THE CAUSES OF SLEEP.

The exciting cause of natural and periodic sleep is undoubtedly to be found in the fact that the brain at stated times requires repose, in order that the cerebral substance which has been decomposed by mental and nervous action may be replaced by new material. There are other exciting causes than this, however, for sleep is not always induced by ordinary or natural influences acting periodically. There are many others, which within the strict limits of health may cause such a condition of the brain as to produce sleep.

Authors, in considering sleep, have not always drawn the proper distinction between the exciting and the immediate cause. Thus Macario,[5] in alluding to the alleged causes of sleep, says:

“Among physiologists some attribute it to a congestion of blood in the brain; others to a directly opposite cause, that is, to a diminished afflux of blood to this organ; some ascribe it to a loss of nervous fluid, others to a flow of this fluid back to its source; others again find the cause in the cessation of the motion of the cerebral fibers, or rather in a partial motion in these fibers. Here I stop, for I could not, even if I wished, mention all the theories which have prevailed relative to this subject. I will only add that, in my opinion, the most probable proximate and immediate cause appears to be feebleness. What seems to prove this view is the fact that exhaustive hot baths, heat, fatigue, too great mental application are among the means which produce sleep.”

Undoubtedly the influence mentioned by Macario, and many others which he might have cited, lead to sleep. They do so through the medium of the nervous system—causing a certain change to take place in the physical condition of the brain. We constantly see instances of this transmission of impressions and the production of palpable effects. Under the influence of fatigue, the countenance becomes pale; through the actions of certain emotions, blushing takes place. When we are anxious or suffering or engaged in intense thought, the perspiration comes out in big drops on our brows; danger makes some men tremble, grief causes tears to flow. Many other examples will suggest themselves to the reader. It is surely, therefore, no assumption to say that certain mental or physical influences are capable of inducing such an alteration in the state of the brain as necessarily to cause sleep. These influences or exciting causes I propose to consider in detail, after having given my views relative to the condition of the brain which immediately produces sleep.

It is well established as regards other viscera, that during a condition of activity there is more blood in their tissues than while they are at rest. It is strange, therefore, that, relative to the brain, the contrary doctrine should have prevailed so long, and that even now, after the subject has been so well elucidated by exact observation, it should be the generally received opinion that during sleep the cerebral tissues are in a state approaching congestion. Thus Dr. Marshall Hall,[6] while contending for this view, also advances the theory that there is a special set of muscles, the duty of which is, by assuming a condition of tonic contraction, so to compress certain veins as to prevent the return of the blood from the heart.

Dr. Carpenter[7] is of the opinion that the first cause of sleep in order of importance is the pressure exerted by distended blood-vessels upon the encephalon.

Sir Henry Holland[8] declares that a “degree of pressure is essential to perfect and uniform sleep.”

Dr. Dickson[9] regards an increased determination of blood to the cerebral mass, and its consequent congestion in the larger vessels of the brain, as necessary to the induction of sleep.

In his very excellent work on Epilepsy, Dr. Sieveking[10] says:

“Whether or not there is actually an increase in the amount of blood in the brain during sleep, and whether, as has been suggested, the choroid plexuses become turgid or not, we are unable to affirm otherwise than hypothetically; the evidence is more in favor of cerebral congestion than of the opposite condition inducing sleep—evidence supplied by physiology and pathology.” Dr. Sieveking does not, however, state what this evidence is.

Barthez[11] is of the opinion that during sleep there is a general plethora of the smaller blood-vessels of the whole body. He does not appear to have any definite views relative to the condition of the cerebral circulation.

Cabanis[12] declares that as soon as the necessity for sleep is experienced, there is an increased flow of blood to the brain.

To come to more popular books than those from which we have quoted, we find Mr. Lewes,[13] when speaking of the causes of sleep, asserting that: “It is caused by fatigue, because one of the natural consequences of continued action is a slight congestion; and it is the congestion which produces sleep. Of this there are many proofs.” Mr. Lewes omits to specify these proofs.

Macnish[14] holds the view that sleep is due to a determination of blood to the head.

