And here I may remark, as a convenient place for the observation, that the brain, like the optic nerve, does not readily part with impressions of great vividness, so as to lapse into sleep; or if sleep does come on, the ideas still continue unaltered. How common is it for persons to say that they have been too much excited to sleep. New company, intensely interesting conversation, new sights, unexpected intelligence, etc., are all sufficient to prevent the accession of sleep, by reason of the unusual vividness of the impressions, - the vividness being dependent on their nature, or their degree, or their mere novelty; a new impression being cæteris paribus, more vivid than one repeated. A person cannot sleep away from home. This may generally be traced to the fresh objects and impressions. But some aver that they cannot sleep in a strange bed, without reference at all to the preceding or accompanying circumstances, or to the comfort or discomfort of the bed; and I do not disbelieve them. The sensibility of some persons is no measure for that of others, as to the corporeal functions, any more than as to mental conditions. A new bed may, in its arrangements, make a number of new impressions on the cutaneous nerves not possible to be individually specified; but, in the aggregate, bringing an amount of new and vivid sensation to the brain, incompatible with the supervention of sleep.

To return. For natural sleep, a certain wholesome amount of cerebral fatigue is necessary. To enter upon all the auxiliary or interfering agencies, would take me into a discussion far too protracted; and I am treating the subject in its scientific, rather than in its practical aspects. A very few words may suffice.

The effect of posture was touched upon when we spoke of the blood vessels, in relation to the nervous centres. I shall now add, that general plethora tends to somnolency by the pressure on the brain, and that this must not be confounded with the effect of excited capillary circulation, which causes excessive functional activity of the brain, and sometimes obliges the subject of it to support the head on higher pillows than what are ordinarily used.

The influence of food is matter of common remark, and, at first sight, seems to be beset with many anomalies and incongruities. Early dinners or late dinners, supper or no supper, - these have their respective advocates. To me the truth of this matter is comprehended in a small compass. During the process of digestion, the brain, unless stimulated by alcoholic liquids, is disposed to quiescence by that law of balance of function to which I have already adverted. There is a diversion of vital energy (through the nerves and blood vessels), from the brain to the stomach. Sleep in this state is natural, and comports with what we observe in the lower animals. Yet we must take care not to rush at once to the conclusion that it is well to take a nap after dinner, and to eat a good supper before retiring to rest. Our whole mode of life, in modern civilization, has become so artificial, that there is need of constant compromise and conciliation. No rule can be laid down without a knowledge of the health and habits of the individual. Digestion is sometimes absolutely disturbing to sleep. When it is accomplished with difficulty, morbid impressions on the nerves of the stomach reverberate in the brain, keeping it awake, or suggesting strange or even miserable dreams. On the other hand, the nerves of the stomach instead of fretting over their work, may be unhappy for want of occupation, and their cravings also echo in the brain, and disquiet that . noble organ! Or without these importunate complainings from the stomach, the brain goes on working, simply because it is enjoying a monopoly of vital energy, while the other has no employment. The very heavy and all but apoplectic sleep of those whose hands have been too generous to their mouths, has its cause in the pressure on the brain, induced by the over-filled blood vessels, to say nothing of the direct narcotic influence of some of the ingredients. But this is a subject too humiliating to the higher part of human nature, to deserve consideration in the transactions of an institution devoted to Philosophy, Literature, and the Fine Arts, and therefore I shall say no more about it!

Though sleep is usually promoted by the absence of light and sound, a certain amount of either may be necessary. A person accustomed to a light in the room, may be unable to sleep without it, - nay, to awake if it is withdrawn. And wakefulness may be induced by the subsidence of an accustomed noise; the habitual impression on the sensory nerve being tantamount to a new impression, because the nerve is in an unusual condition.

But some sounds are absolutely lulling. Unvarying sounds are generally so; - " the droning flight" of the beetle ; the " drowsy tinkling " of the sheep-bell; the rippling of water; the sighing of the wind among trees and sedge; the deep boom of the sea; or, as old Burton says, " to have a basin of water still dropping by one's bedside, or to lye near that pleasant murmur, lene sonantis aquæ, some flood-gates, arches, falls of water, like London bridge, or some continual noise which may benumb the senses."

This effect is prettily described by Spenser: -

And more to loll him in his slumber soft, A trickling stream, from high rock tumbling downe,

And ever drizzling rain upon the loft, Mixed with a murmuring winde, much like the soune Of swarming bees, did cast him in a swoune."

C. I., Stanza 41.

Not only undulating sounds, but also undulating sights conduce greatly to sleep. Those regular and almost rhythmical movements of the hands before the eyes, sometimes practised by magnetizers, have a stupefying effect, and induce sleep, though it is usually of an imperfect kind, like that of the somnambulist, and, as such, productive of strange phenomena in the nervous system, which prove very exciting to the speculations and fancies of those who delight in mysticism. Gentle .friction of the skin, inducing, as in the other cases, a succession of uniform impressions, is also very sedative.