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248 ANATOMY.

Superior Vena Cava, Innominate, and Thyroid Veins.

The Superior or Descending Vena Cava (see Fig. 106) is the
large vessel that receives all the blood from the upper extremities, the
head, and the walls of the thoracic cavity. It is from 2 to 3 inches in
length, and commences at the junction of the right and left innominate
veins, internal to and just below the attachment of the first costal carti-
lage of the right side to the sternum. It passes almost directly down-
ward, curving slightly to the left, and enters the right auricle opposite
the third costal cartilage at its upper anterior portion, its orifice look-
ing downward and forward.
The Innominate or Brachio-Cephalic Veins are two in num-
ber, right and left. They each originate at the junction of the subclavian
and the internal jugular veins, which are situated posterior to the sternal
extremities of the clavicles, and extend downward to the origin of the
superior vena cava, which they form by their union. They are of
unequal length, and diifer from mpst of the other veins of the body by
being destitute of valves.
The Right Innominate Vein is the shorter of the two, being but
slightly over 1 inch in length. It extends from its commencement
almost vertically downward external to the origin of the subclavian
and innominate arteries, the pleura being interposed between it and the
lung on the right side. The vessels which empty into it are the right
thoracic (lymphatic) duct, the right vertebral vein, right mammary,
right inferior thyroid, and the right superior intercostal veins.
The Left Innominate Vein is larger and longer than the right,
beiuo; about '4 inches in leny-th. It extends from its origin on the left
side of the sternum from left to right across the superior and anterior
portion of the chest, inclining slightly downward to its union with the
right innominate vein. It is in relation with the sterno-clavicular artic-
ulation, the ui)per portion of the manubrium, from which it is sepa-
rated only by the lower extremities of the sterno-hyoid and sterno-
thyroid muscles and the thymus gland, or its remains in the adult. The
three arteries arising from the arch of the aorta and the phrenic and
pneumogastric nerves pass down the neck in close proximity to it, the
transverse portion of the arch of the aorta being situated below it.
The Trihiitarie.s of the Left Id nominate Vein are the thoracic (lym-
phatic) duct, the left vertebral, left inferior thyroid, and the left supe-
rior intercostal veins.
The Inferior Thyroid Veins are generally two in number, right
and left, tinJugh occasionally there are three or even four. They are
formed by the union of numerous small veins which originate in the
lower portion of the thyroid body, and which anastomose with similar
branches from the middle and supi-rior thyroid wins. They jiass down-
ward, and form a plexus in fi-ont of the trachea below the istlnnus of
the thyroid gland. This plexus often causes trouble from hemorrhage
in the operation of tracheotomy. The inferior thyroid vein (or veins,
if there are more than one) of the right side are situated a little to
the right of the median line, while those of the left side are usually
directly in the median line. As they descend in front of the trachea