In order to answer this question, we must obtain, if possible, a clear idea respecting this element of our being, and know what kind of a machinery it is that we are using. And, in truth, we are, in this respect, constructed in a way eminently adapted both to excite and to reward our curiosity. Beautiful, and even mysterious, as many of the exhibitions of nervous activity appear, and wonderful as are its aggregate results, as displayed in the varied processes of human life, there is hardly any thing in the whole range of science better ascertained, or more simple, than are many of its fundamental principles. In this respect the study of the nervous system is like that of astronomy, in which, while the great moving force still remains unexplained, yet many laws are clearly known, and these scarcely more interesting for their practical importance than for their simplicity. "If," says Sir Charles Bell, "I could address my reader with the same freedom, and with the same examples before me, with which I speak to my pupils on this subject, I think I could interest him in it." And no one who has once experienced the fascination of the study can help having the same feeling. But it must be remembered that our knowledge extends only to a certain point. While much can be explained with certainty, many problems still remain unsolved, many questions which we naturally ask can receive only a partial answer.

It was at one time thought that the presence of a nervous system constituted a distinction between the animal and the vegetable. But this opinion does not seem to be correct. The lowest animals have no discoverable nerves; they lead merely a sort of vegetative life, and their simple structure does not demand any special mechanism for bringing into union the actions of different parts. Yet, although this is the case, the nervous system is one of the chief characteristics of animal life, and it makes its appearance immediately there is exhibited in the animal scale any complexity of structure. It is by its ineans, indeed, that various organs are blended into a whole; and thus the animal is a unit or individual, while the plant always remains a mere bundle of more or less similar parts. The proper life of the animal consists in an ability to react in a definite manner upon objects that affect it from without, not only by a motion of the part immediately affected, but by the combined movements of many, and it may be distant, organs. In this lies the primary need for a nervous system. It is in its simplest aspect merely a channel by which the affections of one portion of the body are enabled to call out the activity of another. Keeping this idea in view, we shall find there is no difficulty in following, in their gencral principles, the structure or the functions of the nervous system, even in its most highly developed and complicated forms.

It was an ancient notion that man is a microcosm, a little world, combining in himself all

throughout the greater world around him. In physiology the same idea has found a place in the representation that man embodies, and is a union of, all the lower animal natures. These ideas may have been mere dreams; yet they were dreams that contained an element of truth. The most rigid examination with the dissecting knife confirms them in a certain sense. In his nervous system man does present a combination of the structures and activities of the various forms of life below him. We live, in respect to our nerves, distinct and separate lives, and unite in our own person opposite existences. The spinal cord has one life of its own; the lower part of the brain another; and by means of its upper part we live a third kind of life, higher than the other two.

The effects, and the proof also, of this diversity of life within us are partially seen in the variety of actions which we are capable of carrying on at the same time without their interfering with each other. By this means it is that, without taking any thought, we breathe regularly fifteen times in the minute; that we maintain ourselves in the erect position without any consciousness of effort; that (almost equally without consciousness when our attention is otherwise engaged) we walk, or eat, or perform other habitual motions, and at the same time carry on a distinct train of thought, or perform complicated and delicate manual operations. We are able to do all these things at once because, besides distinct groups of muscles, we have distinct nervous systems operating within us, each regulating its own circle of activities.

But elaborate as is the structure thus provided as the condition of our varied life, and diverse as are the results which ensue from the action of its different parts, it is all constructed on one plan. Its operations when combined, as they are in our experience, make up a whole of which we can not think without wonder, and the intricacy of which seems to defy comprehension. But simplicity comes with analysis. The various elements which make up the nervous activity are presented to us by nature in various classes of animals, separated, and, as it were, distinctly exposed to view, while through them all there runs an identity of character which makes them easily reducible to a single law.

What are nerves wanted for? Not, in the first place, to make the body alive, or to give it the power of acting. The various structures of which it is composed, each for itself, have their own active properties, their own power of responding to stimulus. The muscle contracts when it is touched, or when it is galvanized, though no nerve be present; the gland pours forth its secretion under the like conditions. due supply of blood alone is necessary for all these operations. But for animal life, except in its lowest grades, this kind of activity is not enough. The sensitive plant possesses as much as this; and indeed, so far as we can judge, this "irritability" (as the tendency to perform a mo

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tion on being touched is termed) is essentially the same property in the plant and in the animal. In fact, if we suppose such a mechanism to be connected with a sensitive plant, that on any given leaf being touched, not that leaf only, but others also, and those in a distant part of the plant, should be thrown into motion-say in such a way as to guard the irritated part-we should have a pretty good imitation of the animal activity. Such a result might be brought about if there were introduced into the plant a system of tubes, or fibres, which should convey the impulse from each point to various others; or more completely still, if these fibres were connected with a central apparatus that should gather up the impulses transmitted from every leaf, and pass them on in an orderly sequence to the rest. By such an arrangement it is evident a sort of animal intelligent-looking activity might be grafted on to the mere vegetable "irritability" of the plant. No fresh power would be needed in these fibres or in the central apparatus; only a capability of receiving, and transmitting unimpaired, the impulses conveyed to it from every quarter. No fresh power would be needed, only a "susceptibility" and a definite arrangement. In truth, owing to the greater amount of the action induced in the leaves of the sensitive plant than that of the stimulus by which they are excited-a mere breath being sufficient often to produce a long contractile motion-these actions might go on by means of such an arrangement of fibres, continually multiplying, until a slight touch might suffice to throw the whole tree into - we will venture to say-convulsions. It is evident, however, that if any complicated series of actions were desired; if a touch (or other stimulus) applied to any single leaf were meant to call forth a corre

sponding action in distant parts; and especially if any large number of these actions were to be combined together, and this in many or varied groups, then the arrangement of the fibres would need to be exceedingly exact and complex. There would need to be points also at which the various impulses might be transferred from one set of fibres to another, or their progress altogether arrested for a time. In brief, the arrangements would be somewhat like those of an elaborate telegraphic system.

