620 NERVOUS SYSTEM 



nations 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 that accords with 

 the personal experience of all who have reflected on 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 sec- 

 onds. A person is awakened by a certain impression, which undoubt- 

 edly has given rise to a dream that seemed to occupy hours or days, and 

 yet the period of time between the impression and the awakening was 

 hardly more than a few seconds ; and persons will drop asleep for a 

 few minutes, and yet have dreams with elaborate details and appar- 

 ently of great length. 



Condition of the Brain and Nervous System during Sleep. During 

 sleep the brain may be in a condition of absolute repose - at least, so 

 far as there is any subjective knowledge of mental operations or there 

 may be more or less connected trains of thought. There is, also, as a 

 rule, absence of voluntary effort, although movements may be made to 

 relieve discomfort from position or external irritation, without awakening. 

 The sensory nerves retain their properties, although the. general sensi- 

 bility is somewhat blunted ; and the same may be said of the special 

 senses of hearing, smell and probably of taste. There is every reason 

 to believe that the action of the sympathetic system is not disturbed or 

 affected by sleep, if the influence of the vasomotor nerves on the circu- 

 lation in the brain be excepted. 



Two opposite theories were formerly held in regard to the immediate 

 cause of sleep. In one, this condition was attributed to venous conges- 

 tion and increased pressure of blood in the brain, and this view probably 

 had its origin in the fact that cerebral congestion induces stupor or coma. 

 Stupor and coma, however, are distinct from natural sleep ; for in the 

 former the action of the brain is suspended, there usually is no conscious- 

 ness, no dreaming, and the condition is manifestly abnormal. In animals 

 rendered comatose by opium, the brain when exposed is found deeply 

 congested with venous blood. The same condition often obtains in pro- 

 found anesthesia by chloroform, but a state of the brain nearly resem- 

 bling normal sleep is observed in anesthesia by ether. These facts have 

 been demonstrated by experiments on living animals and have been 

 observed in the human subject in cases of injury of the head. When 

 opium is administered in large doses, the brain is congested during the 

 condition of stupor or coma ; but this congestion is relieved when the 

 animal passes, as sometimes happens, from the effects of the agent into 

 a natural sleep. In view of these facts and others that will be stated 



