Areas of the Thorax

Overview
The mediastinum forms the central space within the thoracic cavity and contains essential structures of the cardiovascular, respiratory, and lymphatic systems. This section describes the four principal divisions of the mediastinum, offering a structural framework for understanding thoracic organisation and the positional relationships of its major organs.

Key Structures
This section covers the superior, anterior, middle, and posterior mediastinum, outlining the boundaries, principal contents, and anatomical relationships of each. Information is presented in a clear, clinically focused manner to facilitate effective learning.

Clinical Relevance and Learning Focus
A thorough grasp of mediastinal anatomy is crucial for interpreting imaging studies, identifying the spread of infection or malignancy, evaluating acute chest presentations, and planning thoracic interventions. This section lays the groundwork for clinical assessment and diagnostic decision-making.

The Superior Mediastinum

Contents

The mediastinum is the central space within the thoracic cavity, positioned between the two pleural sacs. It contains the majority of thoracic organs and serves as a passageway for structures travelling through the chest en-route to the abdomen.

From an anatomical perspective, the mediastinum is divided into two regions by an imaginary line extending from the sternal angle (formed at the junction of the manubrium and the body of the sternum) to the level of the T4 vertebra:

• Superior mediastinum – extends upward to the superior thoracic aperture.
• Inferior mediastinum – extends downward to the diaphragm and is further divided into the anterior, middle, and posterior mediastinum.

This article focuses on the superior mediastinum, outlining its boundaries, contents, and relevant clinical associations.

Borders

The superior mediastinum is enclosed by the following anatomical landmarks:

• Superior: Thoracic inlet
• Inferior: Continuous with the inferior mediastinum at the level of the sternal angle
• Anterior: Manubrium of the sternum
• Posterior: Vertebral bodies of T1–T4
• Lateral: Pleurae of the lungs

Fig 1 – The mediastinal compartments of the thorax.

Contents

The superior mediastinum contains a variety of neural, vascular, and respiratory structures that pass between the neck and the thorax, as well as structures continuing toward the abdomen via the inferior mediastinum.

Great Vessels

The major systemic blood vessels associated with the heart are located within the superior mediastinum. Their principal branches arise here before passing through the superior thoracic aperture.

Arch of the Aorta

Three main branches originate from the aortic arch within the superior mediastinum:

• Brachiocephalic artery – supplies the right side of the head and neck and the right upper limb
• Left common carotid artery – supplies the left side of the head and neck
• Left subclavian artery – supplies the left upper limb

Superior Vena Cava

Several tributaries of the superior vena cava are found within the superior mediastinum:

• Brachiocephalic veins – drain venous blood from the upper body
• Left superior intercostal vein – drains the left second and third intercostal spaces into the left brachiocephalic vein
• Supreme intercostal vein – drains blood from the first intercostal space directly into the brachiocephalic veins
• Azygos vein – receives blood from the right posterior intercostal veins; left-sided intercostal veins first drain into the hemiazygos and accessory hemiazygos veins before joining the azygos vein around the T7–T9 vertebral levels

Fig 2 – Major vessels within the superior mediastinum.

Nerves

Vagus Nerve

Within the superior mediastinum, the vagus nerves follow distinct courses on each side:

• Right vagus nerve – descends alongside the trachea, passing posterior to the superior vena cava and the right primary bronchus
• Left vagus nerve – enters the superior mediastinum between the left common carotid and left subclavian arteries, descends anterior to the aortic arch, and then passes posterior to the left bronchus

The left recurrent laryngeal nerve branches from the left vagus nerve as it crosses the aortic arch. It loops beneath the arch, to the left of the ligamentum arteriosum, and then ascends toward the larynx within the tracheo-oesophageal groove.

Phrenic Nerve

Originating from the anterior surface of the anterior scalene muscle, the phrenic nerves (C3–C5) enter the superior mediastinum lateral to the great vessels. They then pass anteriorly into the middle mediastinum, travelling in front of the lung hila.

Other Nerves

• Cardiac nerves – arise from the superior, middle, and inferior cardiac ganglia and contribute to the superficial and deep cardiac plexuses. The superficial plexus lies between the aortic arch and right pulmonary artery, while the deep plexus is located on the surface of the trachea at its bifurcation.
• Sympathetic trunk – runs bilaterally along the vertebral bodies for the entire length of the vertebral column.

Fig 3 – Overview of the anatomical course of the left vagus nerve.

Other Structures in the Superior Mediastinum

Thymus

The thymus is the most anterior structure in the superior mediastinum. It lies directly against the posterior surface of the sternum, extends into the anterior mediastinum, and may reach superiorly into the neck.

Trachea

The trachea divides into the right and left primary bronchi posterior to the ascending aorta at the level of the sternal angle.

Oesophagus

The oesophagus ascends toward the pharynx, which it joins at the level of the C6 vertebra.

Thoracic Duct

Within the superior mediastinum, the thoracic duct travels to the left of the oesophagus as it ascends toward its termination at the junction of the left internal jugular and subclavian veins.

