Gall says, "sleep is merely the inactivity, the perfect repose of the brain in health. During this suspension of the cerebral functions, the brain acquires new force, and on awaking, its functions take place rapidly."

Dendy says, "sleep expresses that condition which is marked by a cessation of certain mental manifestations, coincident with the degree of oppression; for it is an error to say that the body sleeps; it is the brain only, perhaps I may say the cerebrum, or the fore lobes; for. I believe the lower part of it (that which imparts an energy to the process of breathing and blood circulation) is never in a complete sleep, but merely in a state of languor, or rather of repose, sufficient for its restoration; if it were to sleep, death would be the result." And again, "sleep is, indeed, the reality of another existence."

Sir Thos. Brown affirms "that we are somewhat more than ourselves in our sleep, and the slumber of the body seems to be but the waking of the soul. It is the negation of sense, but the liberty of reason; and our waking conceptions do not match the fancies of our sleeps."

Scaliger, an eminent Italian physician of the 15th century, describes sleep to be "a rest or binding of the outward senses, and of the common sense, for the preservation of body and soul." This definition, in some respects, i$ the most philosophical to be found ; for sleep does, so far as we are able to comprehend, consist in this binding of the "outward senses and of the common sense" as a leading characteristic of its phenomena.

But Richerand has more nearly defined sleep than any other philosopher. He says that "sleep is the repose of the organs of sense and voluntary motion." Had he limited this repose to the organs of sense, he would have more nearly approximated the fact of what sleep is, than by involving voluntary motion; which does not necessarily repose during sleep, as somnambulism is a common occurrence, and change of posture is usual with most persons.

To define sleep, qualities negative as well as positive must be embraced. Sleep is that condition of existence, in which the mind is separated from external things; and although the mind is uninfluenced by circumstances from without, still it is not inactive, but (as will afterwards appear) ceaselessly operative. The mind has sources of stimulus in the various internal sensations, which, during sleep, influence and control it, at least so far as their powers extend, with as much vigor as during wakefulness. It is also, modified, as well as diversified in its operations, by the recollection of things past, which have become the subjects of memory. In short, we would say that perfect sleep consists in the abeyance of the functions of the organs of external sense - at least an ordinary activity of the organs of internal sensation - a full perception by the intellectual faculties of their appropriate stimuli, and a loss of memory during its continuance.

Sleep is a part of the animal constitution, and has its necessity in our organization. Sleepiness, ceteris paribus, is in proportion to age. Infants pass most of their time ill sleep; youth requires more than manhood, and manhood more than old age. Variations of this general law exist in idiosyncrasies and pathological conditions which interrupt the normal condition. .

Nutrition and sleep are intimately connected. In the season of life when the growth of the body is most vigorous, more deep is requisite than when this period has been displaced by one in which only the wastes of the organization are to be repaired, and this also requires more than when the recuperative energy has given place to gradual decay.

The necessity for sleep does not depend upon the amount of food consumed, but upon the quantity required for the purposes of nutrition. Many old persons eat enormously; notwithstanding they daily become more emaciated, but are. nevertheless in ordinarily good health, and pass but little time in sleep. Analogous cases are common among the insane. Their flesh wastes; they sleep- but little, and yet consume, when unrestrained, alm6st incredible quantities of food.

Lord Stanhope, in speaking of the causes of the restoration produced by sleep, says that "the refreshment, which it produces is very different from that which is derived from food, which may supply as large a quantity of arterial blood. A person, may live some days without any food, and for a considerable time with only a small quantity, but he cannot long exist without sleep. The refreshment does not arise merely from repose of the body or of the mind, for in some persons the latter is always very active, and the former, though it may continue to be so for many hours, is not revived by sleep, but feels languor, if not lassitude." Probably on some occasion, every one has felt the bad effects of sleeplessness on his physical powers. In some instances of the disease popularly termed "ship fever" (typhus), we have known patients to sleep soundly, and on awaking, experience no relief from the excessive weariness that oppressed them.

But what was most remarkable, they were entirely unconscious of having slept, and denied the fact, although their attendants knew that this, even for hours, had been the case. The physical being did sleep most perfectly, but the influence of the disease counteracted its ordinary beneficial effect.

Duration of sleep is influenced by many causes, and, under certain circumstances, varies as much in one as in different individuals. "Dr. Plot," as copied by Dr. Binns, "relates the case of a poor girl, eight years old, who, being beaten by a severe step-mother, and sent hungry with some refreshments to her father in the fields, could not refrain from eating part of them. Reflecting afterwards on the probable consequences of her conduct, she proceeded no further on her way, but retired to a neighboring wood, and there fell into a profound sleep, being oppressed with fear and sorrow: in this state she remained for seven days, and when discovered, showed no symptoms of life, besides the softness of her flesh and the flexibility of her joints. Dan Ludovicus, from whom Dr. Plot borrows this relation, happening to be present, succeeded in his attempts to recover this poor creature. The same author has also preserved another instance of a sleeper, in the circle of his own acquaintance. This is the history of Mary Foster, of Amsterdam. She remained in a profound sleep for fourteen days and nights, after ah equal period of fear and anxiety, occasioned by her falling casually into a well; and the accident seems to have produced in her a disposition to torpor; for two years after, she slept two nights and a day at Uttoxeter."