Note: A handful of important structures straddle the thoracic plane (sternal angle to intervertebral disc of T4/5), the division between the superior and inferior (anterior, middle, & posterior) mediastina. A mnemonic for remembering the relative positions of these structures is CLAPTRAP:

Cardiac plexus

Ligamentum arteriosum

Aortic arch (concavity)

Pulmonary trunk

Tracheal bifurcation

Right-to-left transition of thoracic duct

Azygos vein joins the superior vena cava

Pre-vertebral & pre-tracheal fascia end

Photo 2. Superior-to-inferior mediastinal transition

2.) Using blunt dissection, remove the parietal plurae from the lateral and posterior thoracic wall.

Photo 3. Removing the parietal pleurae

Find on a donor, a prosection, or a plastinated specimen:

• Thoracic aorta

  • Visceral branches

    • Bronchial aa. (2 left, 1 right)

    • Esophageal aa. (multiple pairs)

  • Parietal branches

    •  Posterior intercostal aa. (intercostal spaces 3-11)

3.) As you continue cleaning along the aorta, you may find bronchial aa., esophageal aa., and posterior intercostal aa. If you cannot locate these arteries, find them on a prosection.

Note: The descending aorta runs from the arch of the aorta (near T4 vertebral level) to its bifurcation into the common iliac aa. (near L4). The descending aorta is divided into thoracic and abdominal parts by the diaphragm (near T12) delimiting the boundary between them.  

Note: The thoracic aorta compliments and anastomoses with branches of the internal thoracic a. (from first part of subclavian a.) to supply blood to the wall of the thorax, and also supplies elements of the thoracic viscera; thus, the thoracic aorta has both parietal (serving the wall) and visceral branches.

Parietal branches:

Parietal brs. of the thoracic aorta typically exit the vessel posterolaterally, with posterior intercostal aa. typically servicing the 3rd - 11th intercostal spaces, and the subcostal aa. supplies the body wall below the 12th rib. Parietal brs. of the thoracic aorta typically anastomose with branches of the internal thoracic aa. (eg anterior intercostal brs. and musculophrenic brs.) to serve the thoracic wall.

Visceral branches:

Visceral brs. of the thoracic aorta typically exit the vessel anterolaterally to supply structures within the thorax, including the: bronchi, esophagus, elements of the mediastinum, pericardium, and the superior surface of the diaphragm.

Note: Typically, there are three bronchial aa.:

• • two left bronchial aa., and

  • one right bronchial a., which may either originate from the right third posterior intercostal a., or from one of the two left bronchial aa.

The bronchial aa. supply the bronchi, tissues of the lung, bronchopulmonary (hilar) lymph nodes, the proximal portion of the esophagus, etc.

Note: Distal to the bronchial aa. are:

• • mediastinal brs., which serve tracheobronchial lymph nodes and other tissues of the posterior mediastinum,

  • pericardial brs., which serve the posterior portion of the pericardium,

  • esophageal aa., which supply the remainder of the esophagus, and

  • superior phrenic aa., which service the diaphragm.

Photo 4. Posterior intercostal arteries

Photo 5. Left bronchial arteries

Find this structure:

• • Posterior mediastinum

4.) Explore the posterior mediastinum and adjacent regions.

Note: The posterior mediastinum is the space anterior to the T4-T12 vertebrae, posterior to the pericardium, superoposterior to the diaphragm, inferior to the transverse thoracic plane, and medial to the parietal pleurae.

Boundaries of the posterior mediastinum:

The major features transmitted through the posterior mediastinum are the:

• thoracic aorta,

• thoracic duct,

• azygos v., accessory hemi-azygos v., and hemi-azygos v.,

• esophagus,

• esophageal nerve plexus, and

• vagus nn. (CN X); vagal trunks.

Photo 6. Posterior mediastinum

Identify the vagus nn., and examine the esophagus and thoracic duct.

