1. External sensory stimuli

The younger Strumpell, the son of the philosopher, whose work on dreams has already more than once served us as a guide in considering the problems of dreams, has, as is well known, recorded his observations of a patient afflicted with general anaesthesia of the skin and with paralysis of several of the higher sensory organs. This man would laps into sleep whenever the few remaining sensory paths between himself and the outer world were closed. When we wish to fall asleep we are accustomed to strive for a condition similar to that obtaining in Strumpell's experiment. We close the most important sensory portals, the eyes, and we endeavour to protect the other senses from all stimuli or from any change of the stimuli already acting upon them. We then fall asleep, although our preparations are never wholly successful. For we can never completely insulate the sensory organs, nor can we entirely abolish the excitability of the sensory organs themselves. That we may at any time be awakened by intenser stimuli should prove to us "that the mind has remained in constant communication with the external world even during sleep." The sensory stimuli that reach us during sleep may easily become the source of dreams.

There are a great many stimuli of this nature, ranging from those unavoidable stimuli which are proper to the state of sleep or occasionally admitted by it, to those fortuitous stimuli which are calculated to wake the sleeper. Thus a strong light may fall upon the eyes, a noise may be heard, or an odour may irritate the mucous membranes of the nose. In our unintentional movements during sleep we may lay bare parts of the body, and thus expose them to a sensation of cold, or by a change of position we may excite sensations of pressure and touch. A mosquito may bite us, or a slight nocturnal mischance may simultaneously attack more than one sense- organ. Observers have called attention to a whole series of dreams in which the stimulus ascertained on waking and some part of the dream-content corresponded to such a degree that the stimulus could be recognized as the source of the dream.

I shall here cite a number of such dreams, collected by Jessen (p. 527), which are traceable to more or less accidental objective sensory stimuli. Every noise indistinctly perceived gives rise to corresponding dream- representations; the rolling of thunder takes us into the thick of battle, the crowing of a cock may be transformed into human shrieks of terror, and the creaking of a door may conjure up dreams of burglars breaking into the house. When one of our blankets slips off us at night we may dream that we are walking about naked, or falling into water. If we lie diagonally across the bed with our feet extending beyond the edge, we may dream of standing on the brink of a terrifying precipice, or of falling from a great height. Should our head accidentally get under the pillow we may imagine a huge rock overhanging us and about to crush us under its weight. An accumulation of semen produces voluptuous dreams, and local pains give rise to ideas of suffering ill-treatment, of hostile attacks, or of accidental bodily injuries....

"Meier (Versuch einer Erklarung des Nachtwandelns, Halle, 1758, p. 33) once dreamed of being attacked by several men who threw him flat on the ground and drove a stake into the earth between his first and second toes. While imagining this in his dream he suddenly awoke and felt a piece of straw sticking between his toes. The same author, according to Hemmings (Von den Traumen und Nachtwandlern, Weimar, 1784, p. 258), "dreamed on another occasion, when his nightshirt was rather too tight round his neck, that he was being hanged. In his youth Hoffbauer dreamed of having fallen from a high wall, and found, on waking, that the bedstead had come apart, and that he had actually fallen on to the floor.... Gregory relates that he once applied a hot-water bottle to his feet, and dreamed of taking a trip to the summit of Mount Etna, where he found the heat of the soil almost unbearable. After having a blister applied to his head, another man dreamed of being scalped by Indians; still another, whose shirt was damp, dreamed that he was dragged through a stream. An attack of gout caused a patient to believe that he was in the hands of the Inquisition, and suffering the pains of torture (Macnish)."

The argument that there is a resemblance between the dream-stimulus and the dream-content would be confirmed if, by a systematic induction of stimuli, we should succeed in producing dreams corresponding to these stimuli. According to Macnish such experiments had already been made by Giron de Buzareingues. "He left his knee exposed and dreamed of travelling on a mail- coach by night. He remarked, in this connection, that travellers were well aware how cold the knees become in a coach at night. On another