That a similar opinion has prevailed from very ancient times, it would be easy to show. I do not, however, propose to bring forward any further citations on this point, except the following, from a curious old black-letter book now before me, in which the views expressed, though obscure, are perhaps as intelligible as many met with in books of our own day:

“And the holy scripture in sundrie places doth call death by the name of sleepe, which is meant in respect of the resurrection; for, as after sleepe we hope to wake, so after death we hope to rise againe. But that definition which Paulus Ægineta maketh of sleepe, in my judgment, is most perfect where he saith: Sleepe is the rest of the pores animall, proceeding of some profitable humour moistening the braine. For here is shewed by what means sleepe is caused; that is, by vapours and fumes rising from the stomache to the head, where through coldness of the braine they being congealed, doe stop the conduites and waies of the senses, and so procure sleepe, which thing may plainly be perceived hereby; for that immediately after meate we are most prone to sleepe, because then the vapours ascende most abundantly to the braine, and such things as be most vaporous do most dispose to sleepe, as wine, milke, and such like.”[15]

The theory that sleep is due directly to pressure of blood-vessels, filled to repletion, upon the cerebral tissues, doubtless originated in the fact that a comatose condition may be thus induced. This fact has long been known. Servetus, among other physiological truths, distinctly announces it in his Christianismi Restitutio, when he says:

“Et quando ventriculi ita opplentur pituita, ut arteriÆ ipsÆ choroidis ea immergantur, tunc subito generatur appoplexia.”

Perhaps the theory which prevails at present, of sleep being due to the pressure of distended blood-vessels upon the choroid plexus, is derived from these words of Servetus.

That stupor may be produced by pressure upon the brain admits of no doubt. It is familiarly known to physicians, surgeons, and physiologists; the two former meet with instances due to pathological causes every day, and the latter bring it on at will in their laboratories. But this form of coma and sleep are by no means identical. On the contrary, the only point of resemblance between the two consists in the fact that both are accompanied by a loss of volition. It is true, we may often arrive at a correct idea of a physiological process from determining the causes and phenomena of its pathological variations, but such a course is always liable to lead to great errors, and should be conducted with every possible precaution. In the matter under consideration it is especially of doubtful propriety, for the reason stated, that coma is not to be regarded as a modification of sleep, but as a distinct morbid condition. Sir T. C. Morgan,[16] in alluding to the fact that sleep has been ascribed to a congested state of the brain, for the reason that in apoplectic stupor the blood-vessels of that organ are abnormally distended, objects to the theory, on the ground that it assimilates a dangerous malady to a natural and beneficial process. He states (what was true at the time he wrote) that the condition of the circulation through the brain, during sleep, is wholly unknown.

It is important to understand clearly the difference between stupor and sleep, and it is very certain that the distinction is not always made by physicians; yet the causes of the two conditions have almost nothing in common, and the phenomena of each are even more distinct.

1. In the first place, stupor never occurs in the healthy individual, while sleep is a necessity of life.

2. It is easy to awaken a person from sleep, while it is often impossible to arouse him from stupor.

3. In sleep the mind may be active, in stupor it is as it were dead.

4. Pressure upon the brain, intense congestion of its vessels, the circulation of poisoned blood through its substance cause stupor, but do not induce sleep. For the production of the latter condition a diminished supply of blood to the brain, as will be fully shown hereafter, is necessary.

Perhaps no one agent so distinctly points out the difference between sleep and stupor as opium and its several preparations. A small dose of this medicine acting as a stimulant increases the activity of the cerebral circulation, and excites a corresponding increase in the rapidity and brilliancy of our thoughts. A larger dose lessens the amount of blood in the brain, and induces sleep. A very large dose sometimes diminishes the power of the whole nervous system, lessens the activity of the respiratory function, and hence allows blood which has not been properly subjected to the influence of the oxygen of the atmosphere to circulate through the vessels of the brain. There is nothing in the opium itself which produces excitement, sleep, or stupor, by any direct action upon the brain. All its effects are due to its influence on the heart and blood-vessels, through the medium, however, of the nervous system. This point can be made plainer by adducing the results of some experiments which I have lately performed.

Experiment.—I placed three dogs of about the same size under the influence of chloroform, and removed from each a portion of the upper surface of the skull an inch square. The dura mater was also removed, and the brain exposed. After the effects of the chloroform had passed off—some three hours subsequent to the operation—I administered to number one the fourth of a grain of opium, to number two a grain, and to number three two grains. The brain of each was at the time in a perfectly natural condition.

