The aorta is the single great vessel which conveys blood from the left
ventricle. It makes a sharp semicircular curve dorsad (Fig. 118, _a′_)
and to the left, passes caudad at the left side of the vertebral column,
and passes between the crura of the diaphragm to reach the abdominal
cavity. It is divisible into =thoracic aorta= (Fig. 118) and =abdominal
aorta= (Fig. 126).
A. =Thoracic Aorta= (Fig. 118).--The first portion of the thoracic
aorta, curved as above described, is the =aortic arch= (_a′_). It lies
in the thoracic cavity opposite the interval between the third and
fourth or fourth and fifth ribs. It is separated from the vertebral
column on the right by the superior vena cava (_q_), and on the left by
the œsophagus (4). At its beginning it lies a little to the right of the
median plane; but it passes at once to the left side of the vertebral
column.