THE THORACIC AORTA. 753 
vertebra, and finally turns downwards to become continuous with the descending 
aorta. : : 
At its commencement it has the same diameter as the ascending aorta, 14 
inches (28 mm.), but after giving off three large branches, the diameter is reduced 
to a little less than one inch (23 mm.). 
Relations.—It is overlapped in front and on the left side by the right and left lungs 
and pleural sacs, but much more by the latter than the former, and in the interval 
between and behind the pleural sacs it is covered by the remains of the thymus gland. 
As it turns backwards it is crossed vertically on the left side by four nerves in the 
following order from before backwards—the left phrenic, the inferior cervical cardiac 
branch of the left. vagus, the superior cardiac branch of the left sympathetic and the 
trunk of the left vagus, and the left superior intercostal vein passes obliquely upwards 
and to the right between the vagus and phrenic nerves. 
Behind and to the right side of the arch are the trachea, the left recurrent laryngeal 
nerve, the left border of the cesophagus, and the thoracic duct. Above are its three large 
branches—the innominate, the left common carotid, and the left subclavian arteries—and 
crossing in front of the roots of these is the left innominate vein. Below is the bifurcation 
of the pulmonary artery and the root of the left lung; the ligamentum arteriosum, which 
is also below, attaches it to the commencement of the left pulmonary artery, whilst to 
the right of the ligament is the superficial cardiac plexus, and to its left is the left 
recurrent laryngeal nerve. 
Branches.—The three great vessels which supply the head and neck, part of the 
thoracic wall, and the upper extremities—viz. the innominate, the left common carotid, 
and the left subclavian arteries—arise from the aortic arch, 
The descending aorta (aorta descendens).—The thoracic portion of the de- 
scending aorta hes in the posterior mediastinum ; it extends from the termination 
of the arch, at the lower border of the left side of the fourth dorsal vertebra, to 
the aortic opening in the diaphragm, where, opposite the twelfth dorsal vertebra, 
it becomes continuous with the abdominal portion. Its length is from seven to 
eight inches (17:5 to 20 cm.), and its diameter diminishes from 23 mm. at its 
commencement to 21 mm. at its termination. 
Relations.— Immediately Jehind it is the vertebral column and the anterior 
common ligament. It rests also on the vena azygos minor superior and the vena 
azygos minor inferior, whilst from its posterior aspect the aortic intercostal branches are 
given off. 
In front it is in relation, from above downwards, with the root of the left lung, the peri- 
cardium which separates it from the back of the left auricle, the cesophagus with the ceso- 
phageal plexus of nerves, and the crura of the diaphragm which separate it from the Spigelian 
lobe of the liver. On the eft sede are the left lung and pleura. On the right side the 
thoracic duct and the vena azygos major form immediate relations along its whole length. 
The cesophagus also lies to the right of the upper part of the descending aorta, whilst the 
right lung and pleura are in close relation below. 
Branches.—Nine pairs of aortic intercostal arteries, two left bronchial arteries, four 
or five oesophageal, some small pericardial, and a few posterior mediastinal branches 
usually arise from the descending aorta. 
THE ABDOMINAL AORTA. 
The abdominal portion of the descending aorta lies in the epigastric and 
umbilical regions of the abdomen. It extends from the middle of the lower 
border of the last dorsal vertebra to the left side of the body of the fourth lumbar 
vertebra, where it bifurcates into the right and left common iliac arteries. The 
point of division is a little below and to the left of the umbilicus, opposite a line 
drawn transversely across the abdomen on a level with the highest points of the 
iliac crests. 
At its commencement it is 21 mm. in diameter, but after the origin of two 
large branches, the cceliac axis and the superior mesenteric arteries, it diminishes 
considerably, and then retains a fairly uniform diameter to its termination. 
Relations.— Behind, it is in contact with the upper four lumbar vertebrae and 
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