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the same activity is said to become worn out and rendered torpid in the same organ; and, hence, to be incapable of carrying off the fine fluid which is perpetually exhaling from the secernent vessels into the ventricles of the brain.
Nothing, however, can be more unfounded than both these conjectures, and it is difficult to determine which of the two is the most so. But we are in no want of either of them, for we are in no want of the pressure which they are invented to account for. The principle of exhaustion alone will, I trust, be found sufficient to answer every purpose as a general cause of natural sleep; and, were it possible for us to add that of local pressure, the sleep would no longer be natural, but morbid.
Before we proceed farther, however, I will just hint that Dr. Cullen supposes the nervous Auid or power to be disposed by nature to an alternating state of torpor and mobility.* He does not admit that it is ever exhausted and restored as a secretion it and hence in sleep it is only suspended : and in consequence of this suspension the exercise of sense and volition is suspended, also. I Narcotics do not, therefore, in his view, exhaust, but only suspend the nervous power or fluid, and thus induce sleep, which consists in such suspension. The apparently stimulant power of narcotics he derives from the vigilant exertion of the vis medicatrix naturæ,-the instinctive effort of nature to guard against such suspension of vital power as essentially mischievous, and, when carried to an extreme, fatal: and hence, narcotics are with him directly sedative, but only indirectly stimulant. poses both sleep and waking to take place upon each other merely by a law of alternation: an explanation that will satisfy few.
But the chief attention of physiologists, both ancient and modern, has been directed to the subject of dreaming, which has usually but erroneously been regarded as a distinct process from that of sleeping. Let us next, therefore, as briefly as may be, and before we enter into a direct analysis of the phenomena that successively arise, take a glance at a few of the conjectures by which dreaming has hitherto been accounted for.
Among the Greek philosophers we meet with two explanations that are worthy of notice; that of Epicurus, because of its ingenuity, and that of Aristotle, because it has descended to the present times.
According to the Epicurean hypothesis of sensation, all the organs of ex. ternal sense are stimulated to their appropriate functions, by the friction of an effluvium or emanation thrown off from the body perceived. This doctrine, which still holds good, and is uniformly employed in modern times to explain the senses of taste and smell, was equally extended by Epicurus to those of sight and hearing: the former being supposed to depend upon an effluvium of exquisitely fine films, images, or species, as they were technically called, perpetually issuing in every direction from every existing substance, somewhat in the manner in which snakes and grasshoppers cast off their skins annually, but almost infinitely finer, and altogether invisible. And as these rush against the eye, they were conceived to convey to it a perfect image of the object from which they are ejected. While sound was supposed to be excited in like manner by particles of a peculiar kind thrown off from the sonorous body, and rousing the ears by their appropriate stimulus.
These effluvia of every kind were conceived to be so exquisitely attenuate that they can pass, as light, heat, or electricity does, through a variety of solid bodies, without being destroyed in their passage. The effluvia or pellicles of vision were supposed not unfrequently to arise from the very bodies of those that have been long buried; and to be capable not only of transpiercing the soil in which they are inhumed, and of stimulating the organs of external sight, but of winding through the substance of the flesh, and of stimulating the soul itself in the interior of the animal frame, especially when
Dreaming. Sce Transactions of the Association of Fellows and Licentiates of the King's and Queen's
f Ib. p. 223.
Ib. p. 226.
in a state of sleep, in which the external sense is closed, or of deep abstraction, in which it is inattentive; and thus of presenting to the soul in its naked state, as it may be called, pictures of objects no longer in existence. And hence these philosophers, with great ingenuity, though, as it now appears, with great incorrectness, undertook to solve many of the most difficult problems in nature; accounted for the casual appearance of spectres in the gloom of solitude and retirement, and directly unfolded to the world the * stuff that dreams are made of."
It is needless to point out the errors of this system, for it has long sunk into disuse, never to rise again. And I shall therefore proceed to the rival hypothesis of Aristotle, which, though equally unfounded in fact, has been fortunate enough to descend to modern times, and to have met with very powerful advocates in M. Wolff* and M. Formey. It was the doctrine of Aristotle, that external sensations not only produce by their stimulus a variety of INTELLECTUAL FORMS or images in the sensory, somewhat similar to the ideas of Plato, and for all practical purposes not very dissimilar to what is meant by ideas in the present day, but that these forms or ideas are themselves capable of producing another set of forms or ideas, though of a more airy and visionary kind :
As every shadow has itself a shade.
