The theory of the origin of sleep which has gained the widest credence
is the one that attributes it to anæmia of the brain. It has been
shown by Mosso, and many others, that in men with defects of the
cranial wall the volume of the brain decreases during sleep. At the
same time, the volume of any limb increases as the peripheral parts of
the body become turgid with blood. In dogs, the brain has been
exposed, and the cortex of that organ has been observed to become
anæmic during sleep. It is a matter of ordinary observation that in
infants, during sleep, the volume of the brain becomes less, since the
fontanelle is found to sink in. It has been supposed, but without
sufficient evidence to justify the supposition, that this anæmia of
the brain is the cause and not the sequence of sleep. The idea behind
this supposition has been that, as the day draws to an end, the
circulatory mechanism becomes fatigued, the vasomotor center
exhausted, the tone of the blood vessels deficient, and the energy of
the heart diminished, and the circulation to the cerebral arteries
lessened. By means of a simple and accurate instrument (the
Hill-Barnard sphygmometer), with which the pressure in the arteries of
man can be easily reckoned, it has been recently determined that the
arterial pressure falls just as greatly during bodily rest as during
sleep. The ordinary pressure of the blood in the arteries of young and
healthy men averages 110-120 mm. of mercury. In sleep, the pressure
may sink to 95-100 mm.; but if the pressure be taken of the same
subject lying in bed, and quietly engaged on mental work, it will be
found to be no higher. By mental strain or muscular effort, the
pressure is, however, immediately raised, and may then reach 130-140
mm. of mercury. It can be seen from considering these facts that the
fall of pressure is concomitant with rest, rather than with sleep. As,
moreover, it has been determined on strong evidence that the cerebral
vessels are not supplied with vasomotor nerves, and that the cerebral
circulation passively follows every change in the arterial pressure,
it becomes evident that sleep cannot be occasioned by any active
change in the cerebral vessels. This conclusion is borne out by the
fact that to produce in the dog a condition of coma like to sleep, it
is necessary to reduce, by a very great amount, the cerebral
circulation. Thus, both carotids and both vertebral arteries, can be
frequently tied at one and the same time without either producing coma
or any very marked symptoms. The circulation is, in such a case,
maintained through other channels, such as branches from the superior
intercostal arteries which enter the anterior spinal artery. While
total anæmia of the brain instantaneously abolishes consciousness,
partial anæmia is found to raise the excitability of the cortex
cerebri. By estimation of the exchange of gases in the blood which
enters and leaves the brain, it has been shown that the consumption of
oxygen and the production of carbonic acid in that organ is not large.
Further, it may be noted that the condition of anæsthesia is not in
all cases associated with cerebral anæmia. Thus, while during
chloroform anæsthesia the arterial pressure markedly falls, such is
not the case during anæsthesia produced by ether or a mixture of
nitrous oxide and oxygen.
The arterial pressure of man is not lowered by the ordinary fatigue of
daily life. It is only in extreme states of exhaustion that the
pressure may be found decreased when the subject is in the standing
position. The fall of pressure which does occur during rest or sleep
is mainly occasioned by the diminished rate of the heart. The increase
in the volume of the limbs is to be ascribed to the cessation of
muscular movement and to the diminution in the amplitude of
respiration. The duty of the heart is to deliver the blood to the
capillaries. From the veins the blood is, for the most part, returned
to the heart by the compressive action of the muscles, the constant
change of posture and by the respiration acting both as a force and
suction pump. All of these factors are at their maximum during bodily
activity and at their minimum during rest. On exciting a sleeper by
calling his name, or in any way disturbing him, the limbs, it has been
recorded, decrease in volume while the brain expands. This is so
because the respiration changes in depth, the heart quickens, the
muscles alter in tone, as the subject stirs in his sleep in reflex
response to external stimuli. Considering all these facts, we must
regard the fall of arterial pressure, the depression of the
fontanelle, and the turgescence of the vessels of the limbs as
phenomena concomitant with bodily rest and warmth, and we have no more
right to assign the causation of sleep to cerebral anæmia than to any
other alteration in the functions of the body, such as occur during
sleep.
We may well here summarize these other changes in function:
(1) The respiratory movement becomes shallow and thoracic in type.
(2) The volume of the air inspired per minute is lessened by one-half
to two-thirds.
(3) The output of carbonic acid is diminished by the same amount.
(4) The bodily temperature falls.
(5) The acidity of the cortex of the brain disappears.
(6) Reflex action persists; the knee jerk is diminished, pointing to
relaxation in tone of the muscles; consciousness is suspended.
Analyzing more closely the conditions of the central nervous system,
it becomes evident that, in sleep, consciousness alone is in abeyance.
The nerves and the special senses continue to transmit impulses and to
produce reflex movements. If a blanket, sufficiently heavy to impede
respiration, be placed upon the face of a sleeping person, we know
that it will be immediately pushed away. More than this, complicated
movements can be carried out; the postilion can sleep on horseback;
the punkah-wallah may work his punkah and at the same time enjoy a
slumber; a weary mother may sleep, and yet automatically rock her
infant's cradle. Turning to the histories of sleep walkers, we find it
recorded that, during sleep, they perform such feats as climbing
slanting roofs or walking across dangerous narrow ledges and bridges.
The writer knew of the case of a lad who, when locked in his room at
night to prevent his wandering in his sleep, climbed a partition eight
to ten feet in height which separated his sleeping compartment from
the next, and this without waking.
The brain can carry out not only such complicated acts as these, but
it has been found to maintain during sleep its normal inhibitory
control over the lower reflex centers in the spinal cord.
Thus, in sleeping dogs, after the spinal cord has been divided in the
dorsal region, reflexes can be more easily evoked from the lumbar than
from the cervical cord, because the former is freed from the
inhibitory control of the brain.
