Posterior Mediastinum

In Lesson 3, we discussed the subdivisions of the mediastinum as described by anatomists. 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.

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.

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.

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.

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.

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.
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.

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.
The image on the right is a colorized picture of an actual dissection of the paravertebral region of the thorax. You should be able to identify the following:
  • sympathetic chain
  • sympathetic ganglion
  • white communicating ramus (preganglionic neurons)
  • gray communicating ramus (postganglionic neurons)
  • roots of the splanchnic nerves (postganglionic neurons)
  • posterior intercostal veins
  • posterior intercostal arteries
  • intercostal nerves
This would also be a good time to describe what is going on in the sympathetic nervous system.

First, the sympathetic trunk and ganglion belong to the autonomic nervous system. The internal structure of the autonomic nervous system is made up of a series of two neurons:

  • preganglionic neuron - arises in the central nervous system from spinal cord segments T1 to L1 or L2 (the lower limit varies). This neuron synapses on a cell body of a postganglionic neuron.
  • postganglionic neuron - arises in a sympathetic ganglion and travels peripherally to act on smooth muscles, cardiac muscles or glands.

The path of the neurons are:

  • intermediolateral cell column in gray matter of spinal cord
  • ventral root of spinal nerve
  • spinal nerve (intercostal nerve in this case)
  • white communicating ramus
  • sympathetic chain. The preganglionic neuron can travel up and down the sympathetic chain to synapse in adjacent ganglia or synapse on the ganglion that it enters.
  • postganglionic neuron leaves the ganglion by way of the gray communicating ramus to reenter the spinal nerve (intercostal in this case) or, in the thorax by way of a splanchnic nerve to supply structures in the abdominal cavity.

    The postganglionic neurons from the middle cervical ganglion, the stellate ganglion and ganglia T2-T4 enter the cardiac plexus and from there to the heart.

Heart   Superior Mediastinum

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