NERVES ARE CONDUCTORS.

39

Neither feeling nor action will start up at any point on the line of a nervous bundle; but through the instrumentality of transmission, every nerve may be the means of causing either the one or the other.

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 nerve with its numerous branches still remains, but, dissevered from the centres, it has no influence; the entire ramifications might as well be torn out of the limb. 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, an influence is generated and is made to appear by causing 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 precisely the same. The pricking originates an impression or stimulus, which the nerve conveys through its whole length; wherever that nerve ramifies, in each place do we note feeling or movement, or both. The nerve neither begins nor swallows up the influence; but carries forward and discharges it. Vary the experiment as we may, the interpretation to be put upon the result is of one unvarying tenor.*

It seems singular that such an action as pricking a nerve with the point of a needle, pinching it, electrifying it, dropping pungent liquids upon it, should set a-going the same kind of influence as comes from its own proper centres. But this is from the peculiarity of the nerve, and not from any identity between the influence of a nerve centre and the influence of a mechanical irritation, electrical action, or chemical corrosion. A nerve is so constituted that it will carry one set of influences and no other. If we are able to disturb it at all, so as to propagate any kind of influence, this will be the influence that the nerve is accustomed to propagate. It will be an influence either setting some part in motion, or producing sensation somewhere. Because we burn it with an acid at one point, it will not therefore convey to the extremities a corroding influence, as if acid were poured out at the end of every fibre; it will simply cause either a convulsive action of the muscles, or a strong sense of pain in the parts where its terminations lie. The fibres are formed to transmit a peculiar and distinct influence, and they will either take on the bent requisite for

13. This property of communication or conduction, the exclusive function of the nerve threads, belongs to all the fibrous masses, that is, to the white matter of the nervous system. The conveying structure is the fibrous; a different function is reserved for the grey matter, as we shall presently see, that of originating influence. Every separate fibre is a wire, and carries its own independent stimulus, although bound up with thousands of other fibres in the same cord or tract. Wherever white matter exists, lines of communication are established. We recognise lines of conduction or transmission not only between the remote organs of the body and the cerebro-spinal centre, but also throughout the different parts of the encephalon and spinal cord; in other words, we must admit the existence of currents and counter currents in the interior of the brain itself. All those connecting bands of fibres, or white substance, known as peduncles of the cerebrum and cerebellum, the commissures of the cerebrum, and all the white matter in the interior of the hemispheres underneath the convolutions, must be looked upon as employed in receiving influences from one nervous centre or portion of the cerebral mass, to discharge them in another. A bundle of white fibres in the main serves no other purpose in the heart of the cerebrum than a similar bundle serves in passing along the arm or over the face.

14. We have remarked of the nerves that they convey influence for the two distinct ends of causing action and causing feeling. For action, the influence must proceed outwards from the centres to the active organs; a stimulus from the brain or spinal cord has to be transmitted to the limbs, trunk, head, eyes, mouth, voice, or other parts that are to be set in

transmitting that influence, or remain dormant. It so happens, however, that the substance of nerve is extremely susceptible, and very readily falls into the active propagating attitude; whence influences very unlike those coming from the proper sources of influence may disturb the quiescent condition and set on the very current belonging to the legitimate stimulus. This gives to the fibrous substance of nerve, not an originating but a determining power, it will take on only one kind of influence, only one sort of message will be carried by it.

MOTOR AND SENTIENT NERVES.

41

motion. For feeling, the influence must pass inwards. In a sensation of hearing, for example, an impression made on the sensitive surface of the ear is conveyed by the nerve of hearing towards the cerebral centres. Now it is found that different sets of nerves are employed for those two purposes; one class being exclusively devoted to the outward transmission of stimulants to action or movement, while the other class is equally confined to the office of conveying influence centrewards, for the ends of sensation or feeling. The first of these two classes is that named efferent (out-carrying) nerves, the second comprises the afferent (in-carrying) nerves. In every individual fibre it would appear that the influence always follows one direction. No single nerve can combine both functions.

It is further known, since the discovery of Sir Charles Bell, that one of the two roots of the spinal nerves is entirely composed of nerves conveying the outward stimulus; these are, therefore, purely nerves of motion, motor nerves.' The other root consists of fibres transmitting influence from the various parts of the body inwards to the centres; these are called the sentient nerves. (They are not all sentient in the full sense of word, as will be afterwards explained.) The anterior roots are the motor nerves; the posterior roots are the in-carrying or afferent nerves. On these last roots, the posterior, the ganglionic swellings occur; and both in the spinal nerves and in those emerging at once from the brain by openings in the cranium, the occurrence of a bead is a proof that the nerve is of the in-carrying or sentient class.