At first the circulation of the blood in the brain was rendered more active, and the respiration became more hurried. The blood-vessels, as seen through the openings in the skulls, were fuller and redder than before the opium was given, and the brain of each animal rose through the hole in the cranium. Very soon, however, the uniformity which prevailed in these respects was destroyed. In number one the vessels remained moderately distended and florid for almost an hour, and then the brain slowly regained its ordinary appearance. In number two the active congestion passed off in less than half an hour, and was succeeded by a condition of very decided shrinking, the surface of the brain having fallen below the surface of the skull, and become pale. As these changes supervened, the animal gradually sank into a sound sleep, from which it could easily be awakened. In number three the surface of the brain became dark, almost black, from the circulation of blood containing a superabundance of carbon, and owing to diminished action of the heart and vessels it sank below the level of the opening, showing, therefore, a diminished amount of blood in its tissue. At the same time the number of respirations per minute fell from 26 to 14, and they were much weaker than before. A condition of complete stupor was also induced from which the animal could not be aroused. It persisted for two hours. During its continuance, sensation of all kind was abolished, and the power of motion was altogether lost.

It might be supposed that the conditions present in numbers two and three differed only in degree. That this was not the case is shown by the following experiment:

Experiment.—To the dogs two and three I administered on the following day, as before, one and two grains of opium respectively. As soon as the effects began to be manifested upon the condition of the brain, I opened the trachea of each, and, inserting the nozzle of a bellows, began the process of artificial respiration. In both dogs the congestion of the blood-vessels of the brain disappeared. The brain became collapsed, and the animals fell into a sound sleep, from which they were easily awakened. If the action of the bellows was stopped and the animals were left to their own respiratory efforts, no change ensued in number two, but in number three the surface of the brain became dark, and stupor resulted.

In order to be perfectly assured upon the subject, I proceeded as follows with another dog:

Experiment.—The animal was trephined as was the others, and five grains of opium given. At the same time the trachea was opened and the process of artificial respiration instituted. The brain became slightly congested, then collapsed, and sleep ensued. The sleep was sound, but the animal was easily awakened by tickling its ear. After I had continued the process for an hour and a quarter, I removed the nozzle of the bellows, and allowed the animal to breathe for itself. Immediately the vessels of the brain were filled with black blood, and the surface of the brain assumed a very dark appearance.

The dog could no longer be aroused, and died one hour and a quarter after the process was stopped.

I have only stated those points of the experiments cited which bear upon the subject under consideration, reserving for another occasion others of great interest. It is, however, shown that a small dose of opium excites the mind, because it increases the amount of blood in the brain; that a moderate dose causes sleep, because it lessens the amount of blood; and that a large dose produces stupor by impeding the respiratory process, and hence allowing blood loaded with carbon, and therefore poisonous, to circulate through the brain.

It is also shown that the condition of the brain during stupor is very different from that which exists during sleep. In the one case its vessels are loaded with dark blood; in the other they are comparatively empty, and the blood remains florid.

I think it will be sufficiently established, in the course of these remarks, that sleep is directly caused by the circulation of a less quantity of blood through the cerebral tissues than traverses them while we are awake. This is the immediate cause of healthy sleep. Its exciting cause is, as we have seen, the necessity for repair. The condition of the brain which is favorable to sleep may also be induced by various other causes, such as heat, cold, narcotics, anÆsthetics, intoxicating liquors, loss of blood, etc. If these agents are allowed to act excessively, or others, such as carbonic oxide, and all those which interfere with the oxygenation of the blood, are permitted to exert their influence, stupor results.

The theory above enunciated, although proposed in a modified form by Blumenbach several years since, and subsequently supported by facts brought forward by other observers, has not been received with favor by any considerable number of physiologists. Before, therefore, detailing my own experience, I propose to adduce a few of the most striking proofs of its correctness which I have been able to collect, together with the opinions of some of those inquirers who have recently studied the subject from this point of view.

Blumenbach[17] details the case of a young man, eighteen years of age, who had fallen from an eminence and fractured the frontal bone, on the right side of the coronal suture. After recovery took place a hiatus remained, covered only by the integument. While the young man was awake this chasm was quite superficial, but as soon as sleep ensued it became very deep. The change was due to the fact that during sleep the brain was in a collapsed condition. From a careful observation of this case, as well as from a consideration of the phenomena attendant on the hibernation of animals, Blumenbach[18] arrives at the conclusion that the proximate cause of sleep consists in a diminished flow of oxygenated blood to the brain.

Playfair[19] thinks that sleep is due to “a diminished supply of oxygen to the brain.”

Dendy[20] states that there was, in 1821, at Montpellier, a woman who had lost part of her skull, and the brain and its membranes lay bare. When she was in deep sleep the brain remained motionless beneath the crest of the cranial bones; when she was dreaming it became somewhat elevated; and when she was awake it was protruded through the fissure in the skull.

