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BLOOD-VESSEL SYSTEM OF THE HEAD, ETC. 245

what longer than the right, and is more deeply situated in the thoracic
cavity. It usually arises directly from the left extremity of the trans-
verse portion of the arch of the aorta, opposite the second dorsal verte-
bra. From this point it passes almost vertically upward, though slightly
outward, and emerges from the thorax into the neck, where it makes a
sharp curve outward over the apex of the left lung to reach the prox-
imal margin of the anterior scalenus muscle, from which point its direc-
tion and relations correspond to the artery of the right side, and there-
fore need no special description.
Relations.—The first or ascending portion of the left subclavian
artery is situated at its origin behind the pleura and upper portion of
the left lung. It is crossed by the left innominate vein, the internal
jugular, and vertebral veins. The pneumogastric nerve passes down
in front of it, and comes in contact with it near its origin. The phrenic
nerve crosses the artery close to the anterior scalenus muscle and exter-
nal to the thyroid axis. The cardiac nerves also pass in front of the
artery. Its deep surface is in close relation to the vertebrae, a portion
of the oesophagus, the thoracic duct, and longus colli muscle. The cord
of the sympathetic nerve passes up behind this surface. Internally are
the left common carotid artery, trachea, a portion of the oesophagus, and
thoracic duct. Externally is the pleura.
The Branches of the Subclavian Artery are four in number—vertebral,
internal mammary, thyroid axis, and superior intercostal. The first
three of these arise from the first portion of the subclavian before it
reaches the scalenus anticus muscle, while the last is derived from the
second poi'tion. Occasionally a branch arises from the third portion
of the artery, and is known as the posterior scapular.

The Vertebral Artery.
The Vertebral Artery is about 5 mm. (i inch) in calibre, and is usually
the first and largest branch of the subclavian. The artery on the right
side generally arises from the upper and posterior portion, about three-
fourths of an inch from the brachio-cephalic or innominate artery; that
on the left usually arises from the first portion of the subclavian as it
curves to the left at the base of the neck.
Variations.—This artery may arise from any portion of the subcla-
vian or the common carotid, and even from the arch of the aorta, though
this latter abnormality is rare. Cases are reported where the vertebral
artery arises as two branches.
From its origin the vertebral artery passes upward and slightly
backward, and usually enters the foramen in the transverse process of
the sixth cervical vertebra. Occasionally, however, it enters the fora-
men in the fifth, fourth, third, or even as high as the second, cervical
vertebra. Sometimes it enters the foramen in the transverse pro-
cess of the seventh cervical vertebra. It then passes upward through
the remainder of the vertebral foramina, curves backward along the
upper surface of the atlas, and enters the skull through the foramen
magnum of the occipital bone. It passes along the side of the medulla
oblongata to its anterior portion, and joins the vertebral of the opposite