THE CHEMISTRY OF SLEEP. 231 



ena presented in natural and in artificial sleep, including its 

 morbid forms, with those forms in which the mind sleeps with 

 the body awake, and those in which the body may sleep with the 

 mind more or less awake. 



Ansesthesia in general, inclusive of its widely differing suc- 

 cessive stages, as that of exaltation of some mental faculties, with 

 or without full consciousness, or one of the waking dream condi- 

 tions (see further on) that of extinction of full consciousness 

 with the muscular system still awake and, finally, that of com- 

 plete lethargy of the mind and body with extinction of all sensa- 

 tion. 



An attempt, which must needs yet be quite crude, to general- 

 ize something useful from the sparse and scattered array of facts 

 thus found to be available. 



As to a definition of natural sleep, it may be interesting to go 

 back a century and examine the views then held. Having at 

 hand the second edition of the Encyclopsedia Britannica, pub- 

 lished from 177S to 1783, the writer finds therein an article on 

 Sleep, in the tenth volume, which article is brief, being but sup- 

 plementary to that on Dreams, in the fourth volume. To this 

 latter subject we shall return further on. Under Dreams, the 

 following meager definition of sleep is given : " Sleep is a state in 

 which all communication is cut off between our sentient principle 

 and this visible world." By this, taken literally, a blind man 

 would be asleep. But, of course, the word visible was intended 

 to imply the whole world of the senses. Still, allowing such lati- 

 tude, the definition is both inaccurate and inadequate. As to the 

 asserted complete cutting oft' of external impressions from the 

 senses in true sleep : were this so, a sleeping person could not be 

 awakened by the usual means namely, a forcible external im- 

 f)ression upon one or more of the senses. 



Its inadequacy may be briefly illustrated by pointing out that 

 it takes no note of one of the most salient phenomena of sleep 

 that the ivill, though not at all suspended therein, being easily 

 recognized in dreams, altogether loses its rationality and its con- 

 trol over the workings of the mind as well as over those of the 

 body. 



The following may be set forth as an attempt at a reasonable 

 and comprehensive definition, or rather description, of the condi- 

 tions we find in sleep : Sleep is a state in which the impressions of 

 external objects on the senses are dulled, but not annulled or sus- 

 pended ; in which the emotions, the imagination, the memory, 

 and the will are but partially or even not at all suspended, and 

 may even be intensified, while the control of the will over the 

 emotions, the imagination, and the memory is wholly annulled. 