Among the most striking proofs of the correctness of the view that sleep is due to diminished flow of blood to the head, are the experiments of Dr. Alexander Fleming,[21] late Professor of Medicine, Queen’s College, Cork. This observer states, that while preparing a lecture on the mode of operation of narcotic medicines, he conceived the idea of trying the effect of compressing the carotid arteries on the functions of the brain. The first experiment was performed on himself, by a friend, with the effect of causing immediate and deep sleep. The attempt was frequently made, both on himself and others, and always with success. “A soft humming in the ears is heard; a sense of tingling steals over the body, and in a few seconds complete unconsciousness and insensibility supervene, and continue so long as the pressure is maintained.”

Dr. Fleming adds, that whatever practical value may be attached to his observations, they are at least important as physiological facts, and as throwing light on the causes of sleep. It is remarkable that his experiments have received so little notice from physiologists.

Dr. Bedford Brown,[22] of North Carolina, has recorded an interesting case of extensive compound fracture of the cranium, in which the opportunity was afforded him of examining the condition of the cerebral circulation while the patient was under the influence of an anÆsthetic, preparatory to the operation of trephining being performed. A mixture of ether and chloroform was used. Dr. Brown says:

“Whenever the anÆsthetic influence began to subside, the surface of the brain presented a florid and injected appearance. The hemorrhage increased, and the force of the pulsation became much greater. At these times so great was the alternate heaving and bulging of the brain, that we were compelled to suspend operations until they were quieted by a repetition of the remedy. Then the pulsations would diminish, the cerebral surface recede within the opening of the skull, as if by collapse; the appearance of the organ becoming pale and shrunken with a cessation of the bleeding. In fact, we were convinced that diminished vascularity of the brain was an invariable result of the impression of chloroform or ether. The changes above alluded to recurred sufficiently often, during the progress of the operation, in connection with the anÆsthetic treatment, to satisfy us that there could be no mistake as to the cause and effect.”

It will be shown, in the course of the present memoir, that Dr. Brown’s conclusions, though in the main correct, are erroneous so far as they relate to the effect of chloroform upon the cerebral circulation; nor does it appear that he employed this agent unmixed with ether, in the case which he has recorded so well. He has, probably, based his remarks on this point upon the phenomena observed when the compound of ether and chloroform was used—the action of pure chloroform, as regards its effect upon the quantity of blood circulating through the brain, being the reverse of that which he claims for it.

But the most philosophical and most carefully digested memoir upon the proximate cause of sleep, which has yet been published, is that of Mr. Durham.[23] Although my own experiments in the same direction, and which will be hereafter detailed, were of prior date, I cheerfully yield all the honor which may attach to the determination of the question under consideration to this gentleman, who has not only worked it out independently, but has anticipated me several years in the publication, besides carrying his researches to a much further point than my own extended.

With the view of ascertaining by ocular examination the vascular condition of the brain during sleep, Durham placed a dog under the influence of chloroform, and removed with a trephine a portion of bone as large as a shilling from the parietal region; the dura mater was also cut away. During the continuance of the anÆsthetic influence, the large veins of the surface of the pia mater were distended, and the smaller vessels were full of dark-colored blood. The longer the administration of the chloroform was continued, the greater was the congestion. As the effects of this agent passed off, the animal sank into a natural sleep, and then the condition of the brain was very materially changed. Its surface became pale and sank down below the level of the bone; the veins ceased to be distended, and many which had been full of dark blood could no longer be distinguished. When the animal was roused, the surface of the brain became suffused with a red blush, and it ascended into the opening through the skull. As the mental excitement increased, the brain became more and more turgid with blood, and innumerable vessels sprang into sight. The circulation was also increased in rapidity. After being fed, the animal fell asleep, and the brain again became contracted and pale. In all these observations the contrast between the two conditions was exceedingly well marked.

To obviate any possible effects due to atmospheric pressure, watch-glasses were applied to the opening in the skull, and securely cemented to the edges with Canada balsam. The phenomena observed did not differ from those previously noticed; and, in fact, many repetitions of the experiment gave like results.

Durham, in the next place, applied ligatures to the jugular and vertebral veins, with the effect—as was to be expected—of producing intense congestion of the brain, attended with coma. This last condition he very properly separates from sleep, which is never caused by pressure from the veins. He likens sleep to the state induced by preventing the access of blood to the brain through the carotids, but does not allude to Fleming’s researches on this point.

From his observations, Durham deduces the following conclusions:

“1. Pressure of distended veins upon the brain is not the cause of sleep, for during sleep the veins are not distended; and when they are, symptoms and appearances arise which differ from those which characterize sleep.

“2. During sleep the brain is in a comparatively bloodless condition, and the blood in the encephalic vessels is not only diminished in quantity, but moves with diminished rapidity.

