for which visible images are excited in sleep, like to the objects with which we converse when awake, the same reasons will hold for changes of apparent magnitude and position also; and these changes in fixed objects, being constantly associated with motions in ourselves when awake, will infer these motions when asleep. But then we cannot have the idea of the vis inertiæ of our own bodies, answering to the impressions in walking; because the nerves of the muscles either do not admit of such miniature vibrations in sleep; or do not transmit ideas to the mind in consequence thereof; whence we appear to sail, fly, or ride. Yet sometimes a person seems to walk, and even to strike, just as in other cases he seems to feel the impression of a foreign body on his skin.
Those who walk and talk in their sleep, have evidently the nerves of the muscles concerned so free, as that vibrations can descend from the internal parts of the brain, the peculiar residence of ideas, into them. At the same time the brain itself is so oppressed, that they have scarce any memory. Persons who read inattentively, i.e. see and speak almost without remembering, also those who labour under such a morbid loss of memory, as that though they see, hear, speak, and act, pro re nata, from moment to moment, yet they forget all immediately, somewhat resemble the persons who walk and talk in sleep.
Fifthly, Dreams consist chiefly of visible imagery. This agrees remarkably with the perpetual impressions made upon the optic nerves and corresponding parts of the brain during vigilance, and with the distinctness and vividness of the images impressed.
We may observe also, that the visible imagery in dreams is composed, in a considerable degree, of fragments of visible appearances lately impressed. For the disposition to these vibrations must be greater than to others, cæteris paribus, at the same time that by the imperfection and interruption of the associations, only fragments, not whole images, will generally appear. The fragments are so small, and so intermixed with other fragments and appearances, that it is difficult to trace them up to the preceding day; the shortness of our memory contributing also not a little thereto.
It happens in dreams, that the same fictitious places are presented again and again at the distance of weeks and months, perhaps during the whole course of life. These places are, I suppose, compounded at first, probably early in youth, of fragments of real places, which we have seen. They afterwards recur in dreams, because the same state of brain recurs; and when this has happened for some successions, they may be expected to recur at intervals during life. But they may also admit of variations, especially before frequent recurrency has established and fixed them.
Sixthly, It has been observed already, that many of the things which are presented in dreams, appear to be remembered by us,