And to this secondary set, these slighter and more attenuate pictures of things, he gave the name of PHANTASMS. In the opinion of this philosopher, dreams consist alone of these phantasms, or mere creatures of the imagination, first excited by some previous motion or sensation in the brain, and afterward continued in a more or less perfect series, according to the power of the imagination itself. The only difference I am able to trace between this theory, as started by Aristotle, and as restarted by Wolff, is in the greater regularity that the latter assigns to the phenomena of dreaming, than the former does: M. Wolff believing them to be, in their commencement, excited by a sensation, and in their succession and series of representations to be as much controlled by a peculiar system of laws, as the motions of the heavenly bodies. Formey appears to carry this point a little farther: his language is, if the dream be natural, it must necessarily originate agreeably to the law of sensation, and be continued by the law of imagination; and hence he concludes those dreams to be supernatural, which either do not begin by sensation or are not continued by the law of imagination.
It may be sufficient to remark upon this theory, first that the phantasms of Aristotle have as little claim to entity as the species of Epicurus; next, that the assumption of a code of laws, or rather of two distinct codes of laws, to regulate the fleeting train of our ideas in dreaming, is in itself altogether visionary and gratuitous; and that if the term chance or fortuitousness, a very useful term and full of meaning in all languages, can with propriety be applied to any thing, there is no subject to which it can be better applied than to that of dreaming; in which the will
, the only legislator and controller of our ideas, has withdrawn its authority, and left the brain to a temporary law. lessness and misrule; and, lastly, that the distinction wbich is thus attempted to be drawn between natural and supernatural dreams is not only altogether fanciful, but could never be of any possible avail, even if well founded; for, in order to distinguish between the two, it would be necessary to be intimately acquainted with those laws of sensation and imagination which are here stated to regulate our natural dreams, and the suspension of which produce dreams of a superior character.
We are touching upon a delicate, and, perhaps, a dangerous inquiry; but as it has been boldly handled in modern times, and made the foundation of a more daring speculation upon the subject, it must not be flinched from in our present discussion. That total absence of all natural law, which M. Formey supposes occasionally to take place in the act of dreaming, and to distinguish the supernatural from the natural vision, Mr. Andrew Baxter,* and, since his time, Bishop Newton, conceive to take place in every instance of dreaming; and hence, that dreaming is at all times, and on all occasions, a supernatural operation. These excellent men divide dreams into two kinds, good and evil; and conceive two kinds of agents, good and evil spirits, employed in their production; and, consequently, account for the one or the other sort of dreams, in proportion as the one or other kind of agents obtains a predominancy.
* Psychol. Empir. sec. 123
† Mém. de l'Acad. de Berlin, it.
Now it must be obvious that this conjecture is just as destitute of all tangible basis as either of the preceding; that it can make no appeal to facts submitted to the senses. But, beyond this, its very foundation-stone consists of a principle that no man can readily grant who maturely weighs its full import; namely, that dreaming is altogether an unnatural operation; that nearly onehalf of our lives is spent in a direct intercourse with invisible beings; and that during this moiety of his existence man is no longer a free agent; his whole train of thoughts and ideas being not loose and dismantled, but run away with by foreign compulsion, and the work of a demoniacal possession.
The difficulties into which such an explanation throws its adherents are incalculable. Let us confine ourselves to one more example. There can be no doubt that other animals have their dreams as well as man, and that they have them as vigorous and as lively. Every one has beheld his favourite dog, while asleep by the fireside in the winter season, violently stretching out his limbs, howling aloud, and at times starting abruptly, beneath the train of images of which his dream is composed. In what manner will such philosophers account for these various phenomena ? Is dreaming a natural operation ? or are good and evil spirits the natural attendants upon dogs and cats, as well as upon mankind ? The one or the other of these conclusions must follow; and there can be no difficulty in determining which of them will possess the general suffrage.
That dreams, like every other occurrence in nature, may occasionally become the medium of some providential suggestion, or supernatural communication, I am by no means disposed to deny. That they have been so employed in former times is unquestionable ; and that they have been so employed occasionally arnong all nations in former times is highly probable; and the peculiar liveliness with which the trains of our dreaming ideas are usually excited, and from a cause which I shall presently endeavour to explain, seems to point out such a mode of communication as peculiarly eligible. But I am at present attending to the natural phenomena alone, and can by no means enter into a consideration of such foreign interference, which, as it certainly has been, may still therefore be, for all we can prove to the contrary, occasionally introduced into them.