Find these structures:

• • Esophagus

  • Vagus nn. (left & right)

  • Vagal trunks (anterior & posterior)

  • Esophageal plexus

  • Thoracic duct

  • Diaphragm

  • Phrenic nn.

5.) Follow the courses of the vagus nn. through the posterior mediastinum. The left vagus n. assumes a more anterior position relative to the esophagus, whereas the right vagus n. assumes a more posterior position.

Note: The right vagus n. enters the thorax via the superior mediastinum, medial to the phrenic n., and anterior to the subclavian a. Once inside the thorax, the right recurrent laryngeal n. branches from the vagus n., and wraps posteriorly around the right subclavian a. before ascending out of the thorax to the larynx. The right vagus n. descends lateral (right) to the trachea, passing posterior to the root of the right lung. Posterior to the root of the lung, the right vagus supplies bronchial brs. (pre-ganglionic parasympathetic) to the right pulmonary plexus. The right vagus n. further descends the posterior mediastinum, providing branches to the esophageal plexus, while moving to the posterior aspect of the esophagus where it is joined with branches from the left vagus n. to become the posterior vagal trunk. The posterior vagal trunk exits the thorax through the esophageal hiatus (near T10), enters the abdominal cavity, and becomes the major supplier of parasympathetics to the foregut and midgut.

Note: The left vagus n. enters the thorax via the superior mediastinum, medial to the phrenic n., and anterior to the subclavian a. Once inside the thorax, the left vagus n. descends anterior to the arch of the aorta, and spawns the left recurrent laryngeal n., which wraps posteriorly around the concavity of the arch in the vicinity of the ligamentum arteriosum before ascending out of the thorax to the larynx. The left vagus n. descends posterior to the root of the left lung, supplying bronchial brs. (pre-ganglionic parasympathetic) to the left pulmonary plexus. The left vagus n. further descends the posterior mediastinum, providing branches to the esophageal plexus, while moving to the anterior aspect of the esophagus where it is joined with branches from the right vagus n. to become the anterior vagal trunk. The anterior vagal trunk exits the thorax through the esophageal hiatus (near T10), enters the abdominal cavity, and becomes a minor supplier of parasympathetics to the foregut.

Note: A mnemonic to help remember which vagus n. forms the bulk of which vagal trunk is LARP (Left Anterior, Right Posterior). These positions are significant as the vagus trunks traverse the diaphragm (through the esophageal hiatus) to serve structures of the abdominal cavity.

Note: The esophageal plexus is found surrounding the lower esophagus (below the roots of the lungs). It consists of contributions from the vagus nn. and the thoracic sympathetic trunks.

Photo 7. Vagus nerves and esophageal plexus

Photo 8. Vagus nerves and vagal trunks

6.) Examine the esophagus throughout its length in the thorax. It enters the superior mediastinum between the trachea and vertebral column, anterior to the vertebral bodies of T1-T4. It passes through the esophageal hiatus of the diaphragm to enter into the stomach at the level of T10.

Photo 9. Esophagus

7.) Locate the thoracic duct. It is typically located adjacent to the esophagus, and anterior to the vertebral bodies of T6/T7-T12. Trace the thoracic duct to its drainage point, the left venous angle.

Note: The thoracic duct is the major lymphatic vessel and duct of the body, receiving and conducting lymph from all but the right head & neck, right upper limb, and right half of the thorax. The thoracic duct receives lymph from the following sources:

• • Left jugular trunk,

  • Left subclavian trunk,

  • Left bronchomediastinal trunk

  • Cisterna chyli (a variable, dilated confluence of the lumbar and visceral lymphatic trunks of the abdominopelvic region).

The pattern of lymphatic trunk drainage is inconstant, thus it is not unusual for trunks to independently enter the vicinity of the venous angles without first joining a duct.

Note: The thoracic duct enters the thoracic cavity through the aortic hiatus (near T12), ascends lateral (right) to the esophagus, transitions from right-to-left between the esophagus and vertebral column (near T5), ascends lateral (left) to the esophagus, and wraps posteriorly behind the subclavian a. (root of neck) to enter the venous system in the vicinity of the left venous angle.

