OF SLEEP, AND DREAMING.
W'E have now been considering the functions and pow ers of the living body as acting according to certain gene rat laws, and proceeding in a regular course without in terruption or deviation. But this, we are aware, is by no means the case; we are not only subject to a variety of accidental injuries, constituting disease, but the very na ture of the body itself is such as to lead us, by an inevita ble progress, through the successive stages of growth, maturity, decline, and decay. But before we enter upon this subject, there is a peculiar kind of interruption to the actions of the system, which we must attend to,—that which constitutes sleep.
In considering the phenomena of sleep, two topics es pecially present themselves for our consideration ; 1st, In what does sleep essentially consist ? and, 2d, What physi cal change takes place in any part of the muscular or ner vous system, which ought to be regarded as the efficient cause of sleep ? With respect to the first query, if we watch the approach of sleep, and compare this with the state after it is fully formed, we shall find that its most obvious characteristics are the loss or diminution of the power of the will, and the incapacity of receiving impressions from ex ternal objects. The functions that serve for the support of life, both physical and intellectual, are many of them continued with full activity ; the circulation, the respira tion, and the digestion, proceed nearly in their ordinary manner, and our ideas often flow with great rapidity, and are perhaps even more vivid than in our waking hours. But during this time we are insensible to the impression of external objects ; and the power of volition is altogether suspended. It is upon the loss of power over the volun tary muscles that the complete relaxation of all the parts of the body depends which occurs in sleep, for every posi tion which we assume during our waking hours, although from habit we arc unconscious of it, requires the constant exercise of the will. It is, however, to be remarked, that although we are so nearly insensible to all external im pressions in sleep, we retain our sensibility to internal stimuli, as appears both by the continued action of many of our functions, and by the various feelings of pain and uneasiness to which we are subject at this time.
It is upon this combination of circumstances that the curious phenomena of dreaming seem to depend ; we are insensible to external objects, while the nervous system retains a considerable degree of activity, and is liable to be affected by various internal feelings. Dreams consist of a succession of ideas, that pass rapidly through the mind, and frequently excite very vivid emotions, but which differs from our train of waking thoughts in being independent of external objects, and, as it appears, not under the control of the will. Hence, in dreams, we have no conception of time and place, we confound past and present events, annihilate distances, and fall into all kinds of inconsistencies and incongruities. In dreams our ideas would appear to follow each other principally in consequence of association, whereas, while we are awake, although our thoughts arc much influenced by associa still they are perpetually diverted into new channels by the intervention of external impressions, or by our vo luntary control over them. Dreams are, in many cases, influenced by those circumstances which have produced the strongest impression upon us during our waking hours, but we do not find this to be invariably the case ; and it is frequently no easy matter to explain how the first link in the chain of associated ideas has been excited. It probably depends upon some accidental impression made upon the nervous system, at the moment of dropping asleep, by which the previous series of ideas being once broken, the new train acquires complete possession of the mind, to the entire exclusion of the former. It must, however, be remarked, that we are not altogether insensi ble to external impressions during sleep, and that, on some occasions, our dreams are evidently derived from sonic sources of this kind.