PLAN OF THE BRAIN.

29

spheres, are the principal seats of the multiplying corpuscles. For every fibre carrying impressions up from the senses, and every fibre carrying out stimulus to the moving organs, there must be perhaps ten thousand, perhaps a hundred thousand, traversing the brain, involving a great and rapid multiplication in the corpuscles of the grey substance.*

* It will be necessary, in speaking of certain functions closely allied to the mind, that some allusion be made to the portion of the nervous organization called the Sympathetic System, consisting of numerous ganglia, or little knots, together with nerve cords, and united by numerous nerve cords or branches to the cerebro-spinal system.

The sympathetic system consists of two knotted or ganglionated cords or strings, running, inside the trunk, from the neck to the pelvis, one on each side of the spine. The upper end is connected with groups of ganglia in the head and face; and, in the trunk, there are detached interlacements of ganglia, or plexuses having connexion with the great viscera in the chest and abdomen.

The knots, or Ganglia, are the centres or grey masses of the system, being made up of nerve corpuscles of a particular kind (having usually a single projection or tail). They exercise the usual functions of the corpuscles, in forwarding, diverting, reflecting, and concatenating nervous currents. The Cords are, as in the cerebro-spinal system, made up of nerve fibres, but these are of a peculiar sort, described as soft, granular, flattish (as opposed to tubular) fibres, without any surrounding sheaths or investments, and containing many dark nuclei; they are called the gelatinous, and the non-medullated fibres.

United with fibres from the cerebro-spinal system, these branches of the sympathetic are distributed over the whole body. Thus, as regards the head, they are found in the iris and the blood-vessels of the eye, in a muscle of the tympanum, in the nose, the palate, and the salivary glands. The great plexus of the chest (the cardiac) sends fibres to the heart, the great blood-vessels, and the lungs; from the aorta, nerves are continued to the arteries throughout the body. The abdominal plexus (called the solar plexus) supplies the stomach, intestines, liver, kidneys, and other abdominal viscera ; each organ having a small plexus of its own. A still lower plexus contributes fibres to the parts contained in the pelvis. As all the ramifications contain a certain number of cerebro-spinal fibres, so it is believed that the cranial and spinal nerves contain everywhere some sympathetic fibres.

It is presumed from analogy, and from the functions exerted by the sympathetic system, that the fibres are of the two classes-incarrying and outcarrying. The incarrying nerves would receive stimulation from the peripheral surfaces; the outcarrying would convey motor stimuli to muscular fibres. This last function is the one most clearly manifested. The muscular fibres stimulated by the sympathetic nerves are almost all involuntary muscles, as the iris, the heart, the muscular coat of the blood-vessels, the

OF THE CEREBRO-SPINAL NERVES.

9. By the cerebro-spinal nerves are meant the connexions of the cerebro-spinal centre with the different parts of the body. These connexions consist of ramifications of nerve cords, threads, or bundles, arising in the central masses, and distributed like the blood-vessels, by subdividing and spreading themselves over the various organs and tissues, thereby establishing a connexion between the brain and the remotest extremities.

'These nerves are formed of the nerve fibres already described, collected together and bound up in membranous sheaths. A larger or smaller number of fibres inclosed in a tubular sheath form a small round cord, usually named a funiculus; if a nerve be very small, it may consist of but one such cord, but in larger nerves several funiculi are united together into one or more larger bundles, which, being wrapped up in a common membranous covering, constitute the nerve (Fig. 5). Accordingly, in dissecting a nerve, we first come to an outward covering, formed of cellular tissue,

intestines, &c. All these parts are primarily governed by the sympathetic system, with more or less interference from the cerebro-spinal centres, through the fibres intermingling with sympathetic fibres.

The sympathetic system presides over the viscera, which are the organs of the nutritive or vegetative life. It sustains the rhythmical action of the heart, and of the intestines. The fibres distributed to the small arteries everywhere maintain these vessels in a state of permanent contraction, the release from which, by extraneous influence, produces local congestion and the allied results. These fibres and their function, receive the designation vasi-motor.

The fibres of the sympathetic are not the medium of sensation. When pain arises in parts mainly supplied by them, as the intestines, it must be attributed to the irritation of the intermingled fibres of the cerebro-spinal class.

Many of the so-called reflex functions are due to the operation of the sympathetic nerves and ganglia. The extreme contrast to the proper voluntary actions is presented by the movements due to this system-witness the heart, the intestines, and the vasi-motor compression of the blood-vessels. Indeed, the absence of sensation and the absence of voluntary control are essentially the same fact.

but often so strong and dense, that it might well be called fibrous. From this common sheath we trace laminæ passing inwards, between the larger and smaller bundles of funiculi,

FIG. 5.*
A

and finally between the funiculi themselves, connecting them together as well as conducting and supporting the fine blood vessels which are distributed to the nerve.'