Muscles

• The sternohyoid and sternothyroid muscles originate from the posterior surface of the manubrium and belong to the infrahyoid muscle group of the neck.
• The inferior portion of the longus colli muscle also originates within the superior mediastinum.

Fig 4 – Spatial relationships of structures passing from the superior mediastinum to adjacent regions. Note: this schematic simplifies anatomy for clarity and does not include every structure described above.

The Anterior Mediastinum

Contents

The mediastinum is the central region of the thoracic cavity, situated between the two pleural sacs. It contains the majority of thoracic organs and functions as a passageway for structures travelling through the chest toward the abdomen.

Anatomically, the mediastinum is divided into two sections by an imaginary line extending from the sternal angle (formed at the junction between the manubrium and the body of the sternum) to the level of the T4 vertebra:

• Superior mediastinum – extends upward to the superior thoracic aperture.
• Inferior mediastinum – extends downward to the diaphragm and is further divided into the anterior, middle, and posterior mediastinum.

This article focuses on the anterior mediastinum, describing its boundaries, contents, and clinical significance.

Borders

The anterior mediastinum is defined by the following anatomical boundaries:

• Lateral borders: Mediastinal pleura (part of the parietal pleural membrane)
• Anterior border: Body of the sternum and the transversus thoracis muscles
• Posterior border: Pericardium
• Roof: Continuous with the superior mediastinum at the level of the sternal angle
• Floor: Diaphragm

Fig 1.0 – Diagram showing the superior, anterior, middle, and posterior mediastinal compartments of the thorax.

Contents

The anterior mediastinum does not contain any major organs. Instead, it houses loose connective tissue, including the sternopericardial ligaments that anchor the pericardium to the sternum, as well as adipose tissue, small lymphatic vessels, lymph nodes, and branches of the internal thoracic vessels.

In infants and children, the thymus extends downward into the anterior mediastinum. During puberty, the thymus undergoes regression and is largely replaced by fatty tissue in adults.

Fig 1.1 – Anatomical illustration of the thymus, primarily located in the superior mediastinum with possible extension into the anterior mediastinum.

The Middle Mediastinum

Contents

The mediastinum is the central space (compartment) within the thoracic cavity, positioned between the two pleural sacs. It contains the majority of thoracic organs and functions as a passageway for structures travelling through the chest on their route to the abdomen.

From an anatomical standpoint, the mediastinum is divided into two regions by an imaginary line extending from the sternal angle (formed at the junction of the manubrium and the body of the sternum) to the level of the T4 vertebra:

• Superior mediastinum – extends upward to the superior thoracic aperture.
• Inferior mediastinum – extends downward to the diaphragm and is further subdivided into the anterior, middle, and posterior mediastinum.

This article examines the middle mediastinum, focusing on its boundaries, contents, and clinical relevance.

Borders

The middle mediastinum is bounded by the following structures:

• Anterior: Anterior surface of the pericardium
• Posterior: Posterior surface of the pericardium
• Lateral: Mediastinal pleura of the lungs
• Superior: An imaginary line connecting the sternal angle to the T4 vertebra
• Inferior: Upper surface of the diaphragm

Fig 1.0 – Diagram illustrating the superior, anterior, middle, and posterior mediastinal regions of the thorax.

Contents

The middle mediastinum is the largest subdivision of the inferior mediastinum and contains several key organs, vessels, nerves, and lymphatic structures.

Organs

This region contains the heart enclosed within its protective sac, the pericardium. It also includes the bifurcation of the trachea and the left and right main bronchi.

Vessels

The middle mediastinum contains the origins of the major vessels entering and leaving the heart:

• Ascending aorta – the initial segment of the aorta arising from the aortic orifice. It ascends, exits the fibrous pericardium, and enters the superior mediastinum where it continues as the aortic arch. The ascending aorta gives rise to the left and right coronary arteries.
• Pulmonary trunk – a short, wide vessel that divides into the left and right pulmonary arteries, allowing a large volume of blood to pass from the heart to the lungs.
• Superior vena cava – returns deoxygenated blood from the upper half of the body and is formed by the union of the left and right brachiocephalic veins.

Fig 1.1 – The great vessels of the middle mediastinum, showing that only their origins lie within this compartment.

Nerves

Both the cardiac plexus and the phrenic nerves are located within the middle mediastinum.

• Cardiac plexus – a network of sympathetic and parasympathetic nerve fibres situated at the base of the heart. Sympathetic fibres originate from spinal cord segments T1–T4, while parasympathetic innervation is provided by the vagus nerve. The plexus is divided into superficial and deep components.
• Phrenic nerves (left and right) – mixed nerves that supply motor innervation to the diaphragm. They originate in the neck and descend through the middle mediastinum to reach the diaphragm.

Fig 1.2 – Illustration showing the course of the phrenic nerves as they pass through the middle mediastinum to innervate the diaphragm.

Lymphatics

The tracheobronchial lymph nodes are located within the middle mediastinum. These nodes are associated with the trachea and bronchi and are commonly enlarged in certain pulmonary diseases. They are formed from the convergence of bronchial lymph nodes at the lung hila, with individual groups connected by fine lymphatic channels.