“3. The condition of the cerebral circulation during sleep is, from physical causes, that which is most favorable to the nutrition of the brain tissue; and, on the other hand, the condition which prevails during waking is associated with mental activity, because it is that which is most favorable to oxydation of the brain substance, and to various changes in its chemical constitution.

“4. The blood which is derived from the brain during sleep is distributed to the alimentary and excretory organs.

“5. Whatever increases the activity of the cerebral circulation tends to preserve wakefulness; and whatever decreases the activity of the cerebral circulation, and, at the same time, is not inconsistent with the general health of the body, tends to induce and favor sleep. Such circumstances may act primarily through the nervous or through the vascular system. Among those which act through the nervous system, may be instanced the presence or absence of impressions upon the senses, and the presence or absence of exciting ideas. Among those which act through the vascular system, may be mentioned unnaturally or naturally increased or decreased force or frequency of the heart’s action.

“6. A probable explanation of the reason why quiescence of the brain normally follows its activity, is suggested by the recognized analogical fact that the products of chemical action interfere with the continuance of the action by which they are produced.”

Luys,[24] after stating the two opposite views relative to the state of the cerebral circulation during sleep, gives his adhesion on principles of analogy to that which holds to a diminished afflux of blood. Taking the condition of the salivary glands during their periods of inaction as the basis of his argument, he says:

“We are then naturally led, in making the application of known facts to those which are yet unknown, to say that the nervous tissue and the glandular tissue present, between themselves, the closest analogy, so far as circulatory phenomena and the double alternation of their periods of activity and repose are concerned. And that if the period during which the gland reconstitutes its immediate principles corresponds to a period of reduced activity of circulatory phenomena—to a state of relative anÆmia—and that when it functionates it is awakened to a state in which its capillaries are turgid with blood, it is very admissible that the same circulatory conditions should be present in the nervous tissue, and that the period of inactivity, or of sleep, should be characterized by an anemic state. Inversely, the period of activity or wakefulness should be marked by an acceleration of the flow of blood, and by a kind of erethism of the vascular element.”

Having thus, in as succinct a manner as possible, brought forward the principal observations relative to the immediate cause of sleep, which up to the present time have been published, I come, in the next place, to detail the result of my own researches.

In 1854 a man came under my observation who had, through a frightful railroad accident, lost about eighteen square inches of his skull. There was thus a fissure of his cranium three inches wide and six inches long. The lost portion consisted of a great part of the left parietal, and part of the frontal, occipital, and right parietal bones. The man, who was employed as a wood chopper, was subject to severe and frequent epileptic fits, during which I often attended him. In the course of my treatment, I soon became acquainted with the fact that, at the beginning of the comatose condition which succeeded the fits, there was invariably an elevation of that portion of the scalp covering the deficiency in the cranium. As the stupor passed away, and sleep from which he could easily be aroused ensued, the scalp gradually became depressed. When the man was awake, the region of scalp in question was always nearly on a level with the upper surface of the cranial bones. I also noticed on several occasions that during natural sleep the fissure was deeper, and that in the instant of awaking, the scalp covering it rose to a much higher level.

After my attention was thus drawn to this subject, I observed that in young infants the portion of scalp covering the anterior fontanelle was always depressed during sleep, and elevated during wakefulness.

During the summer of 1860 I undertook a series of experiments, with the view of ascertaining the condition of the cerebral circulation during sleep, of which the following is a brief abstract:

A medium-sized dog was trephined over the left parietal bone, close to the sagittal suture, having previously been placed under the full anÆsthetic influence of ether. The opening made by the trephine was enlarged with a pair of strong bone-forceps, so as to expose the dura mater to the extent of a full square inch. This membrane was then cut away and the brain brought into view. It was sunk below the inner surface of the skull, and but few vessels were visible. Those which could be perceived, however, evidently conveyed dark blood, and the whole exposed surface of the brain was of a purple color. As the anÆsthetic influence passed off, the circulation of the blood in the brain became more active. The purple hue faded away, and numerous small vessels filled with red blood became visible; at the same time the volume of the brain increased, and when the animal became fully aroused, the organ protruded through the opening in the skull to such an extent that, at the most prominent part, its surface was more than a quarter of an inch above the external surface of the cranium. While the dog continued awake, the condition and position of the brain remained unchanged. After the lapse of half an hour, sleep ensued. While this state was coming on I watched the brain very attentively. Its volume slowly decreased; many of its smaller blood-vessels became invisible, and finally it was so much contracted that its surface, pale and apparently deprived of blood, was far below the level of the cranial wall.

