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.