In what may be called our own times, there are many valuable writers who have turned their attention to this curious subject, and who concur in the two following important positions: first, that the faculty, or at least the action, of the will is suspended during the influence of sleep: and, secondly, that in consequence of this suspension or discontinuance, the trains of ideas which persevere in rushing over the mind, are produced and catenated by that general habit of association which catenates them while we are awake. The power of the will, it is contended, is not necessary to the existence of ideas, which, therefore, may continue while such power is in a state of abeyance; but which, if they continue at all, must take the general order and succession imprinted upon them by the law of association, excepting in cases in which such law is broken in upon a variety of incidental circumstances, as uneasiness arising from a surcharged stomach or other bodily sensations.
Such are the two fundamental principles upon which the theories of Hart. key, Darwin, and Dugald Stewart, are respectively built; and which, in various ways, and with almost equal ingenuity, they seem very satisfactorily to
An Inquiry into the Nature of the human soul, wherein the Immortality of the Soul is evinced from the Principles or Reason and Philosophy, 4to. 1730.
have established. But there is still a very important question, and which, indeed, constitutes the chief difficulty of the subject, and that which none of them have attempted to answer, or, at least, have satisfied themselves upon, while making such attempt. I mean, whence comes it to pass that ideas can at all exist in the brain during sleep, or that all the internal senses are not as much locked up as the external senses, and the faculty of the will?
In the course of the present lecture it will be my endeavour to account for this most curious phenomenon. But we must first follow up, in the series in which they appear to arise, the train of circumstances which accompany sleep and dreaming. The entire study is highly interesting, but requires close attention, in order to its being fully comprehended. And when we have advanced thus far, we shall obtain a clew, if I mistake not, to those equally abstruse and intimately connected subjects, sleep-walking, revery, and wintersleep; as well as to various other obscurities that ramify from the same source.
The fibres distributed over the moving organs of animals, I have already had occasion to observe, in a preceding lecture,* are of two sorts : those of the nerves, which are called sensitive fibres, and those more properly belonging to the muscles, which are called irritative fibres; which last, however, are always accompanied by a greater or less number of the former; by which, indeed, they become endowed with the sense of touch, as well as are rendered capable of contributing to the other external senses, and of maintaining a communication with the brain, from which the sensitive fibres issue, or in which they terminate.
Both these kinds of fibres become fatigued, exhausted, and torpid, in proportion to the length and violence of their exertion, and recover their power alone by rest. The weariness and flaccidity of the muscles of the arms or legs after extreme exercise, or exercise to which they have not been accustomed, may be adduced as a sufficient proof of the truth of this position.f In like manner, we neither hear, nor see, nor taste, nor feel, with the same accuracy, after any or all the organs of these various functions have been long upon the full stretch of action, with which we do on their first exertion in the morning. Increase or prolong this action, and their power will be still farther obtunded, till at length, like an over-wearied limb, they become persectly inert and insensible, and give no account of whatever is passing around us; and it is this general torpitude or inaction of all the external senses, which we call SLEEP. By the exercise of the will, or by any other strong stimulus, this sleep or sensorial torpitude may be postponed; and, vice versa, by the consent of the will, it may be accelerated.
This, however, is sleep in its first or simplest shape alone: it is that which I shall take leave to call Slumber, and is the mere sleep, or torpitude of the organs of external sense; the will being drowsy, indeed, but still continuing in some degree awake: whence the sleeper, if he lie or sit in any uneasy position, exercises his muscles, which are still under the control of the will, and the position is changed. The other internal senses also, as those of memory, imagination, and consciousness, are in like manner, in a greater or less degree, awake; whence the mind is yet filled with ideas, that crowd upon one another with about an equal degree of regularity and consusion: and, if we be spoken to in this state, we return an answer, which intimates, indeed, that we have heard ; but, by its incongruity with the observations made to us, intimates also that the will has, in some degree, lost its control ;—that it has become drowsy, and is affected by the slumber of the organs of external sense.
