Download Mediastinum

Mediastinum
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.
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.
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.
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.
Dissection
of
Mediastinum
Superior
Identify again if possible the remains
of the thymus embeded in fatty
connective tissue immediately
posterior to the manubrium sterni.
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.
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.
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.
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.
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.
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.
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.
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.
Identify and examine
the right and left vagus
nerves.
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.
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.
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.
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.
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.
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.
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.
Dissection of
Anterior
Mediastinum
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.
Dissection of
Middle
Mediastinum
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.
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.
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, serouslined 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.
Remove the heart by cutting
across the great vessels that enter
and leave the heart. Leave the
pericardium intact.
Dissection of
Posterior
Mediastinum
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.
Examine and clean the esophagus.
Using the scalpel handle, gently
elevate the esophagus from the
vertebral column and the
descending thoracic aorta.
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.
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.
Expose and clean the azygos veins
and
the
posterior
intercostal
arteries arising from the aorta.
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.
Identify the greater and lesser
and least splanchnic nerves as
they arised as branches of each
sympathetic trunk.