MEDIASTINUM
The mediastinum is that portion of the thorax that
lies between the right and left pleural sacs and is bounded ventrally by the
sternum and dorsally by the bodies of the thoracic vertebrae. The superior
boundary of the mediastinum is defined by the thoracic inlet, and its inferior
boundary is formed by the diaphragm. By convention, the mediastinum is divided
into superior and inferior parts by a plane extending
horizontally from the base of the fourth vertebral body to the angle of the
sternum. The superior mediastinum contains the aortic arch; the brachiocephalic
(innominate) artery; the beginnings of the left common carotid and left
subclavian arteries; the right pulmonary artery trunk; the right and left
brachiocephalic (innominate) veins as they come together to form the superior
vena cava; the trachea with right and left vagus, cardiac, phrenic, and left
recurrent laryngeal nerves; the esophagus and the thoracic duct; most of the
thymus; the superficial part of the cardiac plexus; and a few lymph nodes.
The anterior mediastinum lies below
the superior mediastinum in the area bordered by the pericardium posteriorly
and the body of the sternum anteriorly. The anterior mediastinum contains a
small amount of fascia, the sternopericardial ligaments, a few lymph nodes, and
variable amounts of the thymus.
The middle mediastinum contains the
heart and pericardium, the beginning of the ascending aorta, the lower half of
the superior vena cava with the azygos vein opening into it, the bifurcation of
the trachea into right and left bronchi, the pulmonary artery dividing into
right and left branches, the terminal parts of the right and left pulmonary
veins, and the right and left phrenic nerves.
The posterior mediastinum is bordered
anteriorly by the tracheal bifurcation and posteriorly by the vertebral column.
The posterior mediastinum contains the thoracic portion of the descending
aorta, esophagus, azygos and hemiazygos veins, right and left vagus nerves,
splanchnic nerves, thoracic duct, and many lymph nodes.
The relationships among compartments
and their included structures are of great clinical importance because a
space-occupying lesion in any one of these may affect neighboring structures.
These relationships can be appreciated through careful scrutiny of Plates 1-18
and 1-19.
The esophagus passes through the
posterior mediastinum immediately ventral to the thoracic vertebral bodies and
is separated from these by the right intercostal arteries, thoracic duct, and
hemiazygos vein. It partially overlaps the azygos vein to its right side. The
right and left vagus nerves form a plexus around the esophagus, with the left
vagus trunk on its anterior surface and the right vagus trunk on its posterior
surface. The trachea passes through the superior mediastinum anterior to the
esophagus. This relationship continues as the trachea passes into the middle
mediastinum to bifurcate.
In the superior and anterior
mediastinum, the remnants of the thymus gland are present in adults. The right
and left brachiocephalic veins and the superior vena cava are the most anterior
of the major structures in the mediastinum followed in sequence (from anterior
to posterior) by the aortic arch, the brachiocephalic artery, and the
beginnings of the left common carotid and left subclavian arteries.
RIGHT THORACIC CAVITY
The hilum of the right lung contains
the right main bronchus with the right pulmonary artery trunk anterior and the
right pulmonary veins anteriorly and inferiorly. The azygos vein arches over
the root of the right lung at the hilum to empty into the superior vena cava.
As the azygos vein begins to arch, it receives the right superior intercostal
vein, which accepts blood from the upper three or four intercostal spaces.
The visceral pleurae reflect onto the
parietal mediastinal surface immediately below the hilum of the right lung to
form the pulmonary ligament.
The thoracic portion of the right
ganglionated sympathetic trunk courses vertically near the necks of the ribs
and is connected with each intercostal nerve by a
gray and a white ramus communicans. The splanchnic nerves branch from the fifth
(or sixth) to the twelfth ganglia and course medially and inferiorly to pierce
the crus of the diaphragm and enter the abdominal cavity. The right phrenic
nerve and the pericardiacophrenic artery and vein pass vertically between the
mediastinal parietal pleura and the pericardial sac to supply the diaphragm.
The medial “wall” of the right
thoracic cavity is formed by the thoracic vertebral bodies posteriorly and
anteriorly by the mediastinum, dominated by the pericardial sac containing the
heart. The posterior, lateral, and anterior walls of the right thoracic cavity
comprise the thoracic cage, which is limited inferiorly by the diaphragm.
LEFT THORACIC CAVITY
The structures forming the hilum of
the left lung are the left main bronchus, left pulmonary artery, and left
pulmonary veins. The pulmonary artery is located superior to the left main
bronchus with the left pulmonary veins posterior and inferior.
The aorta arches over and descends posterior
to the left hilum. As it descends, it lies at first to the left of the thoracic
vertebral bodies (starting with the lower border of the fourth vertebra); it
then approaches the anterior aspect of the vertebral bodies, where it lies as
it pierces the diaphragm. The aorta gives off nine pairs of intercostal
arteries. They supply the lower nine intercostal spaces.
The ligamentum arteriosum (the remnant
of the embryonic ductus arteriosus) runs between the left pulmonary artery and
the aortic arch.
The thoracic portion of the left
ganglionated sympathetic trunk is similar to the portion on the right side and
does not need special description here.
The left phrenic nerve and the left
pericardiacophrenic artery and vein cross the aortic arch and descend between
the mediastinal parietal pleura and the pericardial sac to pass through the
muscular part of the diaphragm.
The left vagus nerve passes in front
of the arch at the aorta, giving off its recurrent branch, which passes under
the arch to course upward to the larynx. The vagus nerve continues caudally on
the posterior aspect of the root of the lung to enter the esophageal plexus,
from which the left vagal trunk emerges to follow the esophagus into the
abdomen.
The left superior intercostal vein
typically drains blood from the upper three or four intercostal spaces. It
crosses the aortic arch and the beginnings of the left subclavian and left
common carotid arteries and empties into the left brachiocephalic vein, often
anastomosing with the accessory hemiazygos vein.
The medial wall of the left thoracic
cavity is formed by the thoracic vertebral bodies posteriorly and the
mediastinum containing the pericardial sac and the heart. As with the right
thoracic cavity, the posterior, lateral, and anterior walls of the left
thoracic cavity are formed by the thoracic cage and limited inferiorly by the
diaphragm.