'The funiculi of a nerve are not all of one size, but all are sufficiently large to be readily seen with the naked eye, and easily dissected out from each other. In a nerve so dissected into its component fasciculi, it is seen that these do not run along the nerve as parallel insulated cords, but join together obliquely at short distances as they proceed in their course, the cords resulting from such union dividing in their further progress to form junctions again with collateral cords; so that, in fact, the funiculi composing a single nervous trunk have an arrangement with respect to each other similar to what we find to hold in a plexus formed by the branches of different nerves. It must be distinctly understood, however, that in these communications the proper nerve fibres do not join together or coalesce. They pass off from one nervous cord to enter another, with whose fibres they become intermixed, and part of them thus intermixed may again pass off to a third funiculus, or go through a series of funiculi and undergo still further intermixture. But through all these successive associations, the nerve fibres remain, as far as known, individually distinct, like interlaced threads in a rope.'

'Represents a nerve consisting of many smaller cords or funiculi, wrapped up in a common cellular sheath. A, the nerve. B, a single funiculus drawn out from the rest (after Sir C. Bell).'-QUAIN.

FUNCTIONS OF THE NERVOUS SYSTEM.

The Nerves.

10. The Nerves are divided into two classes, according as they proceed from the Spinal Cord, or issue direct from the brain. The first class, called the Spinal Nerves, is the most numerous. It is not implied that these nerves have no connexion with the brain, but merely that their place of emergence or superficial origin' is in the Spinal Cord. The arrangement is to be looked upon as a matter of local convenience. The nerves destined for the lower limbs do not leave the general trunk until they approach the neighbourhood that they are to supply; that is, they are prolonged within the spine to its lower extremity; whilst those branching towards the arms emerge in the neck and between the shoulders. On the other hand, the nerves that supply the face and head leave the brain at once by openings in the skull; these are the Cerebral Nerves. There is no difference of nature between the two classes.

In the mode of junction of the Spinal Nerves with the Spinal Cord, a peculiarity is observed of great importance in the present subject. I have already adverted to the fact that they issue from the spine in pairs, one pair between every two vertebra; there are in all thirty-one couples. Each couple contains a right and a left member, for distribution to the right and left sides of the body. This part of the arrangement is likewise a matter of local convenience. But, further, when one individual of these emerging couples is examined, say a right branch, we find that this branch does not arise from the cord single; it springs from two roots, and these, after proceeding apart for a short way, unite in the one single nerve that is seen to issue from between the vertebræ on the right side. The same holds of any left branch that may be fixed upon; the connexion with the cord is not single, but double. The smaller of the two roots, in each case, proceeds from the fore part of the cord, and is called the anterior root;

FUNCTION OF THE NERVES.

33

the other or larger proceeds from the hinder portion of the cord, and is called the posterior root. This last root, the posterior, is distinguished in another point, besides its greater size. Just after leaving the cord, there is a ganglion or little swelling formed upon it, composed in part of grey matter, and being to appearance of the nature of a nerve centre. Beyond the ganglion, the two roots mingle and constitute the one nerve seen to emerge from the spine.*

11. Having thus noticed two classifications of the Nerves, the one-into Spinal and Cerebral-unimportant as respects function, the other-into Anterior and Posterior roots-highly important, as will be seen; we now proceed to illustrate the precise function of a nerve. The function of a nerve is to transmit impressions, influences, or stimuli, from one part of the system to another.

The experimental proofs of this position are numerous, and they are now reckoned conclusive. If a main trunk nerve supplying a limb be cut through, all sensation in the limb ceases, and also all power of movement. The blood circulates and the parts are nourished, but, for the purposes of feeling or action, the member is excommunicated, dead. The telegraph wire is cut.

If, instead of cutting the nerve through, we prick or irritate it, we cause both feeling and movement. Whether the irritation is applied high or low, near the nervous centres or near the extremities of the body, the effect is the same. The pricking originates an impression or stimulus, which the nerve conveys through its whole length; wherever that nerve ramifies, there is feeling or movement, or both. It appears, however that the influence increases as it passes along the nerve, presenting a marked contrast to the conduction of electricity by a wire, for the electric current diminishes by transmission. The nerve is not a passive, but an active conductor.

12. We have remarked of the nerves that they convey influence for the two distinct ends of causing action and of *See Fig. 2, p. 17.