Photo 10. Thoracic duct

Identify the azygos system of veins

Find these structures

• • Azygos v.

  • Hemi-azygos v.

  • Accessory hemi-azygos v.

  • Posterior intercostal vv. (segmentals)

8.) Locate the posterior intercostal vv. on the posterior thorax wall, in the costal grooves of the ribs. Trace these veins to their tributaries.

Note: The azygos venous system is a series of unpaired (a- = not, zygous = yoke) veins into which the posterior intercostal, subcostal, and lumbar vv. of the thoracic and abdominal walls drain.

Note: Intercostal neurovascular bundles consist of a vein, artery, and nerve (vein nearest to the rib). Anterior intercostal vv. drain into the internal thoracic vv., whereas posterior intercostal vv. drain into a variety of sources. See table below for details.

Photo 11. Azygos system of veins

Note: The azygos system of veins can exhibit a great deal of variation. The azygos v. typically runs along the right side of the vertebral column in this region, arching over the root of the R. lung to drain into the superior vena cava.

Photo 12. Azygos v.

Note: The hemi-azygos and accessory hemi-azygos are inconstant. Both vessels may be present; however, each vessel or both vessels may be absent. The accessory hemi-azygos v. is the superior partner of the azygos v. It typically runs along the left side of the vertebral column between T5-T8 and drains into the azygos v.

Note: The hemi-azygos v. is the main tributary of the azygos v. and is often referred to as the inferior partner of the azygos v. It typically ascends as far as T9 and drains into the azygos v.

Photo 13. Accessory hemi-azygos v. and hemi-azygos v.

Identify and clean the sympathetic trunks, ganglia, and the greater splanchnic nn.

Find these structures:

• • Thoracic sympathetic trunk

  • Thoracic sympathetic ganglia

  • Cervicothoracic (stellate) ganglion (if present)

  • Greater splanchnic n.

9.) Locate the thoracic sympathetic trunks and the greater splanchnic nn. Carefully remove any fat or soft tissue surrounding the sympathetic trunks. The trunks run lateral to the vertebral bodies and against the heads of the ribs in the thoracic region.

Photo 14. Thoracic sympathetic trunks

Photo 15. Sympathetic trunk, ganglia, and rami communicantes

10.) Follow the sympathetic trunk, and observe the thoracic ganglia. If present, locate the cervicothoracic (stellate) ganglion at approximately the C7-T1 level.

Note: The thoracic sympathetic trunks typically consist of 11 (but may consist of 10-13) ganglia. Usually, the T1 ganglion fuses with the inferior cervical ganglion to become the cervicothoracic (stellate) ganglion, and thoracic ganglia are successively numbered inferiorly from the cervicothoracic ganglion. In humans, each thoracic ganglion connects with two successive spinal nerves. Connections between ganglia and spinal nerves take the form of white rami communicantes (nerves → ganglia; pre-ganglionic; myelinated) and gray rami communicantes (ganglia → nerves; post-ganglionic; unmyelinated).

Note: The cervicothoracic (stellate) ganglion is noticeably larger than the other ganglia in the thoracic region. It is formed by the fusion of the inferior cervical ganglion (C7) and the first thoracic ganglion (T1). In some instances, the two ganglia do not fuse and can be observed as separate ganglia.

Photo 16. Cervicothoracic (stellate) ganglion

Photo 17. Separate C7 and T1 ganglia

11.) Expose the branches from the sympathetic trunks that form the greater splanchnic nn.

Note: The thoracic splanchnic nn. include the:

Note: The greater splanchnic n. consists of preganglionic sympathetic fibers passing through (but not synapsing) sympathetic ganglia 5-9/10. The greater splanchnic n. travels through the ipsilateral crus of the diaphragm to innervate the celiac ganglion (and plexus), aorticorenal ganglion (and plexus) and the suprarenal gland.

Photo 18. Greater splanchnic n.