There is no difference between a motor, and a sensitive nerve as regards structure. Ehrenberg, indeed, endeavoured to establish that the vari cose character of the fibre belonged to nerves of special sense; but subsequent observation showed this to be incorrect. We can attribute the difference of endowment of the fibres to no other cause, but to the nature of their peri pheral and centml connections. The same nervous force is propagated by the fibres of each kind, but whether that force is to excite motion or sensation must depend on the connec tion of the fibres with muscles in the one case, and with the centre of sensation in the other.
The terms afferent and efferent have been used in expressing the function of different fibres, and they are convenient terms to a cer tain extent. But the use of them tends to con vey erroneous ideas respecting the change which takes place in a nerve when stimulated, as if that change took place only in one direction. It is true that, in a motor nerve, the stimulus ordinarily acts from the centre, and the nervous force is propagated peripherad; and on the other hand, in the sentient nerve, the stimulus is usually applied at the periphery, and the nervous force proceeds centrad. It is the place at which the stimulus is applied which usually determines the direction in which the nervous force travels. But there are no good grounds for supposing that the molecular change con sequent upon the stimulation of a nerve is limited to that part of the nerve-fibre which is included between the point stimulated, and tlie centre or the muscles, where the effect f the stimulation appears; on the contmry, t is not improbable that, at whatever point he stimulus be applied, the whole length I f the nerve-fibre participates in the change.
i !his is not unlikely in the case of motor lierves. For a continued or violent irritation of motor nerve, in some part of its course, cans rig spasm or convulsive movement of the -tuscle.s it supplies, may be propagated along its whole length to the centre, and may there give rise to irritation of neighbouring fibres, whether motor or sensitive, exciting more con vulsion and pain. The phenomena of many cases of epilepsy, in which the fit begins with irritation of a few muscles, may be referred to in illustration of this position.* And it is also very probable as regards sensitive nerves. If the ulnar nerve be irritated when it passes be hind the internal co»dyle, a sensation of tingling is excited, which is referred to the sentient surface of the ring and little fingers ; and if the irritation is kept up, the skin of those fingers becomes tender to the touch, its sensibility being very much exalted. This fact cannot be explained unless upon the supposition that the molecular change in the nerve-fibres, pro duced by the irritation, extended to the peri phery as well as to the centre, exalting the excitability of their distal extremities.
It is a highly interesting physiological fact, which has an important practical bearing, that at whatever part of their course sentient nerve fibres be irritated, the same sensation will be produced, whether the seat of the irritation be the centre, the periphery, or the middle of their course, provided only the same fibres are irri tated .in the same degree. Thus it frequently happens that sensations are referred to the ex tremities of a nerve when the existing irritation is situated at its point of implantation in the centre. The sensation of tingling or formica tion, in the hand or foot, arm or leg, is fre quently an indication of cerebral or spinal disease ; but the practitioner should not forget that precisely the same sensation may be caused by an irritation taking place in the course of the nerve. I have frequent occasion to estimate the importance of this fact in the treatment of cases of Sciatica. This disease genemlly con sists in an irritated state of the nerve in some part of its course by a gouty matter, and it may be treated with the best effects by blisters ap plied over the nerve. As, however, the morbid impregnation may have taken place at any part of the course of the nerve, it is a very useful practice, when a single application fails, to apply the blisters over different parts in succession, instead of confining the vesication to one region.
This law of action of sensitive nerves gives the clue to the explanation of the extraordinary but well-attested fact, that persons who have suffered amputation will continue to feel a con sciousness of the presence of the amputated limb, immediately after, and often for a long time, or even always, after its removal. I have met with two cases, in one of which the arm, in the other the leg, was amputated so long before as forty years ; yet each person declared that he had the sensation of his fingers or toes as distinctly as before the operation. And not only does the consciousness above referred to exist, but likewise, when the principal nerve of the limb is irritated, the patient complains of pains or tingling, which he refers to the fingers or toes.* In such cases the central segments of the amputated nerve-fibres remain; if they retain their healthy condition, they continue to represent in the sensorium the various points on the surface of the amputated limb, and likewise the muscles which they were destined to supply. lf, however, the inte grity of the nerve-fibres has been impaired in consequence of any morbid action which may have followed the operation, then the sensation exists imperfectly or not at all.