Such a system of tubes or fibres would closely represent in some essential characters the nervous system. If we look at the human brain, we find that it consists mainly of a vast mass of fibres. Their number, tenuity, and variety of direction are so great, that no skill has hitherto availed to trace them in detail, though their

general course has been pretty well made out. The subjoined figure may give a general conception of their multitude, and the intricacy of the web they form. Emanating from the brain and spinal cord, long lines of fibres pass to each region of the body, and distribute themselves in a minute net-work around and within the substance of every organ. So fine is this net-work that, if we could see it by itself, it would appear before us a perfect image of the body, all pure

nerve.

We have thus, in our own persons, to do with a structure similar to that which has been supposed. Our body is not primarily dependent upon its nerves; it is active in itself, instinct and throbbing with force almost in every part, but waiting the touch of the master's hand before, in health, its ordered activities are set free. Take away from a man his nervous system (if it could be done with impunity), and there were left not lifeless clay, not even a mere inanimate and passive mechanism; there were left a body physically alive, endowed with active powers as containing in every part more or less of nature's force; but a body worthless as a body, with no unity in its action, nor possibility of ordered movement to any definite purpose; a structure in the whole or in the parts of which more or fewer actions might go on, and go on with vigor, but in which these actions could be made subservient to no end.

The fibres which constitute the chief mass of the nervous system are simple in their structure, so far as the microscope can reveal it, and pre

FIGURE 1.THE FIBRES OF THE BRAIN.

ules-Nature's first and roughest work, her very starting-point in the organic kingdom-strewn in a mere mass with no appreciable order over the ends of a multitude of fibres, and loosely folded up, as it seems, for convenient stowage! This is what meets the eye. Is this the laboratory of reason; the birth-place of thought; the home of genius and imagination; the palace of the soul? Nay, is this even the source and spring of bodily order-the seat of government and control for the disorderly rabble of the muscles? Should we not have expected when we came thus to the inmost shrine of life, and penetrated to the council-chamber of the mind, to find all that had before appeared of skillful architecture and elaborate machinery surpassed and thrown into the shade? But it is all cast away. Mechanical contrivances for mechanical effects! Skillful grouping and complex organization there may be for the hand, the eye, the tongue; for all parts and every function where the mind is not. But where the spirit comes, take all

that scaffolding away.

Whether this suggestion be a true one or not, we do not know. Most probably it is not true; because it is a guess, and expresses ignorance, which ought to be deceived. But it remains a noteworthy fact, nevertheless, and surely puts our anticipation somewhat at fault, that at the very summit of the organic world, every thing that we are accustomed to call structure, and to admire as beautiful either to the eye or to the intellect, sinks to its lowest pitch. The gray matter of the brain, however, is very abundantly supplied with blood.

But to descend again to terra firma-what is the part played by the gray or cellular matter, so far as we can discover it? In order to gain clear ideas on this point, we must consider the general plan on which the nervous system is arranged, and regard it first in its simplest forms. Omitting the lowest members of the animal series in which nerves are found (and in which precisely the same principles prevail), we find in the class of insects a pattern to which all the higher forms may be referred. Figure 5 is a diagram of the nervous system of the centipede. It consists of a series of little groups of nervous cells, arranged on each side of the middle line, a pair in every segment of the body, and additional ones in the head, connected with the organs of sight, smell, touch, etc. These are all united to each other by bands of fibres, and each one sends out nerves to the organs contained in the segment in which it is placed. The nervous system of the highest animals is but a repetition, in an enlarged and condensed form, of this simple type. Figure 6 represents the brain and spinal cord of man. The masses of cells, we perceive, have become joined together, and constitute not a series of double knots, but a continuous column of varying size, and those in the head have become enormously developed. But the parallel between the two structures remains, in spite of these changes. The spinal cord of man is a series of groups of cells, giving off

nerves on each side, and connected by communicating fibres with each other, and with the larger groups in the brain, which also give off nerves to the nose and eye, the skin and muscles of the face, and other parts.

Thus in man and all animals alike masses of gray matter, or cells, are placed at the centre, and nerve fibres connect them with the organs of the body. It has been proved also, by the beautiful experiments of Sir Charles Bell, that the nerve fibres are of two kinds-some conveying an influence from the organs to the centres where the nerve cells are placed, and others carrying back an influence from them to the organs. So these groups of cells evidently answer to the stations of the electric telegraph. They are the points at which the messages are received from one line and passed on along another. But besides this, the cells are the generators of the nervous power. For the living telegraph flashes along its wires not only messages, but the force also which insures their fulfillment. A nerve bears inward, say from the hand or foot, an impression, it may be of the slightest kind; but the cells (richly bathed as they are by air-containing blood) are thrown into active change by this slight stimulus, and are thus able to send out a force along the nerves leading to large groups of muscles, and excite them all to vigorous motion. Just so a message from one line may, by its stimulus to human

They are called "ganglia" in scientific language; but this word has no deep meaning: it signifies a knot, and was applied to them simply with reference to the form they present at some places. Where a nerve passes through a small group of cells, the latter looks something like a knot tied in it.