Mediastinum The mediastinum extends superiorly to the

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Mediastinum
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The mediastinum extends superiorly to the
thoracic inlet and the root of the neck and
inferiorly to the diaphragm. It extends
anteriorly to the sternum and posteriorly to the
12 thoracic vertebrae of the vertebral column.
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It contains the remains of the thymus, the heart
and large blood vessels, the trachea and
esophagus, the thoracic duct and lymph nodes, the
vagus and phrenic nerves, and the sympathetic
trunks.
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The mediastinum may be divided up, for
descriptive purposes, into superior and inferior
mediastina by an imaginary plane passing from the
sternal angle anteriorly to the lower border of
the body of the fourth thoracic vertebra
posteriorly.
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The inferior mediastinum is further subdivided
into the middle mediastinum, which consists of
the pericardium and heart the anterior
mediastinum, which is a space between the
pericardium and the sternum and the posterior
mediastinum, which lies between the pericardium
and the vertebral column.
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Dissection of Superior Mediastinum
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Identify again if possible the remains of the
thymus embeded in fatty connective tissue
immediately posterior to the manubrium sterni.
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This organ reaches its greatest size relative to
the remainder of the body at birth. It continues
to enlarge until puberty, when it gradually
starts to atrophy, and very little is present in
old age. It receives its arterial supply from the
internal thoracic arteries.
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Reflect the thymus and its surrounding connective
tissue superiorly and expose and clean the two
brachiocephalic veins. Note that the two
brachiocephalic veins unite to form the superior
vena cava. Note also that the azygos vein arches
forward over the superior margin of the root of
the right lung to enter the superior vena cava.
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Transect the left brachiocephalic vein at its
junction with the right brachiocephalic vein and
turn it superiorly. Identify the aortic arch and
its branches and the trachea. Clean the aortic
arch and identify the three large arteries that
arise from its superior margin, the
brachiocephalic, the left common carotid, and the
left subclavian arteries. The arch of the aorta
is a continuation of the ascending aorta.
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The brachiocephalic artery passes upward and to
the right of the trachea and divides into the
right subclavian and common carotid arteries
behind the right sternoclavicular joint.
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The left common carotid artery arises from the
aortic arch on the left side of the
brachiocephalic artery. It runs upward and to the
left of the trachea and enters the neck posterior
to the left sternoclavicular joint.
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The left subclavian artery arises from the aortic
arch behind the left common carotid artery. It
runs upward along the right side of the trachea
and the esophagus to enter the root of the neck.
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Identify and examine the right and left phrenic
nerves. Follow the right phrenic nerve as it
descends in the thorax along the right side of
the right brachiocephalic vein and the superior
vena cava.
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Note that it passes in front of the root of the
right lung and runs along the right side of the
pericardium. It then descends on the right side
of the inferior vena cava to the diaphragm. Its
terminal branches supply the musculature of the
right half of the diaphragm and pass through the
caval opening of the diaphragm to supply the
central part of the peritoneum on its under
aspect.
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Identify and examine the right and left vagus
nerves.
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Follow the right vagus nerve as it descends in
the thorax, first lying posterolateral to the
brachiocephalic artery, then lateral to the
trachea and medial to the terminal part of the
azygos vein. Note that it passes behind the root
of the right lung and assists in the formation of
the pulmonary plexus. On leaving the plexus, the
vagus passes onto the posterior surface of the
esophagus and will be followed later in the
posterior mediastinum.
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Now follow the left vegus nerve as it descends to
the thorax between the left common carotid and
left subclavian arteries. Note that it then
crosses the left side of the aortic arch and is
itself crossed by the left phrenic nerve. The
vagus then turns posteriorly behind the root of
the left lung and assists in the formation of the
pulmonary plexus. On leaving the plexus, the
vagus passes onto the anterior surface of the
esophagus and will be followed later in the
posterior mediastinum.
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Identify the ligamentum arteriosum, which is a
fibrous band that connects the bifurcation of the
pulmonary trunk to the lower concave surface of
the aortic arch.
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Pick away the connective tissue around the
ligamentum arteriosum and identify the left
recurrent laryngeal nerve, which is an important
branch of the left vagus. The left recurrent
laryngeal nerve hooks around the lower border of
the ligamentum arteriosum and eventually ascends
in the interval between the trachea and the
esophagus to reach the larynx.
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The trachea divides into the right and left
bronchi at the carina, at about the level of the
sternal angle. Identify the tracheobronchial
lymph nodes along the walls of the bifurcation of
the trachea.
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The esophagus lies immediately posterior to the
trachea. Carefully transect the lower end of the
trachea and remove it to expose the esophagus.
Note that the esophagus lies anterior to the
vertebral column and the left margin projects
slightly to the left of the trachea.
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Identify the thin-walled thoracic duct as it
emerges from behind the left margin of the
esophagus. It ascends into the root of the neck,
where its termination will be dissected with the
head and neck.
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Dissection of Anterior Mediastinum
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The anterior mediastinum is small it lies
between the body of the sternum and the
pericardium. Expose the remains of the thymus in
the loose connective tissue in the anterior and
superior mediastina.
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Dissection of Middle Mediastinum
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Pericardium. Open the pericardium with an
inverted-T incision and clean the pericardial
cavity. Note the outer dull fibrous pericardium
and the inner shiny serous pericardium. Note also
that the visceral layer of serous pericardium is
reflected onto the heart and closely covers it.
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Insert your finger into the transverse
pericardial sinus, which is a serous
pericardial-lined tunnel situated between the
superior vena cava and the ascending aorta.
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Now lift up the apex of the heart and push a
finger posterior to the heart up into the oblique
pericardial sinus. This closed-ended,
serous-lined sinus is bounded on the right by the
inferior vena cava and two right pulmonary veins
and on the left by the two left pulmonary veins.
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Remove the heart by cutting across the great
vessels that enter and leave the heart. Leave the
pericardium intact.
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Dissection of Posterior Mediastinum
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Remove the remains of the pericardium and that it
lies immediately anterior to the esophagus and
descending thoracic aorta. Note also the fusion
of the fibrous pericardium with the central
tendon of the diaphragm.
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Examine and clean the esophagus. Using the
scalpel handle, gently elevate the esophagus from
the vertebral column and the descending thoracic
aorta.
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Clean the descending thoracic aorta and identify
the paired posterior intercostal arteries to the
lower nine intercostal spaces. Arising from the
anterior surface of the aorta arte the bronchial
arteries and a number of small esophageal and
mediastinal branches.
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The thoracic duct is a very important lymphatic
channel. It is grayish white in color and is
thin-walled. Identify the duct as it ascends
through the posterior mediastinum behind the
esophagus. It enters the thorax through the
aortic opening in the diaphragm.
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Expose and clean the azygos veins and the
posterior intercostal arteries arising from the
aorta.
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Although strictly speaking the sympathetic trunks
are not placed within the mediastinum, they
should be cleaned and examined now. The
sympathetic trunk has segmentally arranged
ganglia, each with a white and gray ramus
communicans passing to the corresponding spinal
nerve.
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Identify the greater and lesser and least
splanchnic nerves as they arised as branches of
each sympathetic trunk.