Atrophy may result from pressure, from inflammation, or from an interruption in the continuity of the nerve. The myelin sheaths undergo degeneration, and later on the axis-cylinders break down.

Degeneration of the nerves is with difficulty separated from inflammation. When non-inflammatory, is a simple degeneration; otherwise an inflammatory degeneration. The inflammatory ones are known as parenchymatous neuritis, in contradistinction to the interstitial, which involve the nerve-sheaths.

Degeneration is found in injuries, in infectious diseases, and in intoxications. Certain nerves or sets of nerves may be involved in the different conditions, as those of the pharynx in diphtheria, those supplying the extensor muscles of the forearm in lead-poisoning, or the cord in syphilis.

The degenerative changes may appear within twenty-four hours after an injury. The myelin sheaths become granular and cloudy, and finally break down into droplets. Larger drops form, fatty degeneration occurs, and leukocytes make their appearance. The axis-cylinders resist for a longer time, but they become nodular, vacuolated, and break up. The degeneration occurs more rapidly in the distal than in the proximal end. Regeneration may take place, but the nerve is usually replaced by a cord of fibrous tissue.

Neuritis

The so-called parenchymatous neuritis is a degeneraton of the nerve-fibers without involvement of the connective tissue. Interstitial neuritis is a true inflammatory process affecting the connective tissue.

Acute interstitial neuritis is due to the same causes as bring about the degenerations. It is characterized by an exudation into the endoneurium and perineurium. There are edema and congestion, with an infiltration of round-cells and at times pus-cells. At the same time there is commonly degeneration of the nerve-fibers. The nerves are swollen and reddened.

Chronic neuritis is interstitial and follows an acute attack, or is due to various infections and intoxications, as chronic lead- or alcohol-poisoning. There is a marked hyperplasia of the connective tissue, with associated degeneration and atrophy of the nerve-fibers.

Tuberculosis of the nerves is due to secondary involvement through extension. The roots of the nerves are generally affected. There are a hyperplasia of the connective tissue and a secondary degeneration of the nerve-fibers.

Syphilis of the nerves occurs commonly in the nerve-roots. It is characterized by a round-cell infiltration at first, and later by a marked hyperplasia of connective tissue, associated with atrophy and degeneration. Gummata have been observed in the cranial nerves.

Leprosy of the nerves is characterized by nodular lesions along their course. There is a proliferation of the bacilli, with the formation of nodes in the fibers, accompanied by cellular infiltration with connective-tissue hyperplasia and degenerations. The lepra bacilli can be found within the tissues.

Tumors

Neuromata are the most frequent form of them, the false neuroma, which is more common than the true, is a growth taking place within the connective tissue of the nerve; it is similar to a fibroma.

Sarcomata occur, but are rare.

The malignant tumors may occur as secondary growths.