Anatomy of Middle Mediastinum

Aortic Arch and Descending Thoracic Aorta

• The aortic arch ascends anterior to the right pulmonary artery and bifurcation of the trachea, then passes over the hilum of the left lung and descends posterior to the left T4 vertebra.
• Termination: Lower border of the 4th thoracic vertebra, continuing as the descending aorta.

Aortic Arch Relations

• Anteriorly: Left lung and pleura, remains of thymus, left superior intercostal vein, left phrenic nerve, and left vagus nerve and its branches.
• Posteriorly: Trachea, oesophagus, thoracic duct, and vertebral column.
• Superiorly: Brachiocephalic trunk, left common carotid, and left subclavian arteries.
• Inferiorly: Bifurcation of pulmonary trunk, left bronchus, ligamentum arteriosum, and left recurrent laryngeal nerve.

Descending Thoracic Aorta

• Origin: Continuation from the arch of aorta at the left lower border of the 4th thoracic vertebra.
• Course: Descends on the left side of T5 – T12 vertebrae, lying posterior to the root of the left lung, pericardium, and oesophagus.
• Termination: Aortic opening of the diaphragm in the midline.

Descending Thoracic Aorta Relations

• Anterior: Hilum of the left lung, oesophagus.
• Posterior: Vertebral column, accessory hemiazygos, and hemiazygos.
• Right: Thoracic duct and azygos vein.
• Inferior: Mediastinal pleura of both lungs.

Descending Thoracic Aorta Branches

• Posterior intercostal arteries (3rd – 11th post. intercostal arteries on each side).
• Subcostal arteries (both sides) – Running along the lower border of the 12th rib.
• Pericardial branches:
  • Anterior border of pericardium – anteriorly.
  • Posterior border of pericardium – posteriorly.
  • Mediastinal parietal pleura – laterally.

Middle Mediastinum Content

• Heart and pericardium.
• Great vessels:
  • Ascending aorta.
  • Pulmonary trunk with left and right pulmonary arteries and veins.
  • Inferior half of superior vena cava.
• Nerves:
  • Phrenic nerve.
  • Deep part of cardiac plexus.
• Tracheal bifurcation and main bronchi.
• Root of the lungs.
• Subcarinal and lower paratracheal (tracheobronchial) lymph nodes.

Posterior Mediastinum

• Longest and narrowest inferior mediastinum.
• Boundaries:
  • Transverse thoracic plane – superiorly.
  • Posterior 1/3rd diaphragm – inferiorly.
  • Tracheal bifurcation, fibrous pericardium, and pulmonary vessels – anteriorly.
  • 4th/5th – 12th thoracic vertebrae – posteriorly.
  • Mediastinal pleura – laterally.

Posterior Mediastinum Content

• Posterior mediastinal lymph nodes and thoracic duct.
• Descending aorta and its branches.
• Azygos, hemiazygos, and accessory hemiazygos veins.
• Oesophagus (thoracic part).
• Vagus nerve and oesophageal nerve plexus.

Clinical Applications

• Posterior pericardium.
• Oesophageal branches.
• Distal thoracic oesophagus.
• Bronchial arteries.
• Pulmonary areolar tissue and visceral pleura.
• Mediastinal branches.
• Posterior mediastinum.
• Superior phrenic branches – Supplying superior aspect of diaphragm.

Brachiocephalic Trunk

• First and largest branch of the aortic arch, 4 – 5 cm length.
• Anterior to trachea and posterior to left brachiocephalic vein.
• Course: Ascends superolaterally to the right of trachea, dividing into right subclavian and right common carotid arteries behind the right sternoclavicular joint.

Right Common Carotid Artery and Right Subclavian Artery

• Origin: Brachiocephalic trunk, posterior to sternoclavicular joint.
• Course: Ascends superolaterally to 1st rib (posterior to sternal end of clavicle and scalenus anterior), then inferolaterally across superior 1st rib becoming axillary artery at its external border.

Left Subclavian Artery

• Origin: Third branch of the aortic arch, lies posterior to left common carotid artery.
• Course: Ascends into neck (post to scalenus muscle), then inferolaterally to external border 1st rib to become left axillary artery.

Left Common Carotid Artery

• Origin: Second branch of the aortic arch, lies posterior to and to the left of the brachiocephalic trunk.
• Course: Ascends anterior to left subclavian artery to the left of trachea, then passes posterior to left sternoclavicular joint to enter the neck.