In the #supra-clavicular injuries#, the violence applied to the head or shoulder causes over-stretching of the anterior branches (primary divisions) of the cervical nerves, the fifth, or the fifth and sixth being those most liable to suffer.  Sometimes the traction is exerted upon the plexus from below, as when a man in falling from a height endeavours to save himself by clutching at some projection, and the lesion then mainly affects the first dorsal nerve.  There is tearing of the nerve sheaths, with haemorrhage, but in severe cases partial or complete severance of nerve fibres may occur and these give way at different levels.  During the healing process an excess of fibrous tissue is formed, which may interfere with regeneration.

Post-anaesthetic paralysis occurs in patients in whom, during the course of an operation, the arm is abducted and rotated laterally or extended above the head, causing over-stretching of the plexus, especially of the fifth, or fifth and sixth, anterior branches.

A cervical rib may damage the plexus by direct pressure, the part usually affected being the medial cord, which is made up of fibres from the eighth cervical and first dorsal nerves.

When a lesion of the plexus complicates a fracture of the clavicle, the nerve injury is due, not to pressure on or laceration of the nerves by fragments of bone, but to the violence causing the fracture, and this is usually applied to the point of the shoulder.

Penetrating wounds, apart from those met with in military practice, are rare.

In the #infra-clavicular injuries#, the lesion most often results from the pressure of the dislocated head of the humerus; occasionally from attempts made to reduce the dislocation by the heel-in-the-axilla method, or from fracture of the upper end of the humerus or of the neck of the scapula.  The whole plexus may suffer, but more frequently the medial cord is alone implicated.

Clinical Features.—­Three types of lesion result from indirect violence:  the whole plexus; the upper-arm type; and the lower-arm type.

When the whole plexus is involved, sensibility is lost over the entire forearm and hand and over the lateral surface of the arm in its distal two-thirds.  All the muscles of the arm, forearm, and hand are paralysed, and, as a rule, also the pectorals and spinati, but the rhomboids and serratus anterior escape.  There is paralysis of the sympathetic fibres to the eye and orbit, with narrowing of the palpebral fissure, recession of the globe, and the pupil is slow to dilate when shaded from the light.