faculties already specified, must be regarded as results to whose attainment the nervous system is essential: a statement that does not apply to its influence over the processes of digestion and nutrition; which could certainly be performed without its interference, by the vital endowments of the organs immediately concerned in them. But here its control has been elegantly compared to the sway exercised by a rider over his horse; which, although it does not cause the movements of the animal, yet determines their speed, their regularity, and the periods of their commencement or cessation. In like manner the nervous system influences the operations referred to; and its connexion with them, although stopping short of causation, must be placed among the most important of its numerous offices in the body.

The apparatus by whose working these results are obtained, is formed, chiefly, by two kinds of tissue, presenting diversities of tint, at once apparent to the naked eye; and of minute structure, which require the aid of a microscope for their detection. They are distinguished, in consequence of the first, as the grey and the white; and in consequence of the second, as the granular and the tubular matter of the nervous system. These two elements are always found in close and intimate union with each other; but the exact method of their combination and arrangement varies considerably in different parts, and appears to be governed more by an economy of space, than by any principle connected with the proper performance of their operations. They are as distinct in their respective offices as in their colour and formation; the grey matter being the active

portion, and the seat of all those changes by which the functions of the nervous system are performed; while it is the province of the white portion to convey intelligence of these changes from one collection of the grey matter to others, or to the part of the frame where their final effects are to be produced: an arrangement bearing considerable resemblance to that of the electric telegraph; which consists, as is well known, of galvanic batteries, with wires uniting them, and serving to produce in one battery the action which the hand of man has excited in the other. Thus, the movement of the needle that is occasioned by the operator, say in London, is accompanied by the transmission of a force along the wire, which causes the same movement to take place also in Edinburgh, or elsewhere, in the corresponding needle of another dial. Precisely in a similar way (although by the power of a different agent) the changes worked in one portion of the grey matter of the nervous system, under the influence of external conditions, are carried on or propagated, through the white tissue, to other portions; upon which these external conditions cannot exert an immediate effect, any more than the telegraph clerk in London can alter the Edinburgh needle with his hand.

The distinction already made between the functions to which the nervous system is essential, and those to which it is only an important adjunct, points to a corresponding difference between the parts by which these functions are respectively performed. Accordingly, the powers of Sensation, Motion, and Thought, are found to reside in the portion denominated cerebro-spinal, that is, in the brain, the spinal

cord, and the nerves immediately proceeding from them; while the control of the processes of growth and nutrition is vested in another series of nerves, called the sympathetic or ganglionic system. The structure and operations of each of these great divisions will require to be briefly explained; and all notice of the second must be deferred until after the description of the first.

Before commencing an account of the organs by which these great powers are conferred, it may be remarked that two of them, namely, Sensation and Thought, are inherent in the nervous system itself, and might be exercised by it when standing alone, without the intervention of any other part of the body. But Motion, although excited by the nerves, is performed by the Muscles, and is but the expression of an influence exerted upon them. It is, therefore, necessary to preface a description of the nerves themselves, by a description of the instruments which they employ; and to devote a few lines to the essential characters of the muscular system.

A muscle is a bundle of fleshy fibres, lying parallel to each other, like sticks in a faggot; and firmly adherent, at each extremity, to bones or other points of attachment. These fibres are contractile, that is, they possess the power of shortening themselves; and, by the exercise of this power, their points of attachment are necessarily brought nearer together than before, so that, if a joint be placed between them, it will either be bent or straightened, accordingly as the muscle is situated with regard to it. For instance, there are muscles extending from the upper part of

and

the shoulder to the arm just below the elbow ; those of them on the front of the limb bend the elbow by their contractions, while those on the back straighten it. These muscles may be felt to shorten themselves, whenever the action proper to either set is performed, and by their size and hardness the general energy of the muscular apparatus may be pretty fairly estimated. The various muscles constitute the whole bulk of lean flesh; and in cooked meat, especially in that of young animals, such as veal and lamb, their fibrous character is distinctly apparent. They are the direct causes of all sensible movement, by virtue of the above-described contractility; and they are divided into two great classes, one of which comprises those subject to the control of the will; and the other, those independent of it. The former class furnishes the instruments of every act that is a subject of consciousness; and the muscles included in it can be called into action only by an influence transmitted to them through the nerves. The latter is chiefly concerned in effecting certain internal movements, which commonly take place without our knowledge, and which, although they are liable to be influenced by the sympathetic nerves, are not positively known to be dependent upon them for their occurrence. To the former alone, therefore, will reference be made in connection with the brain and spinal cord; and the latter, for the present, will be entirely disregarded.

The first great division of the nervous system consists of masses of grey and white tissue, contained within the cavity of the skull and of the back bone (and known technically as the nervous centres, or

commonly as the brain and spinal cord); and of slender branching filaments or nerves, which ramify extensively throughout the body; and which are closely connected with the centres by one extremity. The nerves themselves are wholly composed of the white structure, and they serve a conducting office only, by supplying the means of communication between the nervous centres and the parts outside of them. For this purpose they are divided into two great classes, namely, the afferent nerves, which convey intelligence to the centres; and the efferent, which bear back mandates from them. Each of these are arranged in bundles, or nervous trunks, for the convenience of packing, and for protection; the trunk consisting of a number of minute filaments, which extend directly from the nervous centre to their outward destination, subdividing themselves, however, into smaller trunks, and, ultimately, into single filaments, before this destination is attained. The afferent filaments proceed for the most part from the skin, or from the organs of special sense,1 to reach that centre within which they terminate; and each of them, at its farther extremity, lies in contact with a minute portion of grey matter, whose

1 The organs of special sense are the eyes, ears, nose and palate. each of which is susceptible of only one kind of impression; and of one that has no other inlet to the brain. Thus the eye is the sole instrument of vision, while it is perfectly insensible to sound; and the ear has a similar monopoly of hearing, although light produces no effect upon it. The word "special" is used in contradistinction to that general sensibility of the surface of the body, which is possessed in some degree by all its parts, and which enables them to take cognizance of a great variety of external changes.