CONDITION OF THE BRAIN DURING SLEEP. 649 



difficult mathematical problems in dreams, etc., some of which are undoubt- 

 edly accurate. If it be true that the mind is capable of forming consecu- 

 tive ideas during sleep which can hardly be doubted there is no good rea- 

 son why these phenomena should not occur and the thoughts should not be 

 remembered and noted immediately on awakening. In most dreams, how- 

 ever, the mind is hardly in a normal condition, and the brain generally loses 

 the power of concentration and of accurate reasoning. 



Condition of the Brain and Nervous System during Sleep. During 

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

 fort from position or external irritation, without awakening. The sensory 

 nerves retain their properties, although the general sensibility 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 vaso-motor nerves upon the circulation in the brain be 

 excepted. 



Two opposite theories have long been in vogue with regard to the imme- 

 diate cause of sleep. In one, this condition is 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 entirely distinct from natural sleep ; for in 

 the former the action of the brain is entirely suspended, there is no con- 

 sciousness, no dreaming, and the condition is manifestly abnormal. In ani- 

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

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

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

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

 demonstrated by experiments upon 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 which will be stated hereafter, it is unneces- 

 sary to discuss the theory that sleep is attended with or is produced by con- 

 gestion of the cerebral vessels. 



The idea that the circulation in the brain is t diminished during sleep has 

 long been entertained by some physiologists ; but until within a few years, 

 it has rested chiefly upon theoretical considerations. The experiments of 

 Durham (1860) seem to demonstrate that the supply of blood to the brain is 

 always greatly diminished during sleep. These experiments were made 

 upon dogs. A piece of the skull was removed with a trephine, and a watch- 

 glass was accurately fitted to the opening and cemented at the edges with 

 Canada balsam. When the animals operated upon were awake, the vessels 