The Posterior Mediastinum

Contents

The mediastinum is the central region of the thoracic cavity, situated between the two pleural sacs. It contains most of the thoracic organs and functions as a passageway for structures passing through the chest on their route to the abdomen.

Anatomically, the mediastinum is separated into two main divisions by an imaginary line extending from the sternal angle (formed at the junction of the manubrium and the body of the sternum) to the level of the T4 vertebra:

• Superior mediastinum – extends upward to the superior thoracic aperture.
• Inferior mediastinum – extends downward to the diaphragm and is further divided into the anterior, middle, and posterior mediastinum.

This article focuses on the posterior mediastinum, outlining its boundaries, contents, and important clinical considerations.

Borders

The posterior mediastinum is defined by the following anatomical limits:

• Lateral: Mediastinal pleura (part of the parietal pleural membrane)
• Anterior: Pericardium
• Posterior: Vertebral bodies of T5–T12
• Roof: An imaginary line connecting the sternal angle to the T4 vertebra
• Floor: Diaphragm

Fig 1 – Diagram showing the superior, anterior, middle, and posterior mediastinal compartments of the thorax.

Contents

The posterior mediastinum contains several major organs, blood vessels, and nerves. Each of these structures is described below in greater detail.

Thoracic Aorta

The thoracic (descending) aorta is the continuation of the aortic arch and begins at the lower border of the T4 vertebra. It descends through the posterior mediastinum along the left side of the vertebral column, gradually moving toward a more central position. At the inferior border of T12, it continues as the abdominal aorta after passing through the aortic hiatus of the diaphragm.

Numerous branches arise from the thoracic aorta within the posterior mediastinum. These branches are organised into three general vascular planes: unpaired visceral branches that project anteriorly, paired visceral branches that extend laterally, and paired segmental parietal branches that pass mainly posterolaterally. The principal branches include:

• Posterior intercostal arteries – paired parietal branches; nine pairs arise from the posterior surface of the aorta to supply the intercostal spaces (excluding the first two). They run posteriorly and laterally alongside the ribs.
• Bronchial arteries – paired visceral branches, usually one or two in number. The left bronchial arteries arise directly from the thoracic aorta, while the right typically originate indirectly from a right posterior intercostal artery. These vessels supply the tracheobronchial tree.
• Oesophageal arteries – unpaired visceral branches arising from the anterior surface of the aorta. Most individuals have two, though up to five may be present. These arteries supply the oesophagus.
• Superior phrenic arteries – arise from the anterior aspect of the thoracic aorta near the aortic hiatus, with variable numbers, and supply the superior surface of the diaphragm.

Oesophagus

The oesophagus is a muscular tube that connects the pharynx to the stomach, allowing ingested material to enter the digestive system. It enters the posterior mediastinum from the superior mediastinum and descends posterior to both the aortic arch and the heart. Initially positioned slightly to the right, it gradually shifts to the left as it travels inferiorly. The oesophagus exits the mediastinum through the oesophageal hiatus of the diaphragm.

Surrounding the oesophagus is the oesophageal plexus, a network of nerves formed by branches of the left and right vagus nerves. Just above the diaphragm, these fibres converge to form the anterior and posterior vagal trunks, which continue along the oesophagus as it leaves the thorax.

Fig 2 – Posterior view of the oesophagus with associated thoracic vasculature highlighted.

Thoracic Duct

The thoracic duct is the largest lymphatic vessel in the body and returns lymph from most of the body—excluding the right superior quadrant—into the venous circulation.

It originates from the cisterna chyli in the abdomen and enters the thorax through the aortic hiatus. Within the posterior mediastinum, it ascends anterior to the T6–T12 vertebral bodies before deviating to the left as it continues into the superior mediastinum. While in the posterior mediastinum, the thoracic duct receives lymph from the intercostal spaces and surrounding structures via multiple tributaries.

Azygos System of Veins

The azygos venous system drains blood from the thoracic wall and mediastinal structures and ultimately empties into the superior vena cava. It is composed of three main veins:

• Azygos vein – formed by the union of the right lumbar and right subcostal veins; it enters the thorax through the aortic hiatus and drains directly into the superior vena cava.
• Hemiazygos vein – formed by the left lumbar and left subcostal veins; it enters the thorax via the left crus of the diaphragm, ascends on the left side, and crosses to join the azygos vein at approximately the T8 level.
• Accessory hemiazygos vein – formed by the union of the fourth to eighth intercostal veins; it drains into the azygos vein at around T7.

Fig 3 – The azygos venous network and its drainage into the superior vena cava.

Sympathetic Trunks

The sympathetic trunks are paired nerve bundles extending from the base of the skull to the coccyx. Within the thorax, they are referred to as the thoracic sympathetic trunks and descend through the posterior mediastinum.

Branches arising from these trunks form the lower thoracic splanchnic nerves, which continue inferiorly to provide autonomic innervation to the abdominal viscera.