THE PHYSIOLOGY OF SLEEP.
Various conjectures have been advanced as to the precise physiological
change that attends the condition of sleep. Some have located the source
of sleep in the heart and others in the head. It was formerly a favourite
theory that the action of the heart slackened and then the blood,
flowing slowly through the brain, caused a kind of congestion there.
This was, in fact, to look upon sleep as a species of coma that produced
unconsciousness by pressure upon the fibres of the brain.
The later and better opinion is, that sleep is produced by the reverse of
this process; that it is not a state of congestion but of collapse; that
the blood flows _from_ the part of the brain that sleeps, which is thus
left in a state of depletion, with a consequent collapse of the brain
fibres.
Observation of the actual brain of a man who had been trepanned and
over a part of whose brain a movable silver plate was placed entirely
confirmed this conjecture. In sleep, the convolutions of his brain were
depressed; when awake, they resumed their normal form; when his mind was
exerted, they swelled visibly.
Any reader who has been suddenly wakened may recal a sensation as of
swelling of the brain by the blood rushing into it. This sensation was
probably the result of the rapid erection of the flaccid brain fibres.
Other facts strongly support this theory. When the action of the heart
is stimulated by any excitement, mental or bodily, sleep will not come.
So long as the brain is busy we court sleep in vain. To induce sleep
we apply remedies that tend to draw the blood from the brain to the
extremities. A full meal engenders sleep; but not, as formerly supposed,
by congesting the brain, but by attracting the blood to the stomach and
so depleting the brain. Rapid motion in a cold wind causes drowsiness
when warmth is restored. Why? The blood is borne swiftly back to the
surface of the body and quits the brain. Many other instances will
readily occur to the reader.
Note in another the process of “falling sleep.” The eyes move more and
more slowly, the eyelids descend, the head nods and droops, the limbs
relax, the book falls from the hand. Usually, before positive sleep
occurs, involuntary endeavours at resistance are made. The eyes open with
a stare. Consciousness is regained with an effort and a start. The thread
of waking thought is resumed. But it is for a moment only. Again the head
nods, the eyes blink and close, the limbs relax. He is _asleep_.
What are our own sensations when we _go to sleep_? Thought wanders.
Ideas come straying into the mind unbidden and with no apparent
association. External objects grow dim to the eye and sounds fall faint
upon the ear. The communications of the senses to the brain are dull and
uncertain. We are conscious that the power of the _Will_ is relaxed. We
strive to retain it. We recover it by an effort. We resume the work on
which we were engaged. Vain the struggle. The thoughts wander still. The
unbidden pictures flit again before the mind’s eye. We are conscious of
the relaxation of the limbs and the closing of the eyelids. Then we cease
to be conscious of external existence. We sleep.
But we are not conscious of _the act_ of falling asleep—for itself is a
suspension of consciousness. With some sleepers sleep is, as they affirm,
a condition of entire unconsciousness. These tell us they have no sense
of existence until the moment of waking and that, however protracted
their slumber, the moment of waking is to them as the moment after having
fallen asleep. It is impossible to contradict those who thus affirm,
for their mental condition in sleep cannot be read. But if a judgment
may be formed from their _actions_ in sleep, as talking and motions of
the limbs, the probable explanation will be that they dream but do not
remember their dreams. _All_ dreams vanish from _their_ memories as
_some_ dreams vanish from the memories of those who habitually dream.
If we observe the aspect of a sleeper, we note the features placid, the
breathing regular, the pulse soft and even, the limbs relaxed, the skin
moist. Occasionally there are quiverings of the limbs and expressions of
the face which betray the presence of mental emotions.
This is the _physiological_ condition of Sleep.
We turn now to its _mental_ condition.