In the experiments above described as made upon trunk nerves of an arm or leg, effects both of movement and sensation were seen to follow; the limb was thrown into convulsive movements, and the animal showed all the symptoms of being in bodily pain. If, now, instead of a main trunk, the trial is made upon one of the roots of a spinal nerve, only one single effect will be produced,-motion without sensation, or sensation without motion of the part. If an anterior root is pricked or irritated, movements of some part of the body will follow, showing that an active stimulus has been discharged upon a certain number of muscles. If a posterior or ganglionic root

is pricked, the animal will show symptoms of pain, and the pain will be mentally referred to the part where the filaments of the nerve are distributed. If the nerve is one proceeding to the leg, there will be a feeling of pain in the leg; but there will be no instantaneous convulsions and contractions of the limb, such as are produced by irritating an anterior root. All the movements that an animal makes under the stimulus of a sentient root, are consequent on the sensation of pain; they are not the direct result of the irritating application. In one of the trunk nerves of an arm or leg, both motor and sentient fibres are mixed up, which is the reason of the mixed effect in the first experiment above mentioned.

15. Experiments with pure nerves, that is, with motor fibres alone, or sentient fibres alone, are best made upon the nerves of the head, the Cerebral Nerves. A certain number of these are exclusively motor, certain others are exclusively sentient, while a third kind are mixed, like the spinal nerves beyond the point of junction of the two roots.

The Cerebral Nerves are divided into nine pairs, some of these being considered as admitting of farther subdivision. Four are enumerated as nerves of pure sensation :-the nerve of smell (olfactory nerve, 1st pair); the nerve of sight (optic nerve, 2nd pair); the nerve of sensation of the tongue and face generally (5th pair); the nerve of hearing (auditory nerve, part of the 7th pair). These nerves, therefore, are exclusively engaged in transmitting influence from the surfaces of special sense, the nose, eyes, ears, tongue, and face, towards the cerebral mass. Five nerves are enumerated as purely motor or out-carrying-the nerve supplying the four recti (or rectangularly arranged) muscles of the eye, and sustaining its ordinary movements (motor communis oculorum, 3rd pair); the nerve supplying the superior oblique muscle of the eye (trochlearis, 4th pair); the nerve distributed to the external rectus muscle of the eye, and serving to abduct the two eyes by an independent stimulus requisite in adjusting the eyes to different distances (abducent, 6th pair); the trunk nerve for setting on the movements of the face and features (2nd part

FUNCTIONS OF THE SPINAL CORD.

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of 7th pair); the nerve for moving the tongue (9th pair). The pair reckoned the 8th, consists of sensitive fibres distributed to the tongue, throat, lungs, and stomach, and also of motor fibres distributed to muscles.

If any one of the four sensitive nerves issuing from the cranium be cut through, sensation in the connected organ is lost; disease will produce the same effect. Injury in the optic nerve causes blindness, in the auditory nerve deafness. If any one of them is irritated by pricking, corrosion, or electricity, a sensation is produced of the kind proper to the nerve; if the olfactory nerve, a smell is felt; the optic, a flash of light; the auditory, a sound; but no movement is generated. If any one of the five motor pairs is cut, the corresponding muscles cease to act; they are said to be paralysed, an effect also produced by nervous disease. If the third pair were cut, the motion of the eyeballs would cease, there would no longer be any power of directing the gaze at pleasure, the most brilliant spectacle would fail to command the sweeping glances of the eye. If the moving portion of the seventh pair were cut on one side, all the muscles of the face on that side would lose their tension, and the equipoise of the two sides being thus destroyed, the face would be set awry, by the action of the unparalysed muscles.

By experiments of this nature the functions of the several cerebral nerves have been successively ascertained. In like manner, the discovery of Sir Charles Bell as to the compound nature of the spinal nerves has been fully confirmed. It has been shown beyond the possibility of doubt, that the nerve fibres are of two distinct classes, with different functions, and that the same fibre never serves both functions; that a current peculiar to each fibre sets in always in one direction; and that nothing beyond a conducting character ever belongs to the nerve bundles, or to the fibrous aggregates, the white substance, of the cerebro-spinal system.

Functions of the Spinal Cord and Medulla Oblongata.

16. We have now to speak of the Centres, or the masses