744 NERVOUS SYSTEM. 



been known to sleep while walking ; men 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 we have become accustomed may fail to disturb our natural rest. Those 

 who have been long habituated to the endless noise of a crowded city frequently find 

 difficulty in sleeping in the oppressive stillness of the country. We must have sleep; 

 and this demand is so imperious, that we soon accommodate ourselves to the most un- 

 favorable surrounding conditions. It is remarkable, also, that prolonged exposure to 

 intense cold induces excessive somnolence, and, if this be not resisted, the sleep passes 

 into stupor, the power of resistance to cold becomes rapidly diminished, and death is the 

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

 favorable to natural sleep. We generally sleep less in summer than in winter, though in 

 summer, perhaps, we are less capable of protracted mental and physical exertion. 



Sleep is preceded by an indescribable feeling of drowsiness, an indisposition to mental 

 or physical exertion, and a general relaxation of the muscular system. It then requires 

 a decided eifort to keep awake ; and, if we yield to the soporific tendency, the voluntary 

 muscles cease to act, the lids are closed, we cease to appreciate the ordinary impressions 

 of sound, and we sometimes pass into a dreamless condition, in which we lose all knowl- 

 edge of existence. We say sometimes, because the mind is not generally inactive during 

 what we may regard as normal sleep. We may have dreams which are not due, as far 

 as can be ascertained, to impressions from the external world received during sleep. 

 Ideas in the form of dreams may be generated in the brain from impressions previously 

 received while awake, or trains of thought may be gradually extended from the moments 

 immediately preceding sleep into the insensible condition. 



There may be, during sleep, mental operations of which we have no consciousness or 

 recollection, unconscious cerebration, as it is called by Carpenter. It is well known that 

 we vividly remember dreams immediately on awakening, but that the recollection of 

 them rapidly fades away, unless they be brought to mind by an effort to remember and 

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

 there is a condition of 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 subordinate to its influence, and we are so subject to unusual 

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

 that are absolutely physiological, and to separate those that are slightly abnormal. We 

 cannot assert, for example, that a dreamless sleep, in which our existence is, as it were, 

 a blank, is the only normal condition of repose of the system ; nor can we 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 influence, disordered digestion, etc. We may assume that an entirely refreshing 

 sleep is normal, and that is all. 



That reflex ideas originate during sleep, as the result of external impressions, there 

 can be no doubt ; and we have already alluded to this point under the head of reflex 

 action. The most remarkable experiments upon the production of dreams of a definite 

 character, by subjecting a person during sleep to peculiar influences, are those of Maury. 

 The hallucinations produced in this way are called hypnagogic, and they occur when 

 the subject is not in a condition favorable to sound sleep. The experiments made by 

 Maury upon himself are so curious and interesting, that we quote the most striking of 

 them in full : 



FIEST OBSERVATION. " I was tickled with a feather successively on the lips and 

 inside of the nostrils. I dreamed that I was subjected to a horrible punishment, that a 

 mask of pitch was applied to my face, and then roughly torn off, tearing the skin of the 

 lips, the nose, and the face. 



