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Posterior
Mediastinum
As a summary, remember that the mediastinum is
subdivided into superior and inferior parts. The inferior mediastinum is further
subdivided into anterior, middle and posterior parts.
The anterior boundary of the anterior part is the
back of the sternum. Its posterior boundary is the pericardium of the heart.
The middle mediastinum is the pericardium and its
cavity.
The anterior boundary of the posterior mediastinum
is the posterior pericardial wall. The posterior boundary is the anterior
surfaces of the bodies of thoracic vertebrae T5-T12.
In the dissection of cadavers, the posterior
mediastinum is exposed by removing the pericardial sac and the heart. When this
is done, we see the structures that lie immediately behind the pericardium. The
relationships of the structures in the posterior mediastinum should also be
studied on cross sections through the thorax and shown in the section on cross
sections. In the following images, I have started out Level 1 with the
pericardium.
| As you work your way from
anterior to posterior in the posterior mediastinum, your first task will be
to clean away the posterior wall of the pericardial cavity.
After the heart has been removed, you will see
the cut borders of the major vessels entering and leaving the heart.
The oblique pericardial sinus is clearly seen
within the four pulmonary veins and inferior vena cava at this point.
The arrow at the top represents the division
between the superior mediastinum above and the inferior mediastinum below. |
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| In Level 2, the pericardium
has been removed and the esophagus can seen along with its plexus derived
from the left and right vagus nerves as well as branches from the
sympathetic chain.
As the esophagus passes through the diaphragm to
enter the abdomen, the left vagus emerges from the plexus as the anterior
vagal trunk and the right vagus becomes the posterior vagal trunk.
You can also see that the trachea splits into
the right and left primary bronchi at the level of the junction between the
superior and inferior mediastinae. |
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| Once the esophagus has been
studied, the next level to check out is the thoracic aorta.
The thoracic aorta starts at the T4-T5 junction
and extends the full length of the posterior mediastinum passing through the
aortic hiatus of the diaphragm into the abdomen.
The thoracic aorta gives rise to the posterior
intercostal arteries which pass posterior and lateral to join the posterior
intercostal veins and intercostal nerves to form the neurovascular bundle
that travels along the inferior border of a rib or the upper aspect of an
intercostal space, however you want to remember it. It also gives rise to
bronchial branches that enter the root of the lung to supply the bronchi and
lung tissue. |
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| Behind and between the
esophagus and thoracic aorta, you will find the thoracic duct, a fragile
lymphatic duct that begins in the abdomen as the cisterna chyli.
At the T4-T5 level, it veers to the left and
travels through the superior mediastinum on its way into the neck, where it
will enter the venous system at the junction of the internal jugular and
left subclavian veins. |
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| Alongside and behind the
thoracic duct, you will find the veins that drain the walls of the thorax,
the azygos and hemiazygos veins. Each of these veins begin in the abdomen as
the ascending lumbar veins.
The hemiazygous veins are quite variable in
their makeup but the classic description is that the upper intercostal
spaces are drained by the superior hemiazygos vein and the lower the
inferior hemiazygos vein. These two veins may join as one hemiazygos vein
that passes behind the thoracic duct to empty into the azygos vein.
The two veins may just as frequently pass into
the azygos separately, forming two hemiazygos veins.
Or, there may be multiple veins crossing into
the azygos vein, whereby a true hemiazygos vein doesn't exist at all. |
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| If you could reflect or
remove the azygos system of veins, the next structures in the posterior
mediastinum would be the splanchnic nerves, specifically the greater
splanchnic nerves.
These nerves are derived from the sympathetic
chains, thoracic ganglion T5 to T10.
There is also lesser splanchnic nerves that are
derived from ganglion T10 and T11.
Some people may even describe a least splanchnic
that is derived from ganglion T12.
Remember, according to anatomists, the
sympathetic chain is not part of the posterior mediastinum. |
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| The most posterior item in
the posterior mediastinum is the anterior longitudinal ligament which
completely covers the anterior surfaces of the bodies of the vertebrae.
This extends from the sacral part of the
vertebral column all the up to the skull. |
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Although not part of the posterior mediastinum, this
is a good time to present the structures found in the posterior thoracic cavity.
This is mainly the sympathetic chains and the posterior intercostal spaces.
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Potete
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