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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 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 these 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 (outcarrying) nerves, the second comprises the afferent (incarrying) nerves. In the individual fibres, it would appear that the influence always follows one direction. No single. nerve combines both functions.

It is further known, since the discoveries of Bell and others, 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 the word, as will be afterwards explained.) The anterior roots are the motor nerves; the posterior roots are the incarrying or sentient 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 such a bead is a proof that the nerve is of the incarrying or sentient class.

In the experiments above described, as made upon trunknerves of an arm or leg, effects both of movement and of 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,

TWO CLASSES OF NERVES.

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the trial is made upon one of the roots of a spinal nerve, only a 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 a leg, both motor and sentient fibres are mixed up, which is the reason of the mixed effect in the first experiment above mentioned.*

13. 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

• When an anterior root is cut through, irritation of the farther segment produces movements; irritation of the upper segment (nearest the brain) has no effect. If a posterior root is cut, irritation of the farther segment gives rise to no signs of sensation or of motion; irritation of the nearer segment causes signs of pain. The irritation of the farther segment of an anterior or motor root (whose result is movement) may, however, be accompanied with slight indications of pain; the explanation of which is, that the cramping or violent contraction of the muscles stimulates the sensory muscular fibres, which proceed to the brain by the undivided posterior, or proper sensory roots.

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)-(this nerve contains also a motor portion distributed to the muscles of mastication); the nerve of Hearing (auditory nerve, part of the 7th pair). These nerves, therefore, are 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 outcarrying :-the nerve supplying three of the four recti (or rectangularly arranged) and one of the oblique 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 of 7th pair); the nerve for moving the tongue (9th pair). The pair reckoned the 8th has three divisions:-(1) the glossopharyngeal or sensory nerve of the tongue and throat; (2) the vagus or pneumo-gastric, the sensory nerve concerned in respiration, circulation, deglutition, and digestion; (3) the spinal accessory or motor nerve for regulating the movements of the parts supplied by the vagus-as the throat, larynx, and lungs.

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, there is a smell; 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 paralyzed, an effect also produced by nervous disease. If the

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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 7th 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 unparalyzed muscles.

By experiments of this nature, the functions of the several cerebral nerves have been successively ascertained. In like manner, the discovery of 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.

Functions of the Spinal Cord and Medulla Oblongata.

14. With regard to the Spinal Cord, we find, in the first place, that it is necessary to sensation and to voluntary movement (movement from feeling) throughout the entire trunk and extremities of the body. If the cord is cut across at any part, all feeling is lost, and all power of movement by the will, everywhere below that place, or in every portion of the body where the nerves arising beyond the cut are distributed. If the division is made far down in the back, the lower limbs are the parts principally paralyzed; from them feeling comes no more, nor is it possible to move them by any mental effort. If the cut is in the neck, the paralysis overtakes the arms, trunk, and legs. It becomes evident, that the continuity of the cord with the brain is necessary in order to connect the mental system with the bodily members. The cord by itself will not give the power either of sensation or of voluntary movement. We must regard this portion of the cerebro-spinal axis as a main channel of nervous conveyance for sensation and for voluntary action, between the brain, and the trunk and the extremities of the body. The nerve ramifi

cations are here, as it were, gathered together into one rope or bundle, for convenient transmission to and from the masses of the encephalon. To this extent the cord is the assemblage of the general mass of ramifying or communicating fibres; we may look upon it as the trunk of the tree, the final stream of the river system.*

If now we make experiments upon the cord when dissevered from the brain, we discover that a power of producing movements, though not voluntary, still remains. On irritating any portion of the substance, movements of the limbs are observed. This effect might, no doubt, arise from the continuity of the part with some of the motor nerves; for we have seen that movements in a limb are caused by pinching one of the nerves that supply the limb. But there is a mode of trying the experiment so as to prove decidedly that the spinal cord is itself a source of movement; that is, to prick the skin of the toes. When this is done, a convulsive stimulus instantly returns upon the limb and throws it into action. Hence we infer that an impression arising on the surface of the body and conveyed to the spinal cord, but not to the brain, causes the cord to send forth a motor stimulus to the moveable organs;, a phenomenon, moreover, that ceases on the destruction of the cord.

'In most instances where the spinal cord has been divided, whether by design or accident, it has been found that al

Dr. Brown-Séquard has determined by decisive experiments that the transmission of sensitive impressions, in the spinal cord, takes place chiefly through the grey matter, and partly through the anterior columns; the impressions being conveyed to the grey matter by fibres passing obliquely across the posterior columns. The novel part of this doctrine is the attributing of a conducting function to the grey matter; although the grey substance of the cord contains white fibres, these are comparatively few in number, and the conclusion seems inevitable that a line of nervous communication is maintained by the corpuscles of the cord and their connecting fibres. The communication with the brain is maintained after cutting through the white columns, provided the grey substance remains intact; or if, although cut into at different places, it is nowhere completely severed. In the point of special function, there is much uncertainty as between the anterior and the posterior columns.

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