body. BROUSSAIS asserts, that if a person tells you he suffers in a limb which he no longer has, it is because he experiences irritation in the extremities of the divided nerve, but this, in no respect, removes the difficulty. The sensation is referred to a part, which has no existence except in the imagination.

But to return to sleep. We have said, that the object of sleep is to repair the loss which the nervous system has sustained during the previous condition of waking. This may consequently be regarded as the great exciting cause of sleep; but we have seen also that certain states of the mind may postpone the usual period of its recurrence. If, indeed, we allow the attention to flag, and suspend the due exercise of volition, sleep can be indulged at almost any hour of the day. In the same manner, any monotonous impression, or action of the brain in thought; the rocking of a cradle to the restless child; or the song of the nurse; the murmurs of a bubbling brook, &c. may soothe us to rest. A like effect is produced by substances, as narcotics, which, by a specific action on the nervous system, prevent the ordinary sources of irritation from being appreciated, as well as by certain morbid affections of the brain,—compression, concussion, inflammation, &c. In these cases, however, the sleep is morbid, and is an evidence of serious mischief,—often of fatal disease; whilst true sleep is as natural as the waking state, and is always—

"Man's rich restorative; his balmy bath,

That supples, lubricates and keeps in play,
The various movements of that nice machine,
Which asks such frequent periods of repair!"

Yet Haller, Hartley, and numerous others have supposed that natural sleep is likewise dependent upon an accumulation of blood or other fluids in the vessels of the head, pressing upon the brain and thus impeding its functions. In support of this opinion, it is asserted, that all the phenomena which attend the sleeping state seem to prove a determination of blood to the head. The face is flushed; the head is hotter; the skin more moist; and it is generally during the night, or when first awake, that bleeding from the nose and apoplexy take place: the frequency of erection during sleep is affirmed to be owing to the pressure exerted on the cerebellum, which, in the theory of Gall, is the encephalic organ of generation; and lastly, it is argued, that narcotics and vinous and spirituous liquors produce sleep by causing a similar congestion of blood within the cranium. The case, by no means unique, of the beggar whose brain was exposed, and in whom a state of drowsiness was induced when the brain was pressed upon, which could be increased by increasing the pressure, until at length he became comatose, has also been cited by Hartley and others. But all these are cases of morbid suspension of the animal functions, and are no VOL. II.

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more to be assimilated to true sleep, than the drowsiness, which Flourens found to prevail in his experiments on animals when the cerebral lobes were removed.

The believers in the hypothesis, that congestion of the vessels of the brain is the cause of sleep, consider, that the heaviness and stupor, observable in those who indulge too much in laziness and sleep, are owing to the long-continued pressure injuring the cerebral organs. Other physiologists have assumed the opposite ground, and affirmed that during sleep the blood is distributed to the brain in less quantity, and is concentrated in the abdomen, to augment the action of the nutritive functions; whilst Cabanis holds, that during sleep, there is a reflux of the nervous powers towards their source, and a concentration in the brain of the most active principles of sensibility.

On all these topics our ignorance is extreme. We know nothing of the state of the encephalon in sleep. Its essence is as impenetrable as that of every other vital function. Dr. Bostock asserts, that it is not more beyond our grasp than the other functions of the nervous system. This we admit: he has indeed afforded us in his own work indubitable evidences of our utter want of acquaintance with the essence of all those functions.

The state of sleep is as natural, as instinctive, as that of waking: both are involved in mystery, and their investigation, as Mr. DuGALD STEWART has suggested, is beyond the reach of the human faculties.

Reverie has been considered to resemble sleep, and, in its higher grades, to be not far removed from the condition of somnambulism. It is characterized by the attention or volition being directed so intently towards particular topics, during wakefulness, that the impressions of surrounding objects are not appreciated. Various grades of this condition of the mind may be traced from the slightest degree of absence or brown study, to a state of total abstraction, in which the attention is entirely wound up, and riveted to a particular subject. Most persons must have experienced more or less of this, when any subject of severe study, or any great gratification, anxiety, or distress has strongly occupied the mind. If engaged in reading, they may follow every line with the eye; turn over leaf after leaf, and at length awake from the reverie, which had occupied the imagination, and find that not the slightest impression has been made on the mind, by the pages which the eye had perused, and the hand had run over. If walking in a crowded street, they have probably proceeded some way under the influence of revery, moving the limbs as usual, performing various acts of volition, winding safely among the passengers, avoiding the posts and other obstacles, yet so exclusively occupied by the conceptions of the mind, as to be totally unconscious of all these acts of their volition, and of the objects which they have passed, which must necessarily have impressed their senses so as to regulate

those actions, but, owing to the attention having been bent upon other topics, the perceptions have been evanescent. In elucidation of the power of a high degree of revery to render an individual torpid to all around him, the case of Archimedes, at the time of his arrest, has been quoted by writers. When the Roman army had at length taken Syracuse by stratagem, which the tactics of Archimedes had prevented them from taking by force, he was shut up in his closet, and so intent on a geometrical demonstration that he was equally insensible to the shouts of the victors, and the outcries of the vanquished. He was calmly tracing the lines of a diagram, when a soldier abruptly entered his room, and clapt a sword to his throat. "Hold friend," said ARCHIMEDES, 66 one moment, and my demonstration will be finished." The soldier, surprised at his unconcern at a time of such extreme peril resolved to carry him before Marcellus; but as the philosopher put under his arm a small box full of spheres, dials, and other instruments, the soldier, conceiving the box to be filled with gold, could not resist the temptation, and killed him on the spot.

