648 NERVOUS SYSTEM. 



of resistance to cold becomes rapidly diminished, and death is the result. 

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

 able 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 impressions of sound are not 

 appreciated, and sometimes there is a dreamless condition, in which all 

 knowledge of existence is lost. 



There may be, during sleep, mental operations of which there is no con- 

 sciousness or recollection, unconscious cerebration, as it was called by Car- 

 penter. It is well known that dreams are vividly remembered immediately 

 on awakening, but that the recollection of them rapidly fades away, unless 

 they be brought to mind by an effort to recall and relate them. Whatever 

 be the condition of the mind in sleep, if the sleep be normal, there is repose 

 of the cerebro-spinal system and an absence of voluntary effort, which re- 

 store 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 the 

 organism is so subject to unusual mental conditions, that it is difficult to 

 determine with exactness the phenomena of sleep that are absolutely physio- 

 logical and to separate those that are slightly abnormal. It can not be as- 

 sumed, 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 pos- 

 sible 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 upon 

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

 ing sleep to peculiar influences, have been recorded. The hallucinations 

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

 the subject is not in a condition favorable to sound sleep. 



As regards dreams due to external impressions, it is a curious fact, which 

 has been noted by many observers and is one which accords with the per- 

 sonal experience of all who have reflected upon the subject, that trains of 

 thought and imaginary events, which seem to pass over a long period of 

 time in dreams, actually occur in the brain within a few seconds. A person 

 is awakened by a certain impression, which undoubtedly has given rise to 

 a dream that seemed to occupy hours or days, and yet the period of time be- 

 tween the impression and the awakening was hardly more than a few seconds ; 

 and persons will drop asleep for a very few minutes, and yet have dreams with 

 the most elaborate details and apparently of great length. It is unnecessary 

 to cite the accounts of literary compositions of merit, the working out of 



