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 B 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. 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. 35 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 train) 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 #gment 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. |