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 equal'y abstruse and intimately connected subjects, sleep-walking, revery, and winter sleep; 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.† 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 perfectly 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 confusion: and, if *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 1605, 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 there 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 sleep, advanced in the present passage, is that chiefly rested upon by the author of the article on sleep in Dr. Rees's Cyclopadia, though he also adverts to an occasional increased action in the vessels of the brain as a concurrent cause. And thus much of the explanation which will here be found to follow, respecting the nature and phenomena of dreaming, have still more lately been offered to the world by Dr. Spurzheim, and adopted froin him by 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; which, for reasons that will be given in a subsequent lecture (Series 1. Lecture xiii.), the present author cannot admit; and does not conceive 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 Lucretius, give a considerable degree of confirmation to the general basis on which the theory rests. The lecture, as now published, was delivered in the spring of 1811.

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. 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,

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as those of disease or excessive grief, the vital or involuntary organs themselves are fatigued and exhausted; and when the exhaustion is complete, they also, like the organs of external sense, sleep or become torpid: in other words, DEATH ensues, the living principle ceases, and the spirit separates from the body. The resemblance, therefore, between DEATH and SLEEP is not less correct upon the principles of physiology, than it is beautiful among the images of poetry. SLEEP is the DEATH or torpitude of the voluntary organs, while the involuntary continue their accustomed actions. DEATH is the SLEEP or torpitude of the whole.

Every organ of the animal frame recovers from its fatigue or torpor by rest, provided the principle of life continues. Hence the organs of external sense, in a definite period of time, and a period generally proportioned to the degree of their exhaustion, reacquire their accustomed vigour, are alive to the influence of their appropriate stimuli; and the smallest excitement applied to any one of them throws the whole once more into action, in consequence of their habit of acting associately and by common consent. In other words, the man awakes from SLEEP; he rouses himself from the temporary DEATH of the organs of external sense. Were it possible for the principle of life to con tinue during a total rest or torpitude of the vital or involuntary organs, as it does during that of the voluntary, there can be no doubt that these also would, in time, recover from their exhaustion; and that the man would, in like manner, awake from the total torpitude, the sleep or death of the entire frame; but this in man, excepting under very particular circumstances, and circumstances I shall advert to presently, is impossible. The rule of nature is, that as soon as the vital or involuntary functions are discontinued, the principle of life ceases; the soul deserts the body; the laws of chemistry, hitherto held in subjection by a superior control, assert their authority; and the whole visible system falls a prey to corruption and ruin.

When the organs of external sense have recruited themselves by repose, I have already observed that the stimulus that rouses the one rouses at the same time the rest, from a habit of association. From the same habit, the torpitude produced by exhaustion in any single organ is propagated through every other, and the sleep becomes common to the whole: although it is also unquestionable that the whole are fatigued, or partially exhausted, in consequence of the general stock of sensorial power having been borrowed, in a considerable degree, from the rest, and expended at a single outlet.

The sensitive fibres of the organs of external sense are equally affected, and of course become equally exhausted, whether a stimulus be applied at the one end or at the other, the end terminating externally or that connected with the brain: and hence, internal excitements, as those of severe study, intense grief, undue eating or drinking, or febrile diseases, produce the same effect as causes operating from without.

In either case, the sleep or torpitude produced is sound or healthy under a certain degree of exhaustion alone: hence, mankind sleep most refreshingly after a moderate or accustomed fatigue, moderate or accustomed study, moderate or accustomed meals.

If the stimulus be a little increased beyond this medium, an undue and morbid proportion of sensorial power is secreted, which postpones, indeed, the torpitude or sleep for the present, but at the expense of the general strength of the system, and an expense to which the vital organs themselves contribute something: whence a far deeper and heavier sleep or torpitude ensues than would have ensued with a less proportion of fatigue. If such torpitude take place before the vital organs are totally exhausted, it is confined to the organs of sense alone, which hereby progressively recover their accustomed activity and vigour. But if the vital organs be also exhausted before the torpitude ensues, it will be propagated to themselves, the living principle will cease, and the sleep will be the sleep of death. Violent and continued pain or labour, as external stimuli, violent and continued fevers, violent and continued grief, a very inordinate debauch, as internal stimuli, are all liable to produce these effects; and the one or the other will take place in Droportion to their excess and extremity.

If a stimulus affecting the organs of sense, at which end soever applied, be intolerably pungent or forcible, the sensorial power will be exhausted immediately, and the organ directly affected will become instantly torpid. Hence sounds, intolerably loud, make us deaf; excessive light blinds us; acrimonious smells or savours render us incapable of smelling or tasting. And hence an abrupt shock of joy or grief, a sudden and intense paroxysm of fever, large quantities of wine or spirits, as internal causes, produce morbid lethargy, palsy, apoplexy, which are only so many modifications of the sleep or torpitude of the sensitive and irritative fibres. If the same abrupt and violent cause be sufficient to act upon the vital organs, as well as upon those of external sensation, the torpor becomes universal, and the sleep is once more the sleep of death. It is in this manner that death is produced by a stroke of lightning.

