As regards the vascular condition of the cerebrum during natural sleep,
there seems to be at present a virtual agreement among physiologists.
Whatever views may be held of the immediate or proximate cause, it is
generally admitted that during sleep the brain is relatively anæmic.
There are well-attested facts enough on record to substantiate this.
The brain, denuded of a portion of its cranial covering, has been
carefully watched during the waking state and in sleep, and it has been
ascertained that, both in man and in the lower animals, the organ is
comparatively bloodless during sleep, and its circulation more sluggish
than at other times.
In the early part of this century Blumenthal first enunciated this
theory, and supported it by the interesting case of a patient who
had lost a portion of the right frontal bone; during sleep the brain
was seen to be anæmic and in a collapsed condition. Dendy[9] relates
a similar case, which was observed in 1821. But Durham's memoir
on the physiology of sleep, which was published in the volume of
"Guy's Hospital Reports" for 1860, was the first really thorough and
scientific contribution to our knowledge of the vascular state of
the encephalon during sleep, and the relation of that state to the
phenomena of sleep. To Hammond also, many of whose experiments were
made prior to Durham's publication, we are indebted for numerous
original observations, and for the most exhaustive and conclusive
exposition of the subject yet given to the world.[10]
We may see that during sleep all the encephalic blood-vessels are under
a diminished pressure, as proved in fact by the manometer, and that
this lessening of the active flow corresponds with a diminution of
cerebral function. Even if no experiments had ever been made, inductive
reasoning would have led irresistibly to this conclusion. During the
intervals of digestion the gastric mucous membrane is relatively pale
and bloodless; the submaxillary gland does not become turgid with blood
until it begins to secrete saliva; a muscle in action becomes markedly
hyperæmic. It is so with the organs in general. The performance of
function is characterized by vascular activity and fullness. If in any
part there is a call for work, there is a call for more blood. The
nervous system forms no exception to this law, and there is the most
intimate and absolute correlation between the evolution of nervous
energy and the activity of the circulation. So true is this that it is
everywhere admitted that the induction of functional work in any such
apparatus as the digestive, the sexual, or the muscular produces a
degree of hyperæmia of the apparatus called into action sufficient to
prove a serious hinderance to the easy and satisfactory performance of
any severe mental task.
Professor Mosso, of Turin, has lately made some interesting experiments
on persons who had lost portions of the cranial bones, using Marey's
ingenious hydro-sphygmograph. Noting, like others before him, that
during sleep the brain diminished in volume, with shrinkage of its
blood-vessels, and that the lively blush characterizing its surface
during the waking state disappeared, he observed also that any sudden
impression, if sufficient to rouse the brain to partial activity, was
sure to be attended with an increase of its vascularity and its volume.
He has proved, too, that every effort of the intellect is normally
accompanied by a diminution of volume in the peripheral parts, the arm,
for example, and that, on the contrary, when the cerebral activity is
lessened the distant members are augmented in volume. Sleep is always
accompanied by a dilatation of the vessels of the extremities, and
particularly of the forearm, where this dilatation has repeatedly been
measured by Mosso with his registering apparatus. Every excitation
from without causes a contraction of the vessels of the forearm of the
sleeping subject, and the augmented blood pressure at once produces a
renewed afflux of blood to the brain. In this manner the fluctuations
of cerebral activity can be followed: a sound, a touch, a ray of
light falling on the closed lid of the sleeper, all give rise to
modifications of the cerebral circulation--unperceived, doubtless, but
possibly the source of dreams.[11]
The immediate cause of sleep is not simply the shutting off of a
portion of the blood current from the brain. There are more important
factors. Here Vulpian[12] is right. The lessening of the blood supply
to the encephalon is rather the accompaniment than the cause of sleep.
