Page images
PDF
EPUB

indeed as bordering upon a morbid state from the pain liable to attend them; whereas that degree of exertion which is moderate and healthy, unaccompanied by inordinate determination to the head, and occasioning no uneasy sensation, alone disposes to sleep, for the following reasons:

In all moving organs, the state of action, as was formerly shewn, is succeeded sooner or later by a state of comparative relaxation

or rest.

In the voluntary organs, the uneasy sensations which arise from over-exertion prompt us to discontinue our efforts in time; but in the involuntary organs, over which the influence of the will does not prevail, this remission of action is spontaneous.

And accordingly, like other involuntary organs, the capillary vessels at stated periods shew their disposition to spontaneous relaxation; as seen in the swelling of the lower extremities towards evening, in the flush of the face, and the redness of the eyes that accompany drowsiness; a change of action in the vessels, analogous to that which occurs in the muscular fibres of the stomach after repletion, or which was shewn to occasion the spontaneous suspension of hunger upon fasting longer than usual.

Now the exertion which disposes the capillary vessels to take on this state of relaxation must be moderate in degree; for overdistension produces the same effect in them as in other organs, or it occasions resistance and not yielding; as seen in the stomach for instance, which yields to pleasing impressions on taking food, but is roused to resistance by those which are displeasing, or by excessive repletion, and then eructations, nausea, or vomiting is the consequence.

In like manner impelling the blood too forcibly into the capillary vessels rouses them to resistance, and keeps up active circulation; as appears by the increase of vascular action in local inflammation: whilst the unfavourableness of accelerated circulation in disposing to sleep appears from the wakefulness attendant upon inflammation of the brain; and from the absence of sleep, which often continues for whole weeks during the active stage of insanity.

While increasing the stimulus of distention seems to prevent

the spontaneous relaxation in the capillary vessels; the partial abstraction of that stimulus appears to promote it; and hence the simultaneous remission of action in the heart, which causes a retardation of the pulse at night, contributes to that effect; and accordingly sleep appears to proceed from the conjunction of these causes, producing full but slow circulation in the brain.

The proof of this, however, can only be rendered complete by a general survey of the symptoms attending this state, the causes that promote it, and the means that prevent or remove it.

To return, then, the symptoms present must first be shewn to indicate the existence of this state of passive congestion, which they clearly do.

The relaxation of the capillary system during sleep, as already stated, is a general and not a local change; but as the faculties of the mind, and the powers of voluntary motion, are most conspicuously affected, it is in the organs subservient to these functions, which are the brain and spinal chord, that evidence of this change is particularly called for.

Among the circumstances that denote this state of the brain, one of the most obvious and familiar is a symptom usually preceding sleep; to wit, a certain degree of flush in the face. Now this being unattended with any increase in the impulse of the blood, can only be referred to relaxation of the vessels; and the contiguity of the vessels of the face to those of the brain affords a presumptive argument, that a similar change takes place in both.

But a more decisive indication of this state of the cerebral capillaries presents itself in the appearance of the eyes, which acquire a manifest degree of redness as sleep approaches, and are sometimes so completely transfused with blood that their vessels appear as if injected. Since the eyes derive their principal supply of blood directly from the brain, the changes of circulation in the one, may be fairly inferred from those of the other, according to the laws of vascular sympathy, as formerly pointed out.

While the cerebral vessels thus appear to take on a state of relaxation, it is reasonable to expect that those of the spine should participate in the change. For the spinal marrow, though physiologically distinct from the brain in regard to function, is yet when anatomically considered, a part of the same organ, or a prolongation of the brain.

As the situation of this organ precludes the possibility of any visible demonstration of the changes it undergoes, we must look to its function for evidence of their existence, and here they appear sufficiently evident.

It is an admitted fact in pathology, that undue pressure in the spine, is capable of paralyzing the voluntary muscles; and morbid increase of circulation in this part, causing excessive mobility, was shewn in treating of vascular sympathies to be the most probable cause of the convulsive affections termed chorea and

tetanus.

If then the vessels of the spine pass through the same periodical changes of circulation as those of the brain, corresponding fluctuations of function should be observable in the organs dependant upon the spine, or those of voluntary motion.

