The =aorta= is the main systemic arterial trunk. It begins at the base
of the left ventricle and is practically median at its origin. It passes
upward and slightly forward between the pulmonary artery on the left and
right atrium on the right. It then curves sharply backward and upward
and inclines somewhat to the left, forming the =arch of the aorta=
(Arcus aortæ), and reaches the ventral surface of the spine at the
eighth or ninth thoracic vertebra. After passing backward along the
ventral aspect of the bodies of the vertebræ between the lungs it
traverses the hiatus aorticus and enters the abdominal cavity, where it
lies below the vertebral bodies and the psoas minor, just to the left of
the median plane. It divides under the fifth lumbar vertebra into the
two internal iliac or hypogastric arteries.
From the bifurcation a small vessel, the middle sacral artery (A.
sacralis media), sometimes passes backward on the pelvic surface of
the sacrum. It becomes lost in the periosteum or joins the coccygeal
artery, or in exceptional cases is traceable to the sphincter ani
externus.
The caliber of the aorta is greatest at its origin, which is termed the
=bulbus aortæ=. Here it forms three pouch-like dilatations, the =sinuses
of the aorta= (or of Valsalva). These correspond to the cusps of the
aortic valve, and the coronary arteries arise from the left posterior
and anterior sinuses. At the arch the diameter is about two inches (ca.
5 cm.), and beyond this it diminishes gradually in width.
[Illustration: