#Regeneration.#—­Process of Repair when the Ends are in Contact.—­If the wound is aseptic, and the ends of the divided nerve are sutured or remain in contact, they become united, and the conducting paths are re-established by a regeneration of nerve fibres.  There is a difference of opinion as to the method of regeneration.  The Wallerian doctrine is that the axis cylinders in the central end grow downwards, and enter the nerve sheaths of the distal portion, and continue growing until they reach the peripheral terminations in muscle and skin, and in course of time acquire a myelin sheath; the cells of the neurolemma multiply and form long chains in both ends of the nerve, and are believed to provide for the nourishment and support of the actively lengthening axis cylinders.  Another view is that the formation of new axis cylinders is not confined to the central end, but that it goes on also in the peripheral segment, in which, however, the new axis cylinders do not attain maturity until continuity with the central end has been re-established.

If the wound becomes infected and suppuration occurs, the young nerve fibres are destroyed and efficient regeneration is prevented; the formation of scar tissue also may constitute a permanent obstacle to new nerve fibres bridging the gap.

When the ends are not in contact, reunion of the divided nerve fibres does not take place whether the wound is infected or not.  At the proximal end there forms a bulbous swelling, which becomes adherent to the scar tissue.  It consists of branching axis cylinders running in all directions, these having failed to reach the distal end because of the extent of the gap.  The peripheral end is completely degenerated, and is represented by a fibrous cord, the cut end of which is often slightly swollen or bulbous, and is also incorporated with the scar tissue of the wound.

#Clinical Features.#—­The symptoms resulting from division and non-union of a nerve-trunk necessarily vary with the functions of the affected nerve.  The following description refers to a mixed sensori-motor trunk, such as the median or radial (musculo-spiral) nerve.

Sensory Phenomena.—­Superficial touch is tested by means of a wisp of cotton wool stroked gently across the skin; the capacity of discriminating two points as separate, by a pair of blunt-pointed compasses; the sensation of pressure, by means of a pencil or other blunt object; of pain, by pricking or scratching with a needle; and of sensibility to heat and cold, by test-tubes containing water at different temperatures.  While these tests are being carried out, the patient’s eyes are screened off.

After division of a nerve containing sensory fibres, there is an area of absolute cutaneous insensibility to touch (anaesthesia), to pain (analgesia), and to all degrees of temperature—­loss of protopathic sensibility; surrounded by an area in which there is loss of sensation to light touch, inability to recognise minor differences of temperature (72-104 F.), and to appreciate as separate impressions the contact of the two points of a compass—­loss of epicritic sensibility (Head and Sherren) (Figs. 91, 92).