Boundaries and fascial apparatus of the mediastinum
The anatomical space of the mediastinum is tightly bounded on the sides by the pleural cavities of the lungs, which in anatomy are designated as regiones pleuropulmonales.
Particular attention should be paid to the fascial apparatus of this region. The parietal pleura covering the mediastinum closely fuses with the endothoracic fascia (fascia endothoracica). Depending on the level of location, this fascia forms specific structures:
- At the level of the costal pleura, it forms the dense suprapleural membrane — membrana suprapleuralis.
- In the area of contact with the diaphragmatic pleura, the endothoracic fascia is termed the phrenicopleural fascia — fascia phrenicopleuralis.
Topographically, the entire mediastinal space is conventionally divided into two major tiers: superior mediastinum (mediastinum superius) and inferior mediastinum (mediastinum inferius).
Superior mediastinum
The superior mediastinum (mediastinum superius) occupies the space in the upper part of the thoracic cage. Its boundaries are clearly defined:
- Anteriorly, it is protected by the manubrium of the sternum.
- Posteriorly, the boundary is formed by the anterior surface of the vertebral column.
- Superiorly, the region starts at the level of the superior thoracic aperture.
- Inferiorly, the boundary runs along an imaginary horizontal plane drawn precisely through the tracheal bifurcation (the site where it divides into the main bronchi).
A multitude of critically important anatomical structures are concentrated in this compartment. These include the thymus (in children) or its replacement adipose tissue (in adults), the aortic arch with its branching vessels, the superior vena cava, and the brachiocephalic veins. The trachea, upper esophagus, and thoracic duct also pass here. The neural apparatus is represented by branches of the vagus and phrenic nerves, as well as the sympathetic trunks. Additionally, lymph nodes are located in the superior mediastinum.
Inferior mediastinum and its compartments
The inferior mediastinum (mediastinum inferius) lies below the imaginary plane passing through the tracheal bifurcation. For ease of study and clinical application, this extensive area is further subdivided into three independent compartments: anterior, middle, and posterior.
- The anterior mediastinum (mediastinum anterius) is localized between the posterior surface of the sternum and the anterior surface of the pericardium. Visually, it has the shape of a narrow slit. The contents of this space are represented primarily by loose connective tissue containing lymphatic vessels and nodes.
- The middle mediastinum (mediastinum medium) is the most voluminous compartment. Its main content is the heart enclosed within the pericardium. The initial segments of the great vessels are also located here: the ascending aorta and the pulmonary trunk. In addition, the middle mediastinum includes the lower part of the trachea, the main bronchi, tracheobronchial lymph nodes, and the lower half of the phrenic nerves.
- The posterior mediastinum (mediastinum posterius) occupies the position between the posterior wall of the pericardium (anteriorly) and the vertebral column (posteriorly). The thoracic aorta and esophagus pass through this compartment. The lymphatic system is represented by the thoracic duct, and the nervous system by the vagus nerves and sympathetic trunks.
Briefly: answers to topical questions
What branches does the aortic arch give off in the superior mediastinum?
In the superior mediastinum, three large trunks arise from the convex surface of the aortic arch. From right to left, the following branches are located:
- The brachiocephalic trunk (truncus brachiocephalicus) runs superiorly and to the right, dividing into the right common carotid and right subclavian arteries.
- Left common carotid artery (a. carotis communis sinistra) arises directly from the aortic arch.
- Left subclavian artery (a. subclavia sinistra) runs upward and to the left toward the cervical region.
What blood vessels are located in the middle mediastinum?
The middle mediastinum contains the intrapericardial segments of the great vessels, as well as vessels accompanying the nerves. These include:
- The ascending aorta.
- The pulmonary trunk (truncus pulmonalis).
- The pulmonary arteries and veins (aa. et vv. pulmonales).
- The pericardiacophrenic vessels (a. et v. pericardiacophrenica) accompany the phrenic nerves.
What anatomical structures pass through the posterior mediastinum?
Blood and lymphatic vessels, nerves, and organs of the digestive system pass through the posterior mediastinum. The following anatomical structures are located within it:
- The thoracic part of the aorta (descending aorta).
- The azygos vein (v. azygos) and hemiazygos vein (v. hemiazygos).
- The esophagus (esophagus).
- The thoracic duct (ductus thoracicus).
- The vagus nerves (nn. vagi).
- The sympathetic trunks (trunci sympathici) and splanchnic nerves (nn. splanchnici).
- Lymph nodes, connective tissue (adipose tissue), and extensions of the endothoracic fascia.
What nerves and nerve trunks pass through the superior mediastinum?
The following pass through the superior mediastinum:
- The vagus nerves (nn. vagi) and their branches.
- The phrenic nerves (nn. phrenici).
- The sympathetic trunks (trunci sympathici).
What constitutes the contents of the anterior mediastinum?
The contents of the anterior mediastinum, which appears as a narrow slit, are adipose tissue, vessels, lymph nodes, and glandular tissue. It contains:
- Loose connective tissue (adipose tissue) and extensions of the endothoracic fascia.
- The internal thoracic vessels (a. et v. thoracicae internae).
- Lymphatic vessels and nodes (parasternal, prepericardial, anterior mediastinal).
- The bulk of the thymus (thymus).
The union of which veins forms the superior vena cava?
The superior vena cava (v. cava superior) is formed by the union of two large venous vessels collecting blood from the head, neck, and upper extremities. Its formation involves:
- The right brachiocephalic vein (v. brachiocephalica dextra).
- The left brachiocephalic vein (v. brachiocephalica sinistra).
The brachiocephalic veins themselves are formed by the union of the jugular and subclavian veins.
For quick memorization of the boundaries: the main dividing "axis" is the tracheal bifurcation. Everything above it is the superior mediastinum. Everything below it is the inferior mediastinum, which the pericardium (as the central figure) divides into the anterior, middle (the pericardium itself), and posterior compartments.
Class and exam questions
Where is the boundary between the superior and inferior mediastinum?
The boundary is an imaginary horizontal plane drawn at the level of the tracheal bifurcation.
In which mediastinal compartment is the heart located?
The heart, along with the pericardium covering it, is located in the middle compartment of the inferior mediastinum.
What is located in the anterior mediastinum?
This is a narrow slit-like space between the sternum and the pericardium, which contains loose connective tissue, lymphatic vessels, and lymph nodes.
How is the endothoracic fascia related to the pleura?
The mediastinal parietal pleura fuses with the endothoracic fascia. At the level of the costal pleura, the suprapleural membrane is formed, and near the diaphragmatic pleura, the phrenicopleural fascia.
What else to study on this