The strength of stimulus necessary to pass the threshold of
consciousness and to produce an awakening has been measured in various
ways. It has been determined that it takes a louder and louder sound
or a stronger and stronger electric shock to arouse a sleeper during
the first two or three hours of slumber; after that period, the sleep
becomes lighter and the required stimulus need be much less.
The alternative theories which have been suggested to account for the
onset of sleep may be classed as chemical and histological.
In relation to the first, it has been suggested that if consciousness
be regarded as dependent upon a certain rate of atomic vibration, it
is possible that this rate depends on a store of intramolecular
oxygen, which, owing to fatigue, may become exhausted; or it may be
supposed that alkaloidal substances may collect as fatigue products
within the brain, and choke the activity of that organ. Against this
theory may be submitted the facts that monotony of stimulus will
produce sleep in an unfatigued person, that over-fatigue, either
mental or bodily, will hinder the onset of sleep, that the cessation
of external stimuli by itself produces sleep. As an example of this
last, may be quoted the case recorded by Strumpel of a patient who was
completely anæsthetic save for one eye and one ear, and who fell
asleep when these were closed. Moreover, many men possess the power,
by an effort of will, of withdrawing from objective or subjective
stimuli, and of thus inducing sleep.
The histological theories of sleep are founded on recent extraordinary
advances in the knowledge of the minute anatomy of the central nervous
system, a knowledge founded on the Golgi and methylene blue methods of
staining. It is held possible that the dendrites or branching
processes of nerve cells are contractile, and that they, by pulling
themselves apart, break the association pathways which are formed by
the interlacing or synapses of the dendrites in the brain. Ramon y
Cajal, on the other hand, believes that the neuroglia cells are
contractile, and may expand so as to interpose their branches as
insulating material between the synapses formed by the dendrites of
the nerve cells. The difficulty of accepting these theories is that
nobody can locate consciousness to any particular group of nerve
cells. Moreover, the anatomical evidence of such changes taking place
is at present of the flimsiest character.
If these theories be true, what, it may be asked, is the agency that
causes the dendrites to contract or the neuroglia cells to expand? Is
there really a soul sitting aloof in the pineal gland, as Descartes
held? When a man like Lord Brougham can at any moment shut himself
away from the outer world and fall asleep, does his soul break the
dendritic contacts between cell and cell; and when he awakes, does it
make contacts and switch the impulses evoked by sense stimuli on to
one or other tract of the axons, or axis cylinder processes, which
form the association pathways? Such a hypothesis is no explanation; it
simply puts back the whole question a step further, and leaves it
wrapped in mystery. It cannot be fatigue that produces the
hypothetical interruptions of the dendritic synapses and then induces
sleep, for sleep can follow after fatigue of a very limited kind. A
man may sleep equally well after a day spent in scientific research as
after one spent in mountain climbing, or after another passed in
idling by the seashore. He may spend a whole day engaged in
mathematical calculation or in painting a landscape. He fatigues--if
we admit the localization of function to definite parts of the
brain--but one set of association tracts, but one group of cells, and
yet, when he falls asleep, consciousness is not partially, but totally
suspended.
We must admit that the withdrawal of stimuli, or their monotonous
repetition, are factors which do undoubtedly stand out as primary
causes of sleep. We may suppose, if we like, that consciousness
depends upon a certain rate of vibration which takes place in the
brain structure. This vibration is maintained by the stimuli of the
present, which awaken memories of former stimuli, and are themselves
at the same time modified by these. By each impulse streaming into the
brain from the sense organs, we can imagine the structure of the
cerebral cortex to be more or less permanently altered. The impulses
of the present, as they sweep through the association pathways, arouse
memories of the past; but in what way this is brought about is outside
the range of explanation. Perhaps an impulse vibrating at a certain
rate may arouse cells or fibrils tuned by past stimuli to respond to
this particular rate of vibration. Thus may be evoked a chain of
memories, while by an impulse of a different rate quite another set of
memories may be started. Tracts of association are probably formed in
definite lines through the nervous system, as during the life of a
child repeated waves of sense impulses beat against and overcome
resistances, and make smooth pathways here and there through the brain
structure. Thus may be produced growth of axons in certain directions,
and synapses of this cell with that. If the same stimulus be often
repeated, the synapses between groups of cells may become permanent. A
memory, a definite line of action which is manifested by a certain
muscular response, may thus become structurally fixed. If the stimulus
be not repeated, the synapses may be but temporary, and the memory
fade as the group of cells is occupied by a new memory of some more
potent sense stimulus. Many association tracts and synapses are laid
down in the central nervous system when the child is born. These are
the fruits of inheritance, and by their means, we may suppose,
instinctive reflex actions are carried out.
So long as the present stimuli are controlled by past memories and are
active in recalling them, so long does consciousness exist, and the
higher will be the consciousness, the greater the number and the more
intense the character of the memories aroused. We may suppose that
when all external stimuli are withdrawn, or the brain soothed by
monotony of gentle repetition, and when the body is placed at rest,
and the viscera are normal and give rise to no disturbing sensations,
consciousness is then suspended, and natural sleep ensues. Either
local fatigue of the muscles, or of the heart, or ennui, or exhaustion
of some brain center usually leads us to seek those conditions in
which sleep comes. The whole organism may sleep for the sake of the
part. To avoid sleeplessness, we seek monotony of stimulus, either
objective or subjective. In the latter case, we dwell on some
monotonous memory picture, such as sheep passing one by one through a
gap in the hedge. To obtain our object, we dismiss painful or exciting
thoughts, keep the viscera in health, so that they may not force
themselves upon our attention, and render the sense organs quiet by
seeking darkness, silence and warmth.--L.H., in Nature.
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