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 'ts 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. 56). 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 among them is the great sympathetic nerve. There are other ganglions seated in the head, neck, and cavities of the chest and belly, which are very irregular in their situation and form.

The color of the ganglions differs from that of the nerves; it is redder, which is owing to the greater number of blood-vessels: 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. 56.). 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 ones 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 'ts office independently of the brain, or when separated from it.

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. h 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 latter branch, and none on the former; which circumstance also is in exact correspondence with the nerves rising from the spinal marrow. The two branches combine at a short distance from their origin, and are universally distributed to the head and face. Mr. Bell conceived that this nerve is the upper

most of those nerves which confer motion and bestow sensibility. To confirm this opinion, he cut across the posterior branch, or that which has a ganglion, on the face of an ass, and it was found that the sensibility of the parts to which it was distributed was entirely destroyed. Again, he exposed the anterior branch of the fifth pair at its root, in an ass, the moment the animal was killed; and on irritating the nerve, the muscles of the jaw acted, and the jaw was closed with a snap. On dividing the root of the nerve in a living animal, the jaw fell relaxed. Thus its functions were no longer matter of doubt it was at once a muscular nerve and a nerve of sensibility. And thus the opinion was confirmed, that the fifth nerve was to the head, what the spinal nerves were to the other parts of the body.

The muscles have two nerves, which fact had not been noticed previously to Mr. Bell's investigations, because they are commonly bound up together; but whenever the nerves, as about the head, go in a separate course, we find that there is a sensitive nerve and a motor nerve distributed to the muscular fibre, and we have reason to conclude that those branches of the spinal nerves which go to the muscles, consist of a motor and a sensitive filament. The nerve of touch or feeling, ramified on the skin, is distinct from both.

It was formerly supposed that the office of a muscular nerve is only to carry out the mandate of the will, and to excite the muscle to action; but this betrays a very inaccurate knowledge of the action of the muscular system; for before the muscular system can be controlled under the influence of the will, there must be a consciousness or knowledge of the condition of the muscle.

When we admit that the various conditions of the muscle must be estimated or perceived, in order to be under the due control of the will, the natural question arises, Is that nerve which carries out the mandate of the will, capable of conveying, at the sarne moment, an impression retrograde to the course of that influence, which, obviously, is going from the brain towards the muscle? If we had no facts of anatomy to proceed upon, still reason would declare to us, that the same filament of a nerve could not convey

a motion, of whatever nature that motion may be, whether vibration or motion of spirits, in opposite directions, at the same moment of time.

Mr. Bell has found, that, to the full operation of the muscular power, two distinct filaments of nerves are necessary, and that a circle is established between the sensorium and the muscle that one filament or simple nerve carries the influence of the will towards the muscle, which nerve has no power to convey an impression backwards to the brain; and that another nerve connects the muscle with the brain, and, acting as a sentient nerve, conveys the impression of the condition of the muscle to the mind, but has no operation in a direction outward from the brain towards the muscle, and does not therefore excite the muscle, however irritated.

There are four nerves coming out of a track or column of the spinal marrow, from which neither the nerves of sensation, nor of common voluntary motion, take their departure. Experiment proves that these nerves excite motions dependent on the act of respiration.

Under the class of respiratory motions, we have to distinguish two kinds: first, the involuntary, or instinctive; secondly, those which accompany an act of volition. We are unconscious of that state of alternation of activity and rest which characterises the instinctive act of breathing in sleep; and this condition of activity of the respiratory organs, we know by experiment, is independent of the brain. But, on the other hand, we see that the act of respiration is sometimes an act of volition, intended to accomplish some other operation, as that of smelling or speaking. Mr. Bell apprehends that it is this compound operation of the organs of breathing which introduces a certain degree of complexity into the system of respiratory nerves. A concurrence of the nerves of distinct systems will be found necessary to actions, which, at first sight, appear to be very simple.

If we cut the division of the fifth nerve, which goes to the lips of an ass, we deprive the lips of sensibility; so that, when the animal presses the lips to the ground, and against the oats lying