Now the only voluntary muscles, which are constantly and uniformly exerted throughout the day, and which are therefore diurnal in their stages of action, are those which support the head, the lower jaw, the eyelids, those of the trunk, and the muscles of respiration. These although termed voluntary, partake of the nature of involuntary organs, and continuing in action many hours together, they belong to that class in which tone and permanency of contraction predominate rather than mobility. It is therefore to be expected that this character will prevail through their different stages of action.

That stage of action which immediately precedes relaxation, or the third stage of activity, is marked by a tendency to inordinate mobility, or spasmodic action; and accordingly at this period such a tendency is evinced by the affection of those muscles that constitutes yawning and stretching, which clearly appertain to the nature of the tonic spasm, and are the frequent precursors of drowsiness.

This tonic spasm being semi-voluntary, may either be considered as an instinctive effort to obtain relief from an uneasy sensation in the organs, or as the spontaneous result of the spasmodic tendency that denotes the approach of fatigue.

',

Similar also in its nature is the full inspiration which often accompanies a yawn, being somewhat different from a sigh, and less like an effort to obtain relief from accumulation of blood in the lungs, than like a spasmodic expansion of the chest, from the muscles of respiration partaking of the same spastic tendency that affects those of the jaw in yawning.

The next change of function that takes place in these organs, most unequivocally betokens the presence of the stage of fatigue, and the tendency to spontaneous relaxation. The eyelids now close, the head can no longer be held up, but keeps falling forward, and the muscles of the back are so weary, that we are unable to sit erect without pain. In short, a temporary paralysis prevails, an effect which congestion in the spine is well known to be capable of producing; and that this is its cause, in the present instance, will be rendered further apparent by the consideration of the means that promote this state.

Amongst the causes that promote sleep one of the most conspicuous is the recumbent posture, which favours its accession in two ways. In the first place, it withdraws from the heart the principal stimulus that excites it to action, namely, the distending force of the column of blood descending from the head, estimated by Hales, in his Statical Essays, at four or five pounds; the removal of which, promotes retarded circulation, by allowing the heart to relax. And secondly, the horizontal posture promotes sleep, by producing permanent and moderate congestion in the brain and spine, upon the common principles of gravitation, when these parts are placed on a level with the rest of the body; and thus the conjunction of causes favourable to sleep is effected. A moderate degree of exercise during the day is well known to render sleep sounder at night; and according to the principle that the subsequent relaxation of vessels bears a relation to the degree of their previous exertion, it is easily explained how this becomes a predisposing cause to sleep.

Sleep is promoted by most causes that excite a pleasing sensation; which have a tendency to produce relaxation of vessels, upon principles explained in treating of corporeal impressions; provided always that these causes be applied when the vessels are

predisposed to this state of quiescence. Thus after exercise, especially in the open air, and in frosty weather, sleep is promoted by a warm fire, a hearty meal, and a few glasses of wine. When the vessels have not been disposed to rest by previous exertion, exhilaration is the usual effect that results from these causes, instead of sleep.

The darkness and silence of night are evidently conducive to the production of sleep, and apparently owe their influence to the abstraction of stimulus, or the removal of all external impressions, which are calculated to keep up active circulation.

Opium and some other drugs have a powerful effect in promoting sleep; and their operation well accords with the views here offered.

Whether their primary effect be to accelerate or retard circulation, has been a subject of controversy; but its decision is immaterial to the present question, since it is allowed that their ultimate effect is to retard the pulse; and it is then, and not till then, that sleep is produced by them.

The reason why extreme cold is productive of lethargic torpor, may be accounted for without inquiring into its physical influence and mode of operation, Like opium it retards the pulse in a remarkable manner, and this alone is sufficient to account for its influence in promoting sleep.

It is, however, probable, that cold and opium act upon somewhat different principles; opium disposing the vessels to relax by deadening their sensibility; whereas cold, though it also deadens sensibility, appears further to obstruct circulation by constricting the vessels and impairing their power of action.

Whilst the constriction of vessels on the face and surface impedes circulation and retards the pulse; the blood driven inwards promotes at the same time internal congestion in the head; and thus the conjunction of the causes favourable to sleep is effected by extreme cold.

Physical causes act mostly upon distant parts, and extend their influence to the brain through vascular sympathy; but mental impressions act directly upon the brain itself, and their operation becomes therefore a question of greater interest.

« PreviousContinue »