I. _Simple concussion._--Anatomically, or histologically, no information exists as to the changes which give rise to the often transitory symptoms dependent on this condition. We are reduced to the same theories of molecular disturbance and change which have been invoked to account for similar affections of the central nervous system. The causation of concussion is, however, materially influenced in its degree by the velocity of flight of the bullet and consequent severity of the vibratory force exerted. Hence actual contact of the bullet with the nerves is not necessary for its production, as is seen in the temporary complete loss of functional capacity in the limbs in many cases of fracture, where the vibrations are rendered still more far-reaching and effective as the result of their wider distribution from the larger solid resistance afforded by the bone. The relative density and resistance offered by the different parts of the bone acquire great significance in this relation, since local shock due to nerve concussion is far more profound when the shafts are struck than when the cancellous ends furnish the point of impact. The form of concussion which most nearly interests us in this chapter is that affecting single nerve trunks in wounds of the soft parts alone, and here the passage of the bullet is, as a rule, so contiguous to the nerve that there is difficulty in drawing a strict line of demarcation between such cases and those dealt with in the next paragraph. II. _Contusion._--Clinically this was the form of nerve injury both of greatest comparative frequency and of interest from the points of view both of diagnosis and prognosis. The seriousness of a contusion depends on two factors: first, the relative degree of violence exerted upon the nerve, which is dependent on the force still retained by the travelling bullet; and, secondly, on the extent of tissue actually implicated. The range of fire at which the injury was received determines the importance of the first factor; the second varies with the degree of exactness with which the nerve is struck, and on the direction taken by the bullet. Naturally transverse wounds affect a small area; while an oblique or longitudinal direction of the track may indefinitely increase the extent of injury to the nerve trunk, and hence acquire prognostic significance in direct ratio to the amount of tissue which needs to be regenerated.