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The difference between the two classes of animal and nutritive functions is strikingly exhibited in the phenomena we have now to consider. Whilst the former are more or less suspended, the latter continue their action with but little modification.
The functions of sensibility, voluntary motion, and expression, cannot be indulged for any length of time, without fatigue being induced, and a necessity existing for the reparation of the nervous energy which has been expended during their action. After a time, the length of which is somewhat influenced by habit,—the muscles have no longer power to contract, or the external senses to receive impressions; the brain ceases to appreciate; mental and moral manifestations are no longer elicited; and the whole of the functions of relation become torpid, and remain in this state until the nervous system has been renovated, and adapted for the repetition of those functions, which, during the previous waking condition, had been exhausted. This state constitutes sleep; which, consequently, may be defined—the periodical and temporary suspension of all or of most of those functions that connect us with the universe. The suspension occurs in these functions and in these only; and hence the consideration of sleep in many physiological treatises has immediately followed that of the functions of rela
The nutritive functions continue regularly in action from the earliest period of foetal formation; before mental manifestations exist in the embryo, and during sleep. For them there is no cessation, and scarcely any declension of activity, until the decadency of the frame affects them along with the whole of the machinery. Sleep, in the language of poetry, has been compared to death; and Dr. Good has stated that the resemblance between them is not less correct upon the principles of physiology, than it is beautiful among the images of poetry. "Sleep is the death or torpitude of the voluntary organs, while the involuntary continue their accustomed actions. Death is the sleep or torpitude of the whole." Physiologically the difference appears to us considerable. During the whole of sleep a process of renovation is probably going on in the organs of animal life, which adapts them for subsequent activity, and contrasts signally with the state of annihilation that constitutes death; hence the important difference between healthy sleep, and the state of coma, induced by any morbid cause; from which the patient is aroused languid and exhausted, instead of active and recruited. The foetus in utero is also described by some as being in a perpetual sleep, until aroused by the new actions established at birth. It is probable, however, that there are, even in
this case, something like alternations of activity and suspension in the nervous functions. We have seen elsewhere that they are manifestly more or less exerted during intra-uterine existence; nervous energy must therefore be expended; and renovation,—to a much less extent, it is true, than in the new-born child,—be necessary.
Linnæus, under the term somnus plantarum, expresses a peculiar state in the constitution of many plants during the night, evinced by a change of position,—generally a drooping or folding together of their leaves or leaflets; such a change being occasioned by the withdrawal of the stimulus of light, and, probably, it has been conceived, constituting a state of rest to their vital functions; but it is obvious that there can be no similitude between this condition and that of the sleep of animals, which is confined to the functions of relation,—functions that do not even exist in the vegetable.
The approach of sleep is indicated by signs that are unequivocal, and referable to the encephalon. The great nervous centre of animal life, feeling the necessity for renovation, an internal sensation arises in it, as well as in the whole of the nervous system over which it presides, termed sleepiness, or the sensation, or want, or desire of sleep, which, provided the waking state has been protracted, ultimately becomes irresistible, and will draw on sleep in spite of every effort to the contrary. It is affirmed, that boys, exhausted by exertion, dropped asleep amid the tumultuous noise of the battle of the Nile; and the fatigued soldier has been often known to sleep amid discharges of artillery. Noises will at first prevent sleep, but the desire is ultimately so invincible, that they cease to produce any effect. In the noisy inns of large towns, where the perpetual arrivals and departures of travellers keep up an incessant din and confusion, sleep may be for a time withheld, but it ultimately supervenes, although the tumult may be even tenfold; and if the noise should, from any cause, suddenly cease, the individual will probably awake. It is reported of the proprietor of some vast iron-works, who slept close to them, notwithstanding the noise of sledge-hammers, forges and blast-furnaces, that he would immediately awake if any interruption occurred during the night. This effect of habit is seen in the infant, which has been accustomed to the cradle. The moment the motion and noise of the cradle, or the sound of the nurse's voice,—if she has been in the custom of singing the child to sleep,—ceases, it
When the desire for sleep sets in vigorously, the animal functions become more obtuse, until they progressively fail to be exerted. The cessation does not occur in all simultaneously. The power of volition is gradually lost over the muscles; the eyes cannot be kept open; the upper eyelid falls, and if we attempt to raise it again, it appears to be weighed down; the arms fall where gravity would take them; the extensor muscles of the back, deprived of volition,
cease to contract, and the head falls suddenly forward, occasioning nodding, which rouses the brain to momentary action, to be again, however, lost. If the individual be in the erect attitude, his limbs bend under him; and if in the sitting posture, the head gradually falls upon the chest; the extensors of the trunk no longer contract with sufficient force to obviate its tendency to fall forwards; and the attitude, unsupported, can no longer be maintained. The same gradual suspension occurs in the muscular movements concerned in speech and in the production of the voice, which becomes feeble, confused, broken and ultimately lost. All the strictly voluntary muscles have, in short, their action suspended, if we except the orbicularis palpebrarum muscle, which, according to Broussais, now contracts to close the eye and shut off the stimulus of light.
If we determine to resist the desire for sleep, we yawn and stretch, for the reasons elsewhere assigned, and endeavour to arouse the functions to renewed activity. If the state of wakefulness has not been long protracted, we are successful; but all our endeavours fail, if the nervous system be so far exhausted as to render reparation indispensable.
