e same fact. So
many branches spring from all parts of the arch of the subclavian
artery, that the operation of ligaturing this vessel is less successful
than the same operation exercised on others.

The structures which lie in connexion with the arch of the subclavian
also render the operation of tying the vessel an anxious task. It is
crossed and recrossed at all points by large veins, important nerves,
and by its own principal branches. The vagus nerve, S E, Plate 6,
crosses it at B, its root; external to which place the large internal
jugular vein, K, Plate 5, lies upon it; external to this latter, the
scalenus muscle, X, Plate 5, with the phrenic nerve lying upon the
muscle, binds it fixedly to the first rib; more external still, the
common trunk of the external jugular and shoulder veins, U, Plate 5, lie
upon the vessel, and it is in the immediate vicinity of the great
brachial plexus of nerves, P P, which pass down along its humeral
border, many branches of the same plexus sometimes crossing it
anteriorly.

The depth at which the middle of the subclavian artery lies may be
learned by the space which those structures, beneath which it passes,
necessarily occupy. The clavicle at its sternal end is round and thick,
where it gives attachment to the sterno-cleido-mastoid muscle. The root
of the internal jugular vein, when injected, will be seen to occupy
considerable space behind the clavicle; and the anterior scalenus muscle
is substantial and fleshy. The united spaces occupied by these
structures give the depth of the subclavian artery in the middle part of
its course.

The length of the subclavian artery between its point of branching from
the innominate and that where it gives off its first branches varies in
different bodies, but is seldom so extensive as to assure the operator
of the ultimate success of the process of ligaturing the vessel. Above
and below D, Plate 6, the thyroid axis, come off the vertebral and
internal mammary arteries internal and anterior to the scalenus muscle.
External and posterior to the scalenus, a large vessel, the post
scapular, G, Plate 6, R, Plate 5, arises. If an aneurism attack any part
of this subclavian arch, it must be in close connexion with some one of
these branches. If a ligature is to be applied to any part of the arch,
it will seldom happen that it can be placed farther than half an inch
from some of these principal collateral branches.

When the shoulder is depresse