Objects of study may be arranged under two great divisions; one consisting of those which must be sought in a wide investigation of external nature; the other of such as are at all times, and in all places within our reach. The former are spread as far as our bodies, or our senses, aided, or unaided, can extend; the latter we carry about with us. The one class are objects of sensation, or outward observation; the other of consciousness and internal reflection ;-the world without, and the world within ; this embracing the workings of our minds, our emotions, sentiments, affections, and propensities ; the other, all the domain of matter and its attributes, - all that exists, whether we are living to observe it or not. Reviewing these two classes, we cannot help being struck with the overflowing provision which they present to our mental cravings; for while, on the one hand, the perceptive faculties have unbounded and delightful exercise amid the sublime and beautiful objects which the Creator has presented to us, in what we call the realms of Natural History, and Physical Philosophy; on the other hand, when by accident confined to narrow limits of space, deprived of one of our senses, or excluded from the objects of these senses, as in the shades of night, or in the solitude of sickness or captivity, we may turn inwards the mental eye, and see the wonders which the same Almighty Hand has fashioned in the mind and heart of man.

Our present subject belongs in some measure to both of the departments of inquiry which I have thus briefly sketched; for our knowledge of it is in part derived from our own experience, and in part from our observation of it in other beings.

To know something of that condition in which we spend one-third of our lives, is not an unworthy inquiry. And yet the thought may at first occur to you, - What can be better known than sleep? - a thing of which we have all of us common experience. But simple and obvious as it may seem to be, we shall find that the more we investigate it, the more is it productive of topics for interesting and curious speculation, and of questions not very easily answered.

What is Sleep? If we attempt to define it in positive terms, we shall find ourselves insensibly wandering among those metaphorical descriptions familiar to us in the pages of the poets, instead of giving an accurate account of its phenomena; for in fact it is a negative state of the living body, and is only to be correctly represented by the enumeration of various actions which are wanting in that condition, and the presence of which renders a person awake. To sleep perfectly is, - not to see, not to hear, not to smell, not to taste, not to touch, not to speak, not to move; in short, not to exercise one of the faculties which characterize a human being or even an animal. Well, then, may sleep be called "the image of Death," " Death's brother," "so like death," says Sir T. Browne, solemnly, "that I dare not trust it without my prayers." So, too, it was described by Homer,

"Then gentle slumber on his eyelids fell, That deep, sweet sleep, which death resembles well."

In the human body there are two great classes of vital actions. One of them comprehends all that belong to the function of Nutrition, by virtue of which the solid framework is maintained and repaired, and which consists in the continual addition of new particles of matter, and in the removal of those which have become useless. To this class also belongs the circulating function, whereby the materials for the former actions are distributed to the different parts of the body, in the form of blood; also Respiration, a process for purifying the blood, and rendering it otherwise better fitted for the purpose of nutrition; also Secretion, a process by which matters, as in perspiration, are removed from the blood, and by which fluids are formed, which serve important offices in the system.

Now, these several functions, you observe, are all occupied in maintaining the body as a living organic structure, that is, as a structure distinguished by the actions which have been adverted to, from those structures in which there is no growth, no circulation, no respiration, - bodies which, in a word, are inorganic. Moreover, these Amotions are analogous to what are observed in the vegetable kingdom, and therefore they are often called the vegetable or organic Amotions; and as the collection of vital actions is designated the life of the body, so this particular group, of which we have been speaking, is denominated the organic, or the vegetable life of the human body, to distinguish it from another group. This other group comprises sensation, thought and voluntary motion; and as these are possessed only by the members of the animal kingdom, they are called animal functions, and the sum of them the animal life; or, since it is by help of these actions that the animal entertains communications with surrounding objects, we sometimes speak of it as the Life of Relations.

In the state of waking both these lives co-exist, and render each other mutual service. The functions of relation are indicating, through the sensations, certain external means of support for the fabric, while the motor faculties obtain them, and in their turn the vegetable functions are keeping the organs of sensation and motion in a state of efficiency.

But what is the case in sleep? Here we see that the superadded functions which constitute the animal life are withdrawn, and the body, for the time, is reduced to the condition of a vegetable. They are suspended, not extinguished; but there are other states in which extinction of animal life has taken place, though the organic life continues for awhile, as in certain kinds of fatal stupor; that, for example, produced by a poisonous dose of opium. In such irrecoverable sleep animal life is extinct, though the organic may hold out for some time longer.

Ordinary slumber, then, consists in the temporary cessation of the action of voluntary muscles, and their nervous connections, and of the senses. The order in which these are steeped in forgetfulness, those in inertia, is not always the same, nor are they always suspended at the same time.