THEORIES OF SLEEP. 453 



any subjective knowledge of mental operations, or we may 

 have more or less connected trains of thought. There is, 

 also, as a rule, absence of voluntary effort, though move- 

 ments may be made, to relieve discomfort from position or 

 external irritation, without awakening. The sensory nerves 

 retain their properties, though the general sensibility is some- 

 what blunted ; and the same may be said of the special senses 

 of hearing, smell, and probably of taste. The peculiar dreams, 

 induced in the case of Maury by red lights, show that the 

 sense of sight is not entirely lost. There is every reason to be- 

 lieve, however, that the functions of the sympathetic system 

 are not disturbed or affected by sleep, if we except the action 

 of the vaso-motor nerves upon the circulation in the brain. 



Two opposite theories have long been in vogue with re- 

 gard to the immediate cause of sleep. In one, this condition 

 is attributed to venous congestion 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 natu- 

 ral sleep ; for here, the functions of the brain are suspend- 

 ed, there is no consciousness, no dreaming, and the con- 

 dition is manifestly abnormal. In animals rendered coma- 

 tose by opium, the brain may be exposed and is found 

 deeply congested with venous blood. The same condition 

 often obtains in profound anaesthesia from chloroform, but a 

 state of the brain very nearly resembling normal sleep is 

 observed in anaesthesia from ether. These facts have been 

 positively demonstrated by experiments upon living ani- 

 mals, 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. 1 In view of these facts, and 

 others which will be stated hereafter, it is unnecessary to 



1 HAMMOND, Sleep and its Derangements,, Philadelphia, 1869, pp. 26, 32. 