* Series 1. Lecture x. p. 107. + The principles of the theory here advanced were first given to the world, by the author, as far back as 1805, in the comment subjoined to his translation of Lucretius, where the poet is treating of the cause and phenomena of sleep; and may be found in vol. ii. p. 137–141 of that work. Several of the doctrines inere laid down have been since advanced in various forms by different writers, though in some cases, very probably, without their having perused his explanation. Thus the immediate cause of sicep, advanced in the present passage, is that chictly rested upon by the author of the article on sleep in Dr. Rces's Cyclopædia, though he also adverts to an occasional increased action in the vessels of the brain as a concurrent
And thus much of the explanation wluch will here be found to follow, respecting the nature and phenomena of dreaming, have still more lately becii oflerell to the world by Dr. Spurzheim, and adopted from hiin hy Mr. Carmichael of Dublin, with the exception that they have interwoven such views with their peculiar doctrine of a plurality of organs in the brain; whichi, for reasons that will be given in a subsequent lecture (dcries i11. Leciure x11.), the present aullor cannot adınit; and docs not conecive is by any means necessary on the present occasion. Such coincidences of' opinion, however, and especially if they should be accidental, and not derived from his comment on Lucrctius, give a considerable degree of confirmation to the general basis on which the theory rests. The lecture, as now publishod, was delivered in the spring of 1811.
If the general exhaustion be not very considerable, as after dinner, or during the digestion of any other meal, the sleep may not extend beyond this first or simple stage of slumber; though it should be observed that, from the power of association, the internal and external senses have a strong tendency, if in health, to concur or catenate in one common state or action. When the one are in full vigour, the other are usually in full vigour also; and when the one become drowsy, the other incline to the same drowsiness. But if the general exhaustion be more violent than we are now contemplating, the internal senses will unquestionably concur in the effect, and evince, in some or all of them, an equal degree of sleep.
The first of the internal senses that becomes thus influenced is the will itself. It would be easy to show, if we had time, that the will is infinitely more disposed to catenate with the motions of the external senses than any of the other faculties of the mind. It hence gives way first of all, and sleeps along with the exterior organs, while the other faculties of the mind remain awake. We are now arrived at the second stage of sleep; and it is this which we call and which constitutes DREAMING. The will catenates in the sleep of the organs of exterior sense; but all the interior senses, except the will, are still awake. Hence we have ideas of memory, ideas of consciousness, ideas of imagination, ideas of reasoning: but, destitute of a controlling power, they rush forward with a very considerable degree of irregularity, and would do so with the most unshapeable confusion, but that the power of association still retains some degree of influence, and produces some degree of concert in the midst of the wildest and most extravagant vagaries. And hence that infinite variety that takes place in the character of our dreams; and the greater regularity of some, and the greater irregularity of others.
But the general fatigue and exhaustion may be still more violent; and it may also be produced by motions in which the internal senses have principally co-operated: and in such cases, not the will only, but the whole of the internal senses concur in the common torpor or inertness that is produced : and we now advance to a third state, which I shall beg leave to call LETHARGY : dead, senseless sleep, or a stage of sleep without thought or idea of any kind, but still natural and healthy ; the vital organs, though none but the vital organs, still continuing their action.
It has been a question often proposed, whether the mind ever does, or ever can, exist without thinking? But it can only have been proposed by persons who have not paid a due attention to a variety of phenomena, which are perpetually occurring, and which must be conclusive as to the fact. The mind of an infant, or rather of a fetus, must anticipate the thoughts or ideas that are afterward introduced within it. In a complete paroxysm of apoplexy, no man has ever been conscious of a single thought or idea ; in sleepy coma or lethargy in fevers, as opposed to restless coma, the same discontinuity of all thought and idea takes place uniformly; and we meet with it perhaps still more incontrovertibly in all cases of suspended animation from drowning, hanging, or catalepsy. I enter not into an explanation of this state of being; I only advert to the fact : though if we had time I do not think it would be impossible to suggest an explanation that might be satisfactory to every one.
Thus far we have left the vital or involuntary organs, those over which the will exercises no control, in a state of wakefulness, though none but the involuntary organs. For these, in the first place, are far less subject to exhaustion than the organs either of external or internal sense; their actions in a state of health being always more equable and uniform: and hence, secondly, from an independence most wisely ordained, and productive of the utmost benefit to the general system, they never catenate with any other actions, except in cases of extremity. Upon an application, however, of very strong stimuli, whether external, as those of severe pain or labour, or internal,