The nearest approximation, then, towards an answer to our question is, perhaps, this; - that natural sleep is the result of exhausted action in certain portions of the nervous system; that time is required for refitting the materials of the nervous structure itself, that is, of the matter which is to be oxidated, as well as for recruiting the blood; and that no more intercourse can take place between the blood and the specially vital function of the nervous matter, without the consumption of what is required by other parts of the body.

* Dr. Snow on the Action of Narcotic Vapours. - Medical Gazette, April 11, 1851.

But though this is the ultimate change in natural sleep, the sleeping condition may be induced with but a very small degree of such change, by the mere influence of subsidiary circumstances, - some of which, like narcotics, are altogether artificial, while others may be reckoned as natural auxiliaries.

Of these we have now to speak, and the first that I shall notice is vascular pressure. It is familiarly known that compression will benumb the sensibility of a nerve, as in a limb resting on the edge of a hard seat. Analogy would lead us to expect a like result from compression of the nervous centres. But independently of analogy, we know it to be the fact from various circumstances. Thus, a man had lost a portion of his skull by an accident, and the brain, being only covered by soft parts, could be easily subjected to pressure; and at any time he could be sent to sleep by gentle pressure of the finger on this portion of the head. Again, there are certain diseases in which the brain, pent up in its bony case, is liable to pressure. It may be from excessive fulness of the blood vessels themselves, constituting one kind of ap-poplexy, which is the extreme of morbid sleep; or it may be from blood poured out of a ruptured vessel into the brain or its cavities; or it may be the increase of a thin fluid, called the cerebro-spinal fluid, which fills the cavities of the brain, and surrounds its outer surface, as well as that of the spinal cord. The morbid accumulation of this fluid is " water on the brain, " and the existence of the disease is often denoted by extreme somnolence, which passes into what is technically called coma, that is complete insensibility.

From facts of this kind, then, it may be urged that as sleep resembles these states, and is indeed a minor and transient form of that abolition of cerebral functions which belongs to them all, it is probable that sleep has a like causation, and that before its accession a certain amount of pressure takes place in the brain. This seems still more probable, when we observe that people are prone to drowsiness if the system is plethoric, and, also, that recumbence is the natural inducement to sleep; the effect of lying down being to retard the return of blood from the head, and so to produce some relative fulness of the vessels. This state, however, is not essential to sleep, for persons may slumber in the upright posture, though few can do so soundly. There is, then, very abundant reason for believing that a moderate degree of pressure on the brain is a natural disposer to sleep. But this fact has been so strongly impressed on the minds of two physiologists, that they have imagined that there is a special provision for the production of vascular pressure. Thus Dr. Osborne, of Dublin, in a very ingenious paper published some years ago,* endeavours to show that the choroid plexus is the organ of sleep. Now, the objection to this view is, that there is no proof that these vessels are particularly distensible or susceptible of any such degree of fulness, as to warrant our supposing them to have this function, though their situation, lying as they do upon the sensory ganglia, seems to me to be favourable to Dr. Osborne's hypothesis.

Dr. Marshall Hall, to whom the modern physiology of the nervous system is so much indebted, has a notion that a spasm occurs in the deep-seated muscles of the neck, before the supervention of sleep; and that the muscles in this contraction compress the veins, and so produce a temporary turgescence of the vessels of the brain. But it is yet to be proved that this occurs as one of the antecedents of ordinary sleep, however true it may be as to morbid conditions.

* Medical Gazette, Jane, 1849.

We now pass on to the consideration of other circumstances favourable to sleep. After what we have said about the proximate cause, it is almost superfluous to remark, that one of the natural antecedents is a certain amount of exercise of the brain. Its repose is the consequence of its exertion. But in different individuals we observe great differences, as to the amount of exertion necessary to induce this tendency to inaction. Some have so sluggish a nervous system, so little propensity to spontaneous action, that if they are only not stimulated into wakefulness, they are perpetually dozing. In others, the nervous activity is so marked, that it is only after strong and prolonged exertion of mind that sleep visits their eyelids. Minds of equal capacity, acuteness, and vigour, may vary extremely in their capability of sustaining labour and diminution of repose. Those who enjoy a very robust cerebral organization, require a greater amount of mental labour to tire them down sufficiently for sleep ; and they are refreshed by an amount of sleep quite inadequate to the wants of others. It is impossible to lay down a law inclusive of all individualities in this particular. I have known a person of very active mind and literary habits, unable to sleep when leading the life of a sportsman, simply because, though he was fatiguing his muscular system, he had not worked the percipient and reasoning parts of his brain sufficiently ; so that when his limbs were at rest in bed, his brain did not choose to take the same time for repose. It had been slumbering all day. I need not remark that this individual enjoyed an unusual degree of cerebral activity. Many a weary statesman and philosopher would sleep under like circumstances, the brain being ready for slumber whenever it has an opportunity. But there is an altogether different result of excessive mental fatigue not unfrequently met with. In common language, a person goes to bed too weary to sleep. In this state, the over-action of the organ continues; it cannot pass into the natural alternation of inactivity, and it is probable that the cause is to be found in the local circulation. Blood having been attracted to the brain with too much vehemence, and for too long a time, the self-adjustments with which the vessels are provided cannot come at once into action. The vessels cannot at once resume their former dimensions, and the flow through them continues in the same quantity. We see this in other parts of the body. If the eye has been subjected to an inordinate amount of light, and for too long a time, the vessels which had been unduly injected remain in this state for some time after the light has been withdrawn. Or without taking the vessels into account, we have another proof of the same thing, in the luminous impression which remains on the retina.