Two hours subsequently the animal was again etherized, in order that the influence of the ether upon the cerebral circulation might be observed from the commencement. At the time the dog was awake, and had a few minutes previously eaten a little meat and drank a small quantity of water. The brain protruded through the opening in the skull, and its surface was of a pink hue, with numerous red vessels ramifying over it. The ether was administered by applying to the muzzle of the animal a towel folded into the shape of a funnel, and containing a small sponge saturated with the agent.

As soon as the dog commenced to inspire the ether, the appearance of the brain underwent a change of color, and its volume became less. As the process of etherization was continued, the color of the surface darkened to a deep purple, and it ceased to protrude through the opening. Finally, when a state of complete anÆsthesia was reached, it was perceived that the surface of the brain was far below the level of the cranial fissure, and that its vessels conveyed black blood alone.

Gradually the animal regained its consciousness; the vessels resumed their red color, and the brain was again elevated to its former position. In this last experiment there did not appear to be any congestion of the brain. Had this condition existed, it would have been difficult to account for the diminution in bulk, which certainly took place. There was evidently less blood in the cerebral tissue than there had been previously at the etherization; but this blood, instead of being oxygenated, was loaded with excrementitial matters, and consequently was not fitted to maintain the brain in a condition of activity.

The following morning, the dog being quite lively, I removed the sutures which had been placed in the skin, covering the hole in the cranium, with the view of ascertaining the effects of chloroform upon the brain, when introduced into the system by inhalation. Suppuration had not yet taken place, and the parts were in good condition. The opening in the skull was completely filled by the brain, and the surface of the latter was traversed by a great many small vessels carrying red blood. The chloroform was administered in the same way in which the ether had been given the previous day.

In a few seconds the change in color of the blood circulating in the vessels began to take place, but there was no sinking of the brain below the level of the chasm in the skull. On the contrary, its protrusion was greater than before the commencement of the experiment. There was thus not only unoxygenated blood circulating to too great an extent through the brain, but there was very decided congestion.

The foregoing experiments were frequently repeated on other dogs, and also on rabbits, with like results. Within a short period I have in part gone over the ground again, without observing any essential point of difference in the effects produced.

I have never repeated Fleming’s experiment on the human subject, except in one instance, and then sleep, or a condition resembling it, was instantaneously produced. As soon as the pressure was removed from the carotids, the individual gained his consciousness. On dogs and rabbits, however, I have performed it frequently, and though if the pressure be continued for longer than one minute, convulsions generally ensue, a state of insensibility resembling natural sleep is always the first result. Lately, I have had, through the kindness of my friend, Dr. Van Buren, the opportunity of examining a case which affords strong confirmation of the correctness of the preceding views. It was that of a lady in whom both common carotids were tied for a cirsoid aneurism, involving a great portion of the right side of the scalp. One carotid was tied by the late Dr. J. Kearney Rogers, and the other by Dr. Van Buren, seven years ago, with the effect of arresting the progress of the disease. No peculiar symptoms were observed in consequence of these operations, except the supervention of persistent drowsiness, which was especially well marked after the last operation, and which, even now, is at times quite troublesome.

We thus see that the immediate cause of sleep is a diminution of the quantity of blood circulating in the vessels of the brain, and that the exciting cause of periodical and natural sleep is the necessity which exists that the loss of substance which the brain has undergone, during its state of greatest activity, should be restored. To use the simile of the steam-engine again, the fires are lowered and the operatives go to work to repair damages and put the machine in order for next day’s work.

Whatever other cause is capable of lessening the quantity of blood in the brain is also capable of inducing sleep. There is no exception to this law, and hence we are frequently able to produce this condition at will. Several of these factors have been already referred to, but it will be interesting to consider them all somewhat more at length.

Heat.—Most persons in our climate, and in those of higher temperatures, have felt the influence of heat in causing drowsiness, and eventually sleep, if the action is powerful enough and sufficiently prolonged. It is not difficult to understand the mode by which heat acts in giving rise to sleep. During the prevalence of high temperatures the blood flows in increased proportion to the surface of the body and to the extremities, and consequently the quantity in the brain is diminished. Sleep accordingly results unless the irritation induced by the heat is so great as to excite the nervous system. Heat applied directly to the head exerts, of course, a directly contrary effect upon the cerebral circulation, as we see in sun-stroke. Here there is internal cerebral congestion, loss of consciousness, stupor, etc.

