DREAMS. 449 



There may be, during sleep, mental operations of which 

 we have no consciousness or recollection, unconscious cere- 

 bration, as it is called by Carpenter. 1 It is well known that 

 we vividly remember dreams immediately on awakening, 

 but that the recollection of them rapidly fades away, unless 

 they be brought to mind by an effort to remember and re- 

 late them. Whatever be the condition of the mind in sleep, 

 it* the sleep be normal, there is a condition of repose of the 

 cerebro-spinal system and an absence of voluntary effort, 

 which restore the capacity for mental and physical exertion. 



The impressionability and the activity of the human mind 

 are so great, most of the animal functions are so subordinate 

 to its influence, and we are so subject to unusual mental con- 

 ditions, that it is difficult to determine with exactness the 

 phenomena of sleep that are absolutely physiological, and to 

 separate those that are slightly abnormal. We cannot assert, 

 for example, that a dreamless sleep, in which our existence 

 is, as it were, a blank, is the only normal condition of repose 

 of the system ; nor can we determine what dreams are due 

 to previous trains of thought, to impressions from the exter- 

 nal world received during sleep, and are purely physiologi- 

 cal, and what are due to abnormal nervous influence, disor- 

 dered digestion, etc. We may assume that an entirely re- 

 freshing sleep is normal, and that is all. 



That reflex ideas originate during sleep, as the result of 

 external impressions, there can be no doubt ; and we have 

 already alluded to this point under the head of reflex action. 2 

 The most remarkable experiments upon the production of 

 dreams of a definite character, by subjecting a person during 

 sleep to peculiar influences, are those of Maury. The hallu- 

 cinations produced in this way are called hypnagogic, 3 and 



1 CARPENTER, Principles of Human Physiology, Philadelphia, 1853, p. 784. 



2 See page 300 



3 From its derivation, this term is .properly applied only to phenomena ob- 

 served at the instant when we fall asleep, or when we are imperfectly awakened, 

 and not to the period of most perfect repose. 



