Page images
PDF
EPUB

A nerve, says Mr BELL, is a firm white cord, composed of nervous matter and cellular substance. The nervous matter exists in distinct threads, which are bound together by the cellular membrane. They may be likened to a bundle of hairs or threads, inclosed in a sheath composed of the finest membrane.

[graphic][subsumed]

The figure represents a nerve greatly magnified, for the sake of illustration, and consisting of distinct filaments; A, the nerve, enveloped in its membranous sheath; B, onc of the threads dissected out. The nerves in thickness vary from the diameter of a small thread to that of a whip-cord. They are dispersed through the body, and extend to every part which enjoys sensibility or motion, or which has a concatenated action with another part.

The matter of a nerve in health, and in the full exercise of its influence, is of an opaque white; it is soft and pulpy, betwixt fluid and solid, and drops from the probe. When putrid, it acquires a green colour; when dried, it is transparent. Corrosive sublimate and muriate of soda harden it; alkalis dissolve it. Each fibril of a nerve is convoluted, and runs not in a straight line, but zig-zag, like a thread drawn from a worsted stocking, which has by its form acquired elasticity that it would not otherwise have possessed. By want of use, the matter of a nerve is either not secreted in due proportion, or it changes its appearance; for the nerve then acquires a degree of transpa

rency.

There is no evidence that any fluid or spirit circulates in the nerves; nor is there any that the nervous fibrils are tubes.

Nerves are supplied with arteries and veins, and their dependence on the supply of blood is proved by the fact, that if a limb be deprived of blood, the nerves lose their powers, and sensibility is lost. If a nerve be partially compressed, so as to interrupt the free entrance of the blood into it, both the power over the muscles and the reception of sensation through it are interrupted; and when the blood is admitted again, painful tingling accompanies the change. It is not the compression of the tubes of a nerve, but the obstruction of its blood vessels, which produces the loss of power consequent on tying it. The brain, the nerve of the eye, the ear, the nerves of sense and motion, are all affected by the change of circulation; and each organ, according to its natural function, is variously influenced by the same cause -the rushing of blood into it, or the privation of its proper quantity.

A nerve consists of distinct filaments; but there is nothing perceptible in these filaments to distinguish them from each other. One filament serves for the purpose of sensation; another for muscular motion; a third for combining the muscles, when in the act of respiration. But the subserviency of any of all these filaments to its proper office, must be discovered by following it out, and observing its relations, and especially its origin in the brain and spinal In their substance there is nothing particular. They all seem equally to contain a soft pulpy matter, enveloped in cellular membrane, and so surrounded with a tube of this membrane as to present a continuous track of pulpy nervous matter, from the nearest extremity in the brain to the extremity which ends in a muscle or in the skin.

marrow.

The key to the system will be found in the simple proposition, that each filament or track of nervous matter has its peculiar endowment, independently of the others which are bound up along with it; and that it continues to have the same endowment throughout its whole length. There is no interchange of powers betwixt the different filaments;

but a minute filament of one kind may be found accompanying a filament of a different kind, each giving a particular power to the part in which it is ultimately distributed. Some nerves give sensibility; but there are others, as perfectly and delicately constituted, which possess no sensibility whatever. Sensibility results from the particular part of the brain which is affected by the nerve. If the eye-ball is pressed, the outward integuments feel pain, but the retina gives no pain, only rings of light or fire appear before the eye. In the operation of couching the cataract, the needle must pierce the retina; the effect, however, is not pain, but to produce, as it were, a spark of fire; and so, an impression on the nerve of hearing, the papillæ of taste, or any organ of sense, does not produce pain. The sensation excited has its character determined by the part of the brain to which the nerve is related at its root. But there are nerves which have no relation to outward impression. There are nerves purely for governing the muscular frame, these being constituted for conveying the mandate of the will, do not stand related to an organ of sense in the brain; hence no sensibility and no pain will be produced by them. Each of these may be said to be a nerve of exquisite feeling in one sense, that is, it may be a cord which unites two organs in intimate sympathies, so as to cause them to act in unison; yet, being bruised or injured, it will give rise to no perception of any kind, because it does not stand related to a part of the brain, whose office it is to produce either the general impression of pain, or heat, or cold, or vision, or hearing: It is not the office of that part of the brain to which it is related to produce perception at all.

At the conflux of the nervous filaments, small reddish tumours appear, which are named GANGLIONS (See D in fig. p. 61). A ganglion resembles in form the circular swellings which appear on the stalk of a straw or of a cane; but ganglions do not rise at regular intervals on the nerves

like these swellings. Ganglions are laid in a regular succession in the whole length of the body, and, in the vertebral animals, form a regular series down each side of the spinal marrow; the nerve of communication them

among is the great sympathetic nerve. There are other ganglions scated in the head, neck, and cavities of the chest and belly, which are very irregular in their situation and form.

The colour of the ganglions differs from that of the nerves; it is redder, which is owing to the greater number of bloodvessels: They consist of the same matter with the brain.

Wherever we trace nerves of motion, we find that, before entering the muscles, they interchange branches, and form an intricate mass of nerves, which is termed a PLEXUS. A plexus is intricate in proportion to the number of muscles to be supplied, and the variety of combinations into which they enter. The filaments of nerves which go to the skin, and have the simple function of sensation, regularly diverge to their destination, without forming a plexus. From the fin of a fish to the arm of a man, the plexus increases in complexity, in proportion to the variety or extent of motions to be performed in the extremity. It is by the interchange of filaments that combination among the muscles is formed.

Different columns of nervous matter combine to form the SPINAL MARROW, (A B, p. 61.). Each lateral portion of the spinal marrow consists of three tracks or columns ; one for voluntary motion, one for sensation, and one for the act of respiration. So that the spinal marrow comprehends in all six rods, intimately bound together, but distinct in office; and the capital of this compound column is the medulla oblongata.

The anterior column of each lateral division of the spinal marrow is for motion; the posterior column is for sensation; and the middle one is for respiration. The two former extend up into the brain, and are dispersed or lost in it; for their functions stand related to the sensorium: but

the last stops short in the medulla oblongata, being in function independent of reason, and capable of its office independently of the brain, or when separated from it.

[graphic][subsumed][subsumed][subsumed][subsumed]

A B the spinal marrow seen in front; the division into lateral portions appearing at the line A B. The nervous cord C arises from the posterior lateral division, and gives sensibility. The swelling D is its ganglion. The nervous cord E arises from the anterior lateral division, and gives motion. It has no ganglion. These two cords combine at F, and proceed under one sheath to their destinations.

Mr BELL struck a rabbit behind the ear, so as to deprive it of sensibility by the concussion, and then exposed the spinal marrow. On irritating the posterior roots of the nerve, he could perceive no motion consequent on any part of the muscular frame; but on irritating the anterior roots of the nerve, at each touch of the forceps there was a corresponding motion of the muscles to which the nerve was distributed. These experiments satisfied him that the different roots and different columns from which those roots arose, were devoted to distinct offices, and that the notions drawn from the anatomy were correct.

Mr BELL performed certain interesting experiments on the fifth pair of nerves, which originates from the brain. In his Plate I. he represents this nerve rising from two roots, one of them coming from the crus cerebri, corresponding to the anterior column of the spinal marrow; and the other from the crus cerebelli, corresponding to the posterior column of the spinal marrow. There is a ganglion on the

« PreviousContinue »