That the effect of heat is to dilate the vessels of the part subjected to its influence, can be ascertained by putting the arm or leg into hot water. The swelling of the blood-vessels is then very distinctly seen. It will be shown hereafter that one of the best means of causing sleep in morbid wakefulness is the warm-bath.

Cold.—A slight degree of cold excites wakefulness at first, but if the constitution be strong the effect is to predispose to sleep. This it does by reason of the determination of blood to the surface of the body which moderate cold induces in vigorous persons. The ruddy complexion and warmth of the hands and feet produced in such individuals under the action of this influence are well known.

But if the cold be very intense, or the reduction of temperature sudden, the system, even of the strongest persons, cannot maintain a resistance, and then a very different series of phenomena result. Stupor, not sleep, is the consequence. The blood-vessels of the surface of the body contract and the blood accumulates in the internal organs, the brain among them. Many instances are on record showing the effect of extreme cold in producing stupor and even death. One of the most remarkable of these is that related by Captain Cook, in regard to an excursion of Sir Joseph Banks, Dr. Solander, and nine others, over the hills of Terra del Fuego. Dr. Solander, knowing from his experience in Northern Europe that the stupor produced by severe cold would terminate in death unless resisted, urged his companions to keep in motion when they began to feel drowsy. “Whoever sits down will sleep,” said he, “and whoever sleeps will rise no more.” Yet he was the first to feel this irresistible desire for repose, and entreated his companions to allow him to lie down. He was roused from his stupor with great difficulty and carried to a fire, when he revived. Two black men of the party, whose organizations were not so robust as those of the whites, perished. Dr. Whiting[25] relates the case of Dr. Edward Daniel Clark, the celebrated traveler, who on one occasion came very near losing his life by cold. He had performed divine service at a church near Cambridge, and was returning home on horseback, when he felt himself becoming very cold and sleepy. Knowing the danger of yielding to the influence which was creeping over him, he put his horse into a fast trot, hoping thereby to arouse himself from the alarming torpor. This means proving unavailing, he got down and led his horse, walking as fast as he could. This, however, did not long succeed. The bridle dropped from his arm, his legs became weaker and weaker, and he was just sinking to the ground when a gentleman who knew him came up in a carriage and rescued him.

I have often myself noticed this effect of cold in producing numbness and drowsiness, and on one occasion was nearly overcome by it. I was crossing the mountain ridge between Cebolleta and Covero, in New Mexico, when the thermometer fell in about two hours from 52° to 22° Fahrenheit. So great was the effect upon me that if I had had much farther to go I should probably have succumbed. As it was, I reached a rancho in time to be relieved, though several minutes elapsed before I was able to speak. The sensations experienced were rather agreeable than otherwise. There was a great desire to rest and to yield to the languor which was present, and there was a feeling of recklessness which rendered me perfectly indifferent to the consequences. I should have dismounted from my horse and given way to the longing for repose if I had been able to do so. I have several times experienced very similar effects from change of air. A few years since I was so drowsy at the sea-coast, whither I had gone from a hot city, that it was with difficulty I could keep awake, even when engaged in active physical exercise.

Another potent cause of sleep, and one of which we generally avail ourselves, is the diminution of the power of the attention. To bring this influence into action generally requires only the operation of the will under circumstances favorable to the object in view. Shutting the eyes so as to exclude light, getting beyond the sound of noises, refraining from the employment of the other senses, and avoiding thought of all kind, will generally, when there is no preventing cause, induce sleep. To think, and to maintain ourselves in connection with the outward world by means of our senses requires that the circulation of blood in the brain shall be active. When we isolate ourselves from external things, and restrain our thoughts, we lessen the amount of blood in the brain, and sleep results. It is not, however, always easy for us to do this. The nervous system is excited, ideas follow each other in rapid succession, and we lie awake hour after hour vainly trying to forget that we exist. The more the will is brought to bear upon the subject the more rebellious is the brain, and the more it will not be forced by such means into a state of quietude. We must then either let it run riot till it is worn out by its extravagancies, or we must fatigue it by requiring it to perform labor which is disagreeable. Just as we might do with an individual of highly destructive propensities, who was going about pulling down his neighbors’ houses. We might, if we were altogether unable to stop him, let him alone till he had become thoroughly wearied with his exertions, or we might divert him from his plan by guiding him to some tough piece of work which would exhaust his strength sooner than would his original labor.

