When a mixed nerve is divided there occur motor and sensory paralyses in the peripheral parts dependent on it. It sometimes happens that conduction is re-established within a few days, and this must take place by the cut ends uniting by immediate union or by the first intention. Most frequently, however, the restoration of function is tardy, and processes occur in the nerves which have been carefully studied by experimentation on animals. The changes which ensue on the division of a nerve occur mainly in the peripheral portion of it, and they consist in the first place in a Degeneration by which the structure is largely destroyed, and in the second place in a Regeneration by which it is restored. The degenerative process is often called Wallerian, from the observer who first studied it.

The Causation of this peculiar degenerative process is connected with the nutritive arrangements of the nerves. As the axis cylinders of the nerves are but the prolongations of the nerve cells, they are dependent on the nucleus of the cell for their nutrition. Just as any process of a cell will degenerate when cut off from the body of the cell, so do the nerve fibres degenerate when divided from the nerve cells. The sensory fibres of the peripheral nerves are prolongations from the cells of the ganglia of the posterior roots, whilst the motor fibres are prolongations from the cells of the anterior cornua, and hence the growth or degeneration of the fibres takes place in opposite directions and is under the control of different nutritive or trophic centres. Most nerve stems consist of both afferent and efferent fibres, and the division of the stem will cut the fibres off from their cells, whether they be afferent or efferent. It is important to observe that when a posterior nerve root is divided on the proximal side of its ganglion, the fibres degenerate in a central direction, or towards the spinal cord. The small part still attached to the ganglion remains intact. On the other hand, if an anterior root be divided, the proximal portion in connection with the spinal cord remains intact, while the portion distal to the section degenerates.

The changes about to be described have been studied chiefly by experimentation on animals, but similar conditions have been observed in man after division of nerves. In several diseases of nerves also, such as inflammation, leprosy, syphilis, there may be interruptions of the nerve fibres leading tc lesions similar to those occurring in consequence of division, but perhaps mixed up with other phenomena.

The Degenerative process occurs almost simultaneously in the whole peripheral distribution of the divided nerve, affecting both sensory and motor fibres. The most obvious change is in the medullary sheath. It coagulates, breaks up into drops, and through time disappears by absorption. This disintegration of the medullary sheath occurs gradually, and the granular fat into which it breaks up is partly taken up by the nuclei of the nerve fibre, but partly also finds its way out of the primitive sheath, and is found in the surrounding connective tissue and the walls of the capillaries. There is some difference of opinion as to the part taken by the axis cylinder in the process. Erb asserts that it persists after the medullary sheath has been destroyed, but Ranvier states that it is broken up, its interruption corresponding with the abolition of electric conductivity in the nerve. Michael Foster gives his adhesion to the same view. Whether the axis cylinder is destroyed or not, the nerve fibre undergoes a great transformation by the loss of its medullary sheath, and it becomes converted into a pale fibre, interrupted at intervals by the nuclei or by some persisting clumps of myeline.

It is asserted by Ranvier that the nuclei, which we have seen to exist inside the primitive sheath between every two nodes, take an active part in this process. They enlarge and divide, and, by impinging on the medullary sheath, help to break it up. It is by them also, according to this author, that the axis cylinder is interrupted. The enlargement and division of the nuclei is somewhat similar to that which occurs in muscle in certain lesions to be considered afterwards, and it is regarded as inflammatory in its nature.

At the place of division of the nerve, as there is a wound, there are signs of inflammation. Leucocytes collect between and around the cut ends of the nerve, and even penetrate into the primitive sheath for some distance. These leucocytes, which are most abundant soon after the section, attack the medullary sheath, and assist in breaking it up; the myeline is taken up by the leucocytes so as to give them the appearance of compound granular corpuscles. In the central end of the divided nerve, however, the destruction of the medullary sheath is limited, as the invasion of leucocytes generally stops short at the first node.

After a time the inflammation subsides largely, and the wound, including skin and soft parts, is united by a cicatrix formed in the usual way. The divided ends of the nerves are united by a pale cicatricial band, which does not as yet contain any proper nervous elements, and does not effect a restoration of the conductivity.

Conduction* is restored by a process of Regeneration. According to the researches of Ranvier, this occurs entirely by the axis cylinders of the central end budding out and extending first into the cicatrix and then into the peripheral end. The axis cylinder enlarges at its extremity and becomes divided longitudinally into several fine fibres, which grow out into the cicatrix. Arrived at the cut end of the peripheral portion they penetrate into it, and very frequently pass into a primitive sheath. In this way a number of new-formed axis cylinders may be found inside an old nerve-tube, and there may be alongside of them some pieces of persisting myeline. These new axis cylinders after a time acquire medullary sheaths, and the regeneration of the nerve is completed. According to Remak and others, the new fibres are not formed entirely by budding from the central end, but arise also from the remaining axis cylinders of the peripheral end. This view is probably incorrect.

Along with the nerves the Muscles suffer, undergoing marked wasting. The muscular cylinders diminish in diameter, the transverse striation becomes less distinct and the fibres become granular. If regeneration does not occur the muscular fibres lose their transverse stride entirely, become greatly narrower, and may even present hyaline degeneration (see under Coagulation-Necrosis). The atrophy of the muscular fibres is often accompanied by an interstitial inflammation resulting in a new-formation of connective tissue between the muscular fibres, a kind of cirrhosis, leading, in cases where the conduction of the nerve is not restored, to considerable shortening of the muscle and deformity. If regeneration of the nerve occurs then the muscle is restored, but there is frequently some prolonged or permanent damage. The muscular fibres remain partly of smaller diameter and there may be some permanent interstitial overgrowth of connective tissue.

There are also Trophic changes frequently manifested in the skin and other structures, such as will fall to be described later on as Trophoneuroses. They consist of atrophy of the skin, oedema, and occasionally the peculiar condition described as "glossy skin." There may also be swellings of the joint.