As the vaso-motor and trophic nerve-fibres run in the trunks of the cerebro-spinal nerves, destructive lesions of these trunks cut off the influence of the centres with which those fibres are connected, and hence they are followed by changes in the circulation, calorification, and nutrition of the parts to which they are distributed. Thus, the loss of the vaso-motor influence is at first shown in the dilation of the vessels and the unvarying warmth and4 congestion of the part.5 This gives way in time to coldness, due to sluggish circulation and diminished nutritive activity. Marked trophic changes occur in the paralyzed muscles. They atrophy, their fibres becoming smaller and losing the striations, while the interstitial areolar tissues proliferates, and finally contracts cicatricially. The skin is sometimes affected in its nutrition, becoming rough and scaly. Other trophic changes of the skin resembling those produced by irritation of a nerve are very rarely seen, and they may probably be referred to irritation of fibres with which the part is supplied from neighboring trunks.
4 A remarkable exception is seen, however, in the effect of gradual pressure experimentally applied to nerve-trunks until there is complete interruption of sensation and motion, in which case the temperature invariably falls.
5 In a case of gunshot wound that came under the writer's care in 1862, the leg and foot, which were paralyzed from lesion of the popliteal nerve, remained warm and natural in color during repeated malarial chills, which caused coldness and pallor of the rest of the body.
Anatomical Changes in the Divided Nerve and Muscles.—The peripheral portion of a divided nerve separated from its nutritive centres degenerates and loses its characteristic appearance, looking to the naked eye like a grayish cord, and being shrunken to one-fourth of its natural size. The changes which take place in the degeneration of the nerve-fibres, and which proceed from the point of lesion toward the periphery, are, first, an alteration of the white substance of Schwann, which breaks into fragments, these melting into drops of myeline, and finally becoming reduced to a granular mass. The central axis at a later period likewise breaks up, and is lost in the granular contents of the sheath of Schwann. Meanwhile, absorption of the débris of the fibres goes on, until, finally, there remains but the empty and collapsed sheath of Schwann with its nuclei, the whole presenting a fibrous appearance. When this has taken place the degenerated motor nerve-fibres can no longer be excited, and no stimulation applied to them can cause the muscles to contract. At the same time, the muscles atrophy and undergo degenerative changes in their tissue. The fibres become smaller and their transverse striæ indistinct, with the appearance of fatty degeneration, and finally there is proliferation of the interstitial cellular tissue. They do not, however, lose their contractility, and upon a mechanical stimulus being applied directly to them they contract in a degree that is even exaggerated, but with a slowness that is abnormal. If, now, we apply the stimulus of electricity to the muscles themselves, we encounter phenomena of the greatest interest and importance. The application of the faradic current, however strong, elicits no contraction; there is loss of faradic excitability. But if the galvanic current be applied the muscles contract, and that, too, in reply to a current too weak to excite healthy muscles to action; there is increased galvanic excitability. The kind of contraction thus induced is peculiar, differing from that ordinarily seen in muscles. Instead of its being short, and immediately followed by relaxation, as when we make or break the galvanic current in healthy muscles, it is sluggish, long-drawn out, and almost peristaltic in appearance. This is characteristic of degenerated muscles, and is the degenerative reaction. But there is also a change in the manner in which the degenerated muscles reply to the two poles of the galvanic current. Instead of the strongest contraction being elicited, as in the normal condition, by the application of the negative pole to the muscle (C. C. C., cathode closing contraction), an equally strong or stronger is obtained by the application of the positive pole (A. C. C., anode closing contraction), while the contraction normally caused on opening the circuit by removal of the positive pole (A. O. C., anode opening contraction) becomes weaker and weaker, until it is at last exceeded by the contraction upon opening the current by the removal of the negative pole (C. O. C., cathode opening contractions). In short, the formula for the reply of the healthy muscles to galvanic excitation is reversed; there is a qualitative galvanic change in the paralyzed and degenerated muscles.
If no regeneration of the nerve takes place, the reaction of the muscles to the galvanic current is finally lost, and they exhibit those rigid contractions which probably result from a sclerotic condition of the intramuscular areolar tissue.
After complete destruction of the fibres of a nerve at some point of its course, even when a considerable length of it is involved, and after the consequent degeneration of the peripheral portion has taken place, we have, with lapse of time, restoration of its function, consequent upon its regeneration and the re-establishment of its continuity. The histological changes by which the degenerated fibres are restored and the divided ends reunited have not been made out with such certainty as to preclude difference of opinion as to the details. But the process in general seems to be a proliferation of the nuclei in the sheath of Schwann, with increase of the protoplasm which surrounds them, filling the sheath of Schwann with the material from which the new fibre originates. In this mass within the sheath is formed first the central axis of the new fibre, which is later surrounded by the white substance of Schwann. With the regeneration of the nerve-fibres the functions of the nerve return, but in the order of sensation first, and afterward the power of transmitting the volitional impulse to the muscles. Even after regeneration has so far advanced that the muscles may be made to contract by an exercise of the will, the newly-formed fibres fail to respond to other stimuli; thus, the faradic current applied to the nerve does not cause the muscles to contract; the stimulation is not transmitted along the imperfectly restored fibres.
It may be here remarked that after regeneration has restored the functions of a divided nerve the muscles to which it is distributed may still exhibit for a time the degenerative reaction in consequence of unrepaired changes in themselves. In the end we may look for complete restoration in both nerve and muscles.
The time required for the regeneration and reunion of a divided nerve depends somewhat upon the manner in which the destruction has been caused. Thus, a nerve which has been divided by a clean cut, and where the cut ends remain in apposition or close proximity, unites much more readily than one in which bruising, tearing, or pressure has destroyed an appreciable length of its fibres or the divided ends have been thrust apart.
In complete division of a nerve we must not look for regeneration and restoration of its functions, even in favorable circumstances, before the lapse of several months, although cases have been recorded where the process has been much more rapid.