be assumed, for example, that a dreamless sleep, 

 in which existence is, as it were, a blank, is the only normal condition 

 of repose of the system ; nor is it possible to determine what dreams are 

 due to previous trains of thought, to impressions from the external 

 world received during sleep, and are purely physiological, and what are 

 due to abnormal nervous influences, disordered digestion etc. It may 

 be assumed, however, that an entirely refreshing sleep is normal. 



That reflex ideas originate during sleep, as the result of external im- 

 pressions, there can be no doubt ; and many remarkable experiments on 

 the production of dreams of a definite character, by subjecting a person 

 during sleep to peculiar influences, have been recorded. The halluci- 