It is to the capability of indulging to the necessary extent in this kind of mental abstraction, that we are indebted for the solution of every abstruse problem, relating to science or art, and for some of the most beautiful conceptions of the poet. From indulgence, however, in such abstractions, a habit is often acquired, which may be carried so far as to render the individual unfit for society, and to give him a character for rudeness and ill-breeding, of which he may be by no means deserving. Some most amiable and estimable men have, from long habits of abstraction, contracted the disease, (aphelxia,) as Good has constituted it, and have found the cure tedious and almost impracticable: at times, indeed, it appears to have terminated in mental alienation.

The difference between this state and that of sleep is, that the attention and volition are here powerfully directed to one object, so as to be torpid to the impressions of extraneous bodies; whilst sleep is characterized by a suspension or diminished exercise of these faculties.

CORRELATION OF FUNCTIONS.

THE Wonderful and complicated actions of the frame are variously correlated, to accomplish that astonishing harmony which prevails in the state of health, as well as to produce the varied morbid phenomena,—often at a distance from the part originally diseased, -which characterize different pathological conditions. It is not, therefore, simply as a physiological question that the study of the correlation of functions interests the medical inquirer. It is important to him in the study of every department which concerns the doctrine of the healthy or diseased manifestations, and the modes adapted for their removal.

These correlations may be of various kinds;—physical, in which the effect exerted is entirely of a mechanical character; functional, in which the action of one organ is inseparably united to that of another, to accomplish a particular object; and sympathetic, in which there is no physical action or direct catenation of functions; but where an organ, at a distance from one affected, is excited to irregular or regular action in consequence of the condition of the latter.

In the description of the different functions, numerous opportunities occurred for showing the influence, which organs, in the immediate vicinity of each other, may mutually exert so as to modify their functions. The action of the muscles,—particularly those that contract the larger cavities, as the abdomen and thorax,—on the parts with which they come in contact, must be entirely mechanical. In this way, the diaphragm and the abdominal muscles act in vomiting and defecation. During the operation of blood-letting, the flow of blood can be augmented by moving the muscles of the hand; and it is probable that the constant motion of the muscles of respiration impresses a succussion on different organs, which may aid them in accomplishing their functions, although the effect of this is doubtless exaggerated. Every change of position, either of the whole body or of a part, has, likewise, some effect in modifying the actions performed by it or by neighbouring organs, although such effect may not be easily appreciable.

A similar case of mere mechanical influence, which seems to be important to the proper action of certain organs, is exhibited in the pulsation of the different arteries. It has been seen, that a succussion is in this way given to the brain, which appears to be necessary to it; for, if this source of stimulation is in any manner withdrawn, fainting is induced. Perhaps, however, the strongest case that can be offered of modification of function by mechanical

causes, is that of the gravid uterus, which, by its pressure, gives rise to numerous symptoms in other organs, that are often the source of much annoyance during gestation.

The functional correlations or synergies are of much more moment to the physiologist and pathologist. Many of these have also been described in the preceding history; a brief notice of them will be all that is now requisite. For the maintenance of the healthy function we know that certain conditions are necessary, and that if these be modified, in the whole or in any part of the body, disease and death may be the result, even although the derangement may, in the first instance, concern only an apparently unimportant part of the frame; the affection by correlation spreading gradually to more and more essential organs and functions, until the disorder is ultimately too great to allow of a continuance of the vital movements. In this respect, man differs from an ordinary piece of human mechanism, in which the various parts are so adapted to each other as to produce a certain result. If one of these parts be destroyed, the whole machine may have its motion arrested. But the effect is owing to the destruction of one part only, the others remaining sound, whilst death, or the stoppage of the living machine, does not necessarily follow the destruction of any except a few essential organs, and is generally owing to the derangement of many. We shall find, indeed, that except in cases of sudden death, it is extremely difficult to say which of the three truly vital organs has first ceased to act; and that in all such cases death begins in one or other of the organs essential to vitality, and soon extends to the rest.

The essentially vital organs are the respiratory, circulatory, and the organs of innervation; but the great use of respiration is to change the blood from venous to arterial; in other words, to induce a conversion in it by its passage through the lungs, without which it would be inadequate for the maintenance of life in any organ; and the object of the circulation is, to distribute it to the various parts of the frame as the grand vivifying and reparatory material. If, also, the organs of innervation. be destroyed, the nervous influence is no longer conveyed to the different parts of the frame; and as the presence of this influence is everywhere indispensable, the functions may cease from this cause; so that we may regard, as essential elements to the existence of the frame and of every part of the frame, the proper supply of arterial blood and of the nervous influence. In the production and distribution, however, of these agencies, a number of functions is concerned, giving rise to the correlation, which is the object of our present inquiry. If, in any manner, the blood does not meet with the due aeration, as in the ordinary cases of suffocation, death supervenes, in the order elsewhere described; and if a slight degree of aeration is accomplished, but still not enough for the necessities of the system, instead of suffocation, the individual dies more gradually: the functions fail in the same order; black