Page:Encyclopædia Britannica, Ninth Edition, v. 7.djvu/477

DREAM 457 In addition to these simple metaphysical and psycho logical theories of dreaming, there are to be found no less simple physiological hypotheses. Among these we may take the opinion of Hobbes (Leviathan), that the imagina tions of dreams all proceed from "the agitation of the inward parts of a man s body, 1 the disturbance of which parts, owing to their connections with the brain, serves to keep the latter in motion. Another simple physiological hypothesis for explaining dreams is offered by Schopenhauer. According to this writer, the exciting causes of dreams are impressions received from the internal regions of the organism through the sympathetic nervous system. These impressions are afterwards worked up by the mind into quasi-realities by means of its " forms " of space, time, <fec. This simple and "geometric" method of explaining dreams, though it may be valuable up to a certain point, must necessarily fail to account for all the phenomena con cerned. As we have shown in our preliminary description of dreams, their contents vary within very wide limits, and cannot therefore all be referred to one or two simple principles whether mental faculties or bodily stimuli ; also, it is by no means safe to affirm of any mental function that it is universally absent in dreams, since the second mental processes, as Sir H, Holland and M. Maury point out, enter in very unequal degrees into different dreams. A full and exhaustive theory of dreaming would seem to include several distinct lines of inquiry. Among these there are three which have already been well denned by recent writers on the subject. The first relates to the sources of dream-imaginations, or the stimulations which are the immediate causes of these. The second question has to do with the order or form of dream-combinations, and seeks to determine the conditions of the peculiar arrangements, simultaneous and successive, which are observable in dreams. The last problem is that of accounting for the objective reality and generally for the intensity and impressiveness of dream-fancies. In briefly opening up each of these lines of inquiry we shall seek to keep in mind the variable as well as the con stant features of dreaming ; also we shall proceed, as far as possible, according to that double method of study, the psychological and the physiological (subjective and objective), which offers itself to those who accept the idea of a perfect parallelism between mental and bodily events. (A) The Sources of Dream Materials. The numerous images which make up the ever-renewed current of a dream appear sometimes to come from the internal depths of the mind itself. In other cases, as even the ancients recognized, they depend on a stimulation of the brain arising from vary ing conditions of the bodily organs. According to the view that all mental events have their physical accompaniments, the first class of imaginations must also be referred to cer tain conditions of the brain and nervous system. These various sources of dream-activity are roughly classified by Hartley in his Observations on Man. Dream-images, he tells us, are deducible from three causes : (1) impressions and ideas lately received ; (2) present state of the body (especially the stomach and the brain) ; (3) association. The large part played by bodily states in our dream-life is recognized not only by physiologist?, as Maury, but also by those who ascribe dreams in part to occult spiritual faculties, as Schemer. By help of the results of recent researches we are able to improve a little on Hartley s classification. The exciting causes of dream-images full into two main classes : (I.) peripheral, and (II.) central stimulations. The latter arise in the outlying parts of the nervous system, namely, the organs of sense, the muscular apparatus, the internal bodily organs, together with the external portions of the nerves connected with these. Central stimulations are such as arise mainly, if not entirely, within the encephalic region. These again are either (a) direct, or ((3) indirect. The first depend on the condition of the nerve-elements acted upon, and the unknown influences (possibly connected with the condition of the circulation) brought to bear on these at the moment. The indirect stimulations arise as a result of some preceding excitation in a connected region of the brain. The former underlie the apparently spontaneous imaginations of dream ing, as well as those which are the echo of a recent waking experience. The latter are the physical counterpart of images or ideas called up by association with preceding images or thoughts. (I.) Among peripheral stimulations are to be noticed (a) those which arise from the action of external objects on the organs of sensation. Recent researches show that these may play an important part in dreams. Dr Beattie speaks of a man who could be made to dream about a subject by whispering into his ear. Experiments were made by M. Giron do Buzareingnes (Journal de Physiol. torn, viii.) as to the effects of external impressions on dreaming, Thus, by leaving his knee uncovered during sleep, he dreamt he was travelling in a diligence (where knees are apt to get cold). The most elaborate experiments bearing on this point have been carried on by Alf. Maury, with the help of an assist ant. The latter produces some external stimulation while the experimenter sleeps ; he is then wakened up so as to record the dream immediately resulting. By this means important results were reached. When, for example, his lips were tickled, he dreamt that he was subjected to horrible tortures, that pitch plaster was applied to his face and then torn off. Sensa tions of hearing, smell, and taste were also followed by appropriate though greatly exaggerated images. Wundt (Physiologische Psychologic) thinks that cutaneous sensa tions, arising from the varying pressure and temperature of the bodily surface, are frequent excitants of dream-images. (/3) Along with such objective sensations must be reckoned subjective sensations which arise in the absence of external stimuli, and which have as their physical basis certain actions in the peripheral as well as the central regions of the nerves. Of such are the visual images (Schliimmer- bilder) seen by J. Miiller, Goethe, Purkinje, and others, when the body is disposed to sleep. These are called the dream- chaos by Gruithuisen, since they are supposed to form the raw material of dreams. Maury gives a full account of these phenomena, which he terms " hypnagogic hallucina tions," and which appear to include not only visual images but also subjective sensations of sound, touch, &c. He attaches great importance to the action of these subjective sensations in dreams. The predominance of visual imagery in dreaming appears to be connected with the great activity of the organ of sight and its consequent excitability. It is to be added that one can only roughly distinguish these subjective sensations, which involve the peripheral regions of the nervous system, from images supposed to be con fined to the central regions. (y) The conditions of our muscles during sleep, which somehow convey impressions to the brain, affect consciousness, and so influ ence dreaming. To this source we must refer the active phenomena of dreams, as running, flying, resisting, struggling, &c. It is probable, as Wundt remarks, that the movements of the body during sleep, as those of breathing and the extensions and contractions of the limbs, give rise to dream fancies, and painful conditions of the muscles due to an awkward position of the limbs may also serve to excite images. (8) Among the most frequent excitants of dreams are organic or systemic sensations con nected with the varying states of the internal bodily organs. The prominence given to this source of dreaming

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