As violent stimuli produce sudden and occasionally irrecoverable torpitude, either general or local, stimuli less violent induce a tendency to the same effect. Hence the nostrils of persons not accustomed to snuff are more forcibly agitated by its application, than those that have been in the use of it: the eyes of persons accustomed to sleep in the glare of the sun, find no inconvenience from exposure to the light of the morning; while those who usually sleep in total darkness are awoke by its stimulus. And so of the rest. On this account a very small portion of light, of sound, or of exercise, are sufficient sources of exhaustion to those who are not in the habit of using great external or internal activity. Hence savages and quadrupeds, who use but very little internal activity, and no more external activity than is necessary to gratify their passions and satisfy their hunger, become torpid upon very slight excitements. Hence infants become exhausted upon still slighter excitements; as the exercise of being carried, the mere breath of the air, or the digestion of milk alone in the stomach; either of which, but especially the whole collectively, is sufficient to make them sleep soundly :-so soundly, indeed, that no common stimulus is able for a long time to rouse them from their torpor. In other words, it requires a period of many hours for the external organs to recover from their exhaustion. The smallest undulatory motion in the uterus, perhaps, or the very action of the vital organs themselves, may be sufficient to wear out, from time to time, the sensorial power of the fetus on its first formation: and hence the fetus sleeps, with few intermissions, through the whole period of parturition.

For the same reason, persons in advanced age are far less impressed by common stimuli than in any former part of their lives; from a long series of exposure to their influence, the organs of sense are become more torpid, and hence they require less sleep, and at the same time less food. The vital organs partake of the same disposition, and they are in consequence less liable to violent or inflammatory disorders. But the general torpitude increasing, the heart is stimulated with greater difficulty; a smaller portion of sensorial fluid is secreted by the brain; a smaller portion of nutriment is thrown into the circulation from the digestive organs; the pulse and every other power gradually declines, till at length, if ever man were to die of old age alone, he would die from a total torpor or paralysis of the heart. But debilitated as every organ is become long before such a period can arrive, the general frame is incapable of resisting the smallest of the more trivial shocks, whether external or internal, to which man is daily exposed: in other words, there is no reservoir of sensorial power to supply the local or temporary demand; and the man dies, even at last, from sudden exhaustion, rather than from progressive paralysis.

Sleep, then, is a natural torpitude or inertness, induced upon the organs of the body and the faculties of the mind, by fatigue and exhaustion; and in a physiological survey, consists of the three stages of slumber, dreaming, and lethargy. In slumber, the exhaustion is slight, and is almost confined to the organs of external sense, the will only inclining to their inertness: in dreaming, the exhaustion is usually more considerable, the will altogether associating in their inertness: in lethargy, the exhaustion extends to and

embraces the mental faculties. When the system is under the influence of disease, the usual course of the phenomena of sleep and dreaming is often disturbed and interrupted; and when the torpitude extends to the vital organs, the effect produced is death.

But the chief difficulty in the subject of dreaming remains still to be accounted for. How is it possible for thoughts or ideas to exist in the brain, and be continued, while the will, which usually regulates them, and the external senses which give birth to them, have their continuity of action broken in upon? I shall endeavour to explain this difficulty in language as familiar as I can employ.

A certain, but a very small, degree of stimulus applied to any of the cerebral fibres of the human body, whether sensitive or irritative, instead of sensibly exhausting them, seems rather to afford them pleasure; at least the fibres are able to endure it without becoming torpid, or, which is the same thing, requiring sleep or rest.

Hence every gentle sight, and every gentle sound, or any other gentle object in nature, to what sense soever it be directed; the still twilight of a summer evening; the mild lustre of the moon, interwoven with the foliage of forest scenery; the reposing verdure of a spreading lawn; soft playful breezes; the modest fragrance of roses and violets; the light murmurs of a rippling stream; the tinkling of a neighbouring sheepfold, and the sound of village bells at a distance, are all stimuli that produce no sensible exhaustion; and, on this very account, form some of the most agreeable images in nature. In like manner, the orbicular motion of the lips in a sucking infant is a source of so much comfort, and attended with so little exhaustion, that whether sleeping or waking, it will generally be found mimicking the action of sucking, when at a distance from its nurse; and, perhaps, not thinking of such action itself. A person who, from habit, has acquired a particular motion of any one of his limbs, a twirl of the fingers, or a swinging of one leg over the other, perseveres in such motion from habit alone, and feels no torpitude or exhaustion in the fibres that are excited, although it might be intolerably fatiguing to another who has never acquired the same custom.

It is probable, then, that thought, and the action of the vital organs, are of this precise character. We are totally ignorant, indeed, of the mysterious mode by which either the one or the other was produced at first; but we see enough to convince us that the stimulus is, in both cases, equally pleasing and gentle. And hence both actions continue without exhausting us, except when unduly roused; and form a habit too pertinacious to be broken through by any ordinary opposition.

Thought, then, is to the sensory that which the motions I have just spoken of are to the muscles which are the subjects of them. Both continue alike, whether we be reflecting upon the habit or not: but the habit of thinking is so much older, and consequently so much deeper-rooted, than that of any kind of muscular motion, except the muscular motion of the vital organs, that it is impossible for us to subdue it by the utmost efforts of the will: whence, like the action of the vital organs, it accompanies us, not only at all times when awake, but in all ordinary cases during sleep, and is the immediate and necessary cause of our dreaming.

Thought can only be exercised upon perceptions introduced into the sensory by the organs of external sense; and hence the chief bent of our thoughts must be derived, whether sleeping or waking, from the objects or perceptions that most deeply impress us. The train of thoughts, then, that recurs from habit alone, as in sleep or total retirement from the world, must generally be of this description: in the former case, however, by no means correctly or perfectly; because there are others also which have a tendency to recur, and neither the will nor the senses are in action to regulate or repress them. Whence, as I have already observed, proceeds a combination of thoughts or ideas, sometimes only in a small degree incongruous, and at other times most wild and heterogeneous; occasionally, indeed, so fearful and extravagant as to stimulate the senses themselves into a sudden renewal