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244 ANAT03ir.

The first or ascending portion of the right subclavian artery arises from
the innominate or brachio-cephalic artery at its point of bifurcation into the
right subclavian and right common carotid, which latter is situated close
to the trachea posterior to the sterno-clavicular articulation. From this
point it passes upward, outward, and a little backward until it reaches
the proximal border of the scalenus anticus muscle at the base of the
neck.
The second or transverse portion is the shortest, and is situated higher
in the neck than the remainder of the artery, thus forming the dome of
the arch. It commences at the termination of the first portion of the
artery, and passes outward behind the scalenus anticus muscle to its
distal border.
The third or descending portion of the artery passes downward, out-
ward, and forward from the distal border of the scalenus anticus muscle
to a point where it passes from the neck over the first rib and under the
clavicle into the axilla, where it becomes the axillary artery. The third
portion of the subclavian artery is the most superficial. It passes
through a triangular space formed by the clavicle below, the omo-
hyoid muscle externally, and the anterior scalenus muscle internally.
Relations.—The first or ascending portion of the right subclavian
artery is covered by the skin, platysma myoides, deep fascia, outer
attachment of the sterno-cleido-mastoid muscle, the sterno-hyoid and
sterno-thyroid muscles, and the sternal end of the clavicle. The inter-
nal jugular and the vertebral veins cross it on their way to empty into
the right innominate vein. The pneumogastric nerve crosses to the inner
side of the internal jugular vein, while the cardiac branches of the sym-
pathetic and the phrenic nerves also pass over it. Its deep surface is
in close relation to the pleura, and behind it is separated by a cellular
interval from the longus colli muscle and the transverse process of the
seventh cervical vertebra. The right innominate vein is situated below
and slightly anterior to the artery, while the recurrent or inferior laryn-
geal nerve passes over it, returns upon itself, passes under the artery,
and extends upward to the larynx.
The second or transverse portion of tlie subclavian is wholly covered
by the scalenus anticus muscle, while more superficially it is crossed by
the sterno-cleido-mastoid. The left phrenic nerve passes over the second
portion of the left subclavian artery, in this differing from the right
phrenic nerve, which crosses the first portion of the right subclavian.
It is also covered by the integument, platysma myoides, and the deep
fascia. The deep surface of the artery is in relation with the middle
scalenus muscle posteriorly, the brachial plexus of nerves above, and
below with the pleura. The scalenus anticus muscle is between the
subclavian artery and vein, the latter being anteriorly situated.
Eckdkmx.—The third or descending portion of the subclavian artery
is covered l)y the integument, platysma myoides, and deep fascia. The
subclavian vein lies superficial to, though slightly below, it, while the
external jugular and the veins of the shoulder pass over it to enter the
sulx-lavian vein. It is also in close relation to the brachial plexus of
nerves, most of which pass over it, while one or two pass under it.
The first or ascending portion of the left subclavian arterv is some-