We cannot produce normal sleep in a person simply by exsanguinating his
brain, or else we should have in an ice-cap and a hot foot-bath the
speediest and most effective of hypnotics. The brain must first be in
a certain condition. There must be in the constitution of the supreme
nerve centers something that forbids further activity, and with that
cessation of activity there will be a lessening of the blood-flow to
the brain, in accordance with the physiological law before stated. What
is the particular modification of the cortical cells which renders
them less fit for the liberation of their special forces, and finally
compels a suspension of action, with a diminution of the blood supply?
Herbert Spencer has given a very plausible explanation, in accordance
with the theory of evolution:
The waste of the nerve-centers having become such that the stimuli
received from the external world no longer suffice to call forth
from them adequate discharges, there results a diminished impulse to
those internal organs which subserve nervous activity, including more
especially the heart. Consequently, the nerve-centers, already working
feebly, are supplied with less blood and begin to work more feebly,
responding still less to impressions, and discharge still less to the
heart. And so the two act and react until there is reached a state of
profound unimpressibility and inactivity. Between this state and the
waking state the essential distinction is great reduction of waste,
which falls so low that the rate of repair exceeds it.... During the
day the loss is greater than the gain, whereas during the night the
gain is diminished by scarcely any loss. Hence results accumulation;
there is restoration of nerve-tissue to its state of integrity.
According to Mr. Spencer, that rhythmical variation in nervous activity
which we see in sleep and waking is the result of adaptation, due to
survival of the fittest. "An animal so constituted that waste and
repair were balanced from moment to moment throughout the twenty-four
hours would, other things equal, be overcome by an enemy or competitor
that could evolve greater energy during the hours when light
facilitates action, at the expense of being less energetic during the
hours of darkness and concealment."[13]
With some qualification, the foregoing statement is about as
satisfactory as any that has yet been offered as to the proximate cause
of sleep. During the waking hours the vaso-motor center in the medulla
is doubtless under inhibition by the superior centers, and there is
relative relaxation of the cerebral arterioles, with dilatation of the
capillaries; when the cells of the hemispheres are exhausted, they
are no longer able to exercise this inhibition--in common parlance,
they no longer powerfully extract the blood--and the vaso-motor center
"puts on the brakes"; the blood supply is then no longer sufficient for
function, though enough for nutrition.
An ingenious theory has lately been proposed by Preyer, of Jena,[14]
according to which, to use a homely illustration, the fire ceases to
burn because the flues are clogged with cinders.
As Preyer puts it, the activity of the cerebrum is a sort of
respiration, while its repose is a sort of asphyxia of this organ.
It is certain that every psychical act, every thought, involves a
certain consumption of oxygen by the nervous substance. During waking,
this gas is furnished to the brain in the blood. If the blood supply
fails, those forms of activity which we denominate consciousness,
attention, volition, and thought cease. This is easily proved by
compression of the carotids. It is known that in the waking hours the
muscles, as well as the nerves and the nerve-centers, as a consequence
of that activity, produce substances easily oxidizable, among which
is lactic acid. Some have even attributed the sense of fatigue which
we experience after prolonged exertion to the presence of this acid
in the blood.[15] According to Preyer, after the work of the day is
done, and the quiet of sleep is sought, the waste materials of which
we have spoken, and which he proposes to call _ponogènes_ (substances
which cause fatigue), being accumulated in the tissues, little by
little undergo decomposition, by taking oxygen from the blood. They
thus divert a considerable quantity of this gas from the cerebrum, the
cells of which, deprived of this element so indispensable to their
activity, enter into a state of relative repose. These waste matters
are, then, the physical cause of sleep, which will be the more profound
and prolonged the more the blood is charged with the excrementitious
products of function. Preyer has experimented on animals by injecting
varying quantities of lactic acid into their blood, and has produced a
deep somnolent condition which could not be distinguished from natural
sleep. The use of lactate of sodium in the human subject has sometimes
been attended with a like hypnotic effect. Further researches are
needed before the question can be considered as settled.--_N. Y. Med.
Jour._
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