When we view the relative position of the subclavian vessels, A B, Plate
11, to the clavicle, R, we can readily understand why a fracture of the
middle of this bone through that arch which it forms over the vessels,
should interfere with the free circulation of the blood which these
vessels supply to the arm. When the clavicle is severed at its middle,
the natural arch which the bone forms over the vessels and nerves is
lost, and the free moving broken ends of the bone will be acted on in
opposing directions by the various muscles attached to its sternal and
scapular extremities. The outer fragment follows more freely than the
inner piece the action of the muscles; but, most of all, the weight of
the unsupported shoulder and arm causes the displacement to which the
outer fragment is liable. The subclavius muscle, E, like the pronator
quadratus muscle of the forearm, serves rather to further the
displacement of the broken ends of the bone than to hold them in situ.

If the head of the humerus be dislocated forwards beneath L, Plate 11,
the coracoid attachment of the pectoralis minor muscle, it must press
out of their proper place and put tensely upon the stretch the axillary
vessels and plexus of nerves. So large and resistent a body as the head
of the humerus displaced forwards, and taking the natural position of
these vessels and nerves, will accordingly be attended with other
symptoms--such as obstructed circulation and pain or partial paralysis,
besides those physical signs by which we distinguish the presence of it
as a new body in its abnormal situation.

When the main vessels and nerves pass from the axillary space to the
inner side of the arm, they become comparatively superficial in this
latter situation. The inner border of the biceps muscle is taken as a
guide to the place of the brachial artery for the whole extent of its
course in the arm. In plate 11, the artery, b*, is seen in company with
the median nerve, which lies on its fore part, and with the veins called
comites winding round it and passing with it and the nerve beneath the
fascia which encases in a fold of itself all three structures in a
common sheath. Though the axillary vein is in close contact with the
axillary artery and nerves, yet the basilic vein, d*, the most
considerable of those vessels which form the axillary vein, is separated
from the brachial artery by the fascia. The basilic vein, however,
overlies the brachial artery to its inner s