316 SPECIAL PHYSIOLOGY. 



sible to the most powerful external impressions. When sleep is of a 

 light character, very slight stimuli suffice to rouse the individual; in 

 such a condition, many ordinary occurrences are perceived, and not. 

 unfrequently, the resulting ideas interweave themselves in the forma- 

 tion of dreams. 



During sleep, indeed, although the mind is insensible to external 

 impressions, yet the mental faculties may be in a state of internal ac- 

 tivity, simple ideas being formed, or even general notions conceived. 

 This state constitutes dreaming. The current of thought, in this con- 

 dition, is totally independent of the will. Ideas commonly follow each 

 other, in a more or less incongruous manner, sometimes, however, in a 

 uniform and regular order. The character of dreams is influenced by 

 the mental condition in the waking state; hence, when the mind has 

 been busily occupied during the day with certain ideas, these frequent- 

 ly form the subject of dreams at night; so also, when laboring under 

 depressing emotions, dreams during sleep are of a mournful and agi- 

 tating character. The reasoning faculties are sometimes correctly 

 exercised; cases are on record, of mathematicians solving the most 

 difficult problems in their dreams. 



One of the most remarkable and characteristic phenomena of dream- 

 ing, is the rapidity with which ideas pass through the mind, events of 

 a lifetime sometimes appearing to occupy but a few seconds. There 

 is frequently total inability to perform certain movements, however 

 great the wish to do so may be; the inability to strike a desired blow, 

 or to escape from, or avoid an imaginary danger in a dream, notwith- 

 standing all our efforts, affords a familiar example of this fact. This, 

 as well as the simple sensation of weight upon the chest, is a form 

 of incubus or nightmare. It is unknown whether dreaming may be 

 prolonged during the whole period of sleep, or whether it is confined 

 to short intervals between sleeping and waking. The former view is 

 somewhat supported by the occurrence of repeated movements and 

 ejaculations, occasionally observed even in heavy sleepers, and of which 

 they lose all recollection on awaking. But the prevailing opinion is, 

 that dreams are only possible in light imperfect sleep, and that they are 

 incompatible with the condition of the mind in profound sleep. Those 

 dreams which occur during the short interval between sleep and wak- 

 ing, are certainly the best remembered. In light sleep, at the begin- 

 ning of sleep, when the activity of individual parts of the cerebrum 

 has not yielded to the general state of repose, or, at the end of sleep, 

 when those parts have already regained some degree of consciousness, 

 certain mental faculties come into operation, and by a process of com- 

 bination, form an ideal and imaginary world. The whole picture is so 

 unreal, and the dreamer is so conscious of it, that often, even during 

 sleep, he knows that he is dreaming; so conscious, indeed, is he of this, 

 that he can either prolong the dream, or at once put an end to it. This 

 condition is, therefore, hardly one of real dreaming, but approaches more 

 nearly to the waking state. Such dreams remain impressed on the 

 memory, with more or less distinctness. 



Though, in certain dreams, external impressions may be correctly 

 perceived and combined, it often happens that they are misapprehended, 