Many ways of thus tiring the brain have been proposed. The more irksome they are, the more likely they are to prove effectual. Counting a hundred backward many times, listening to monotonous sounds, thinking of some extremely disagreeable and tiresome subject, with many other devices, have been suggested, and have proved more or less effectual. Boerhaave[26] states that he procured sleep by placing a brass pan in such a position that the patient heard the sound of water which was made to fall into it, drop by drop. In general terms, monotony predisposes to sleep. Dr. Dickson[27] quotes Southey’s experience as related in the Doctor,[28] and I also cannot do better than lay it before the reader, particularly as it indicates several methods which may be more efficacious with others than the one he found to succeed so admirably.

“I put my arms out of bed; I turned the pillow for the sake of applying a cold surface to my cheek; I stretched my feet into the cold corner; I listened to the river and to the ticking of my watch; I thought of all sleepy sounds and of all soporific things—the flow of water, the humming of bees, the motion of a boat, the waving of a field of corn, the nodding of a mandarin’s head on the chimney-piece, a horse in a mill, the opera, Mr. Humdrum’s conversations, Mr. Proser’s poems, Mr. Laxative’s speeches, Mr. Lengthy’s sermons. I tried the device of my own childhood, and fancied that the bed rushed with me round and round. At length Morpheus reminded me of Dr. Torpedo’s Divinity Lectures, where the voice, the manner, the matter, even the very atmosphere and the streamy candlelight were all alike somnific; when he who, by strong effort, lifted up his head and forced open the reluctant eyes never failed to see all around him asleep. Lettuces, cowslip wine, poppy syrup, mandragora, hop pillows, spider’s web pills, and the whole tribe of narcotics, up to bang and the black-drop, would have failed,—but this was irresistible; and thus, twenty years after date, I found benefit from having attended the course.”

Frequently the power of the attention is diminished by natural causes. After the mind has been strained a long time in one particular direction, and during which period the brain was doubtless replete with blood, the tension is at last removed, the blood flows out of the brain, the face becomes pale, and sleep ensues. It is thus, as Macnish[29] says, that “the finished gratification of all ardent desires has the effect of inducing slumber; hence after any keen excitement the mind becomes exhausted and speedily relapses into this state.”

A gentleman recently under my care for a paralytic affection, informed me that he could at any time render himself sleepy by looking for a few minutes at a bright light, so as to fatigue the eyes, or by paying particular attention to the noises in the street, so as to weary the sense of hearing. It is well known that sleep may be induced by gentle frictions of various parts of the body, and doubtless the other senses are capable of being so exhausted, if I may use the expression, as to diminish the power of the attention, and thus lessen the demand for blood in the brain. As a consequence, sleep ensues.The cutting off of sensorial impressions aids in lessening the power of the attention and thus predisposes to sleep. Stillness, darkness, the absence of any decided impression on the skin, and the nonexistence of odors and flavors, accomplish this end. In these respects, however, habit exercises great influence, and thus individuals, for instance, who are accustomed to continual loud noises, cannot sleep when the sound is interrupted. As we have already seen, however, the predisposition to sleep is, in healthy persons, generally so great that when it has been long resisted, no sensation, however strong it may be, can withstand its power.

Digestion leads to sleep by drawing upon the brain for a portion of its blood. It is for this reason that we feel sleepy after the ingestion of a hearty dinner. A lady of my acquaintance is obliged to sleep a little after each meal. The desire to do so is irresistible; her face becomes pale; her extremities cold; and she sinks into a quiet slumber, which lasts fifteen or twenty minutes. In this lady the amount of blood is not sufficient for the due performance of all the operations of the economy. The digestive organs imperatively require an increased quantity, and the flow takes place from the brain; it being the organ with her which can best spare this fluid. As a rule, persons who eat largely, and have good digestive powers, sleep a great deal, and many persons are unable to sleep at night till they have eaten a substantial supper. The lower animals generally sleep after feeding, especially if the meal has been large.

Excessive loss of blood produces sleep. We can very readily understand why this should be so if we adopt the theory which has been supported in the foregoing pages. It would be exceedingly difficult to explain the fact upon any other hypothesis. I have seen many instances of somnolency due to this cause. It acts not only by directly lessening the quantity of blood in the brain, but also by so enfeebling the heart’s action as to prevent a due supply of blood being sent to the cerebral vessels.

Debility is almost always accompanied by a disposition to inordinate sleep. The brain is one of the first organs to feel the effects of a diminished amount of blood or a depraved quality of this fluid being supplied, and hence, in old age, or under the influence of a deficient quantity of food, or through the action of some exhausting disease, there is generally more sleep than when the physical health is not deteriorated.

The action of certain medicines, and of other measures capable of causing sleep, not coming within the range of ordinary application, will be more appropriately considered hereafter.