746 NERVOUS SYSTEM. 



rate reasoning. We sometimes commit atrocious crimes in our dreams, without appre- 

 ciating their enormity, and we are often placed in the most absurd and impossible condi- 

 tions, without any idea, at the time, of their extraordinary and unnatural character. 

 This is a fact sufficiently familiar to every one and is one which does not admit of satis- 

 factory explanation. 



We have made no attempt to offer an explanation of the curious psychological phe- 

 nomena presented during sleep, and, indeed, we know little enough of the action of the 

 mind at any time ; but we have merely given the above as examples of what we may call 

 reflex mental phenomena. Somnambulism, general anaesthesia, sleep from hypnotics, the 

 so-called magnetic sleep, ecstasy, catalepsy, trance, etc., are abnormal conditions, which 

 we shall only consider in so far as they resemble natural sleep. 



Condition of the Brain and Nervous System during Sleep. 



As we have already seen, during sleep, the brain may be in a condition of absolute 

 repose at least, as far as we have 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, although movements may be made to relieve discomfort 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. 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 believe, 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 regard to the immediate cause 

 of sleep. In one, this condition is attributed to venous congestion and increased presssure 

 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 here, the functions of the brain are suspended, there is no con- 

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

 comatose 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. In view of these facts and 

 others which will be stated hereafter, it is unnecessary to discuss the theory that sleep 

 is attended with or is produced by congestion of the cerebral vessels. 



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

 entertained by certain physiologists ; but, until within a few years, it has rested chiefly 

 upon theoretical considerations. 



Passing over arguments by the older writers for and against this theory of sleep, we 

 come to the researches of Durham, in 1860, in which it seemed to be demonstrated that 

 the supply of blood to the brain is always greatly diminished during sleep. These experi- 

 ments were made upon dogs. A piece of the skull, about the size of a shilling, 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 in this 

 way were awake, the vessels of the pia mater were seen moderately distended, and the 

 circulation was active ; but, during perfectly natural sleep, the brain retracted and became 

 pale. " The contrast between the appearances of the brain during its period of func- 



