visions) 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.

The _upper-arm type_--Erb-Duc