From the commencement of sleepiness, the action of the senses is enfeebled, and gradually suspended. The sight yields first, the closure of the eyelids preventing the organ from being impressed by its special Irritant. The smell yields after the taste; the hearing after the smell; and, lastly, the touch sleeps; although the special irritants may continue to reach the organs of these senses. All the internal sensations, hunger, thirst, &c., as well as the morbid sensation of pain, are no longer appreciated. The intellectual and moral manifestations exhibit, from the commencement of the feeling of heaviness, the languor which pervades the frame. The will gradually ceases to control the functions that are under its domain, until ultimately the power of volition is lost. In the less perfect kind of sleep, or in slumber, the ideas flit in a disorderly manner, constituting a kind of delirium; but when sleep is complete the whole encephalic organ appears to be at rest, and perceptions are no longer accomplished.
The special irritants, applied to the external senses, excite no sensation. Many physiologists affirm that the internal functions. of nutrition acquire more energy during sleep; but Broussais properly disputes the affirmation, and maintains that the want of action in the senses, muscles, and intellect, must necessarily occasion diminished energy in the nutritive functions. During sleep, circulation and respiration appear to be retarded; perspiration is less active, and digestion more tardy than in the waking condition. The difference in the last respect is so great, that, as Broussais remarks, the appetite recurs many hours before the usual time where long watching is indulged, and an additional meal becomes necessary; proving the truth of the old French proverb,—" qui dort dine." Secretion, nutrition, and calorification are also less
energetically performed than usual. Absorption, alone, according to some is more active; but there seems not to be sufficient reason even for this assertion. This notion of the greater activity of the nutritive organs is as old as Hippocrates, and has been acquiesced in by almost all subsequent writers without examination, especially as it seemed to show a kind of alternation and equipoise between the respective periods of activity of animal and organic life.
During sleep, then, all the animal functions are suspended, and the body generally remains in a state of semiflexion, the one which, as we have elsewhere seen, requires little natural effort. To this, however, there are numerous exceptions depending upon habit. The easiest position for the body is perhaps on the back. It is the one assumed in extreme debility, when the prostration is so great that the individual sinks down in the bed like a dead weight; but the extensor muscles of the thigh and leg, under such circumstances, become fatigued, and relief is obtained by drawing the feet upwards so as to elevate the knees. This is a common attitude in the most debilitating maladies, and is often maintained until within a short time prior to dissolution. Sleep can persist with the exercise of certain muscles. Couriers, on long journeys, will nap on horseback, and coachmen on their boxes. The author has seen a servant boy erect and asleep in the intervals between the demands for his services at the table.
During the first sleep, the suspension of the animal functions is most complete; but, towards morning, some of them become less asleep, or more excitable than others. The intellectual and moral faculties are frequently inordinately active, giving occasion to dreams, which, with some individuals, occupy a great portion of the period allotted to rest. The sense of tact, too, is easily roused. If we lie in a position which is disagreeable, it is soon changed; the limbs are drawn away if irritated in any manner; the clothes are pulled up, if the air is disagreeably cold, &c. The sense of sight and the voluntary motions are least readily aroused, so that those functions which fall asleep the last are most easily awakened, and they gradually resume their activity in the order in which they lost it.
After six or eight hours of sleep,—more or less according to circumstances, the individual awakes, not generally at once, however; a state of slumber, like that which preceded sleep, now succeeding it. The organs, which are the last to resume their activity, require to be excited to the performance of their functions. The eyes are rubbed; stretching is indulged, which recals the nervous influx to the muscles; whilst sighing and yawning arouse the muscles of respiration, and compensate, in some measure, for the minor degree of aeration of the blood accomplished during sleep. The urine is discharged, and the phlegm, that may have collected in the air passages expectorated: these ex
cretions have accumulated during sleep, because, owing to diminished sensibility, the call for their evacuation has not been as urgent. In cases of catarrh, accompanied by copious mucous secretion, and in phthisis pulmonalis, the fluid will collect in surprising quantity in the air-passages during sleep, and it is expectorated as soon as the brain is sufficiently aroused to respond to the sensation.
When the individual is fully awake, the energy, with which the animal functions are exercised, exhibits that the nervous system must have entirely recruited during its state of comparative inaction. The period of sleep, necessary for this purpose, varies in different individuals, and at different ages. Some require eight or ten hours; others not more than three or four; and others are said to have been contented, throughout the whole course of a long life, with not more than one or two. Men of active minds, whose attention is engaged in a series of interesting employments, sleep much less than the lazy and the listless. The great Frederick of Prussia, and the yet more great Napoleon, are said to have spent a surprisingly short time in rest; but with respect to the latter, the fact is controverted by one, who had the best opportunities for observation. It is probable, however, that in these cases, the sleep is more intense, and that such of the animal functions, as require rest indispensably, are completely suspended during the whole period consigned to it. These are the functions of voluntary motion more particularly; the intellectual and moral faculties requiring a much shorter period of repose, as is manifest by their incessant activity during dreaming,—a condition which, with some, continues through almost the whole night. The same individual, too, will spend a shorter time in sleep, when strongly interested in any pursuit, than in the monotonous occurrences of ordinary life, and when any subject occupies us intently, it will frequently keep us awake in spite of ourselves; but, although the period of sleep may be protracted much beyond the accustomed hour by unusual excitation, the effect of the stimulus becomes insufficient, and sleep comes on under circumstances which appear most unfavourable to it. The lunatic affords us a wonderful example of powerful resistance to sleep and fatigue, or rather of the short period which is necessary for the renovation of the nervous system, kept almost incessantly upon the stretch, as it is in many of these distressing cases.
In infancy and youth, whilst the animal functions are extremely active, the necessity for sleep is greatest; in mature age, where time is more valued and the cares are more numerous, it is less indulged; whilst the aged may be affected in two opposite ways; they are either in a state of almost constant somnolency, or their sleep is short and light.
Sleep has been regarded, by the physiologist, as complete, and incomplete. The former is characterized by suspension of all the animal functions; a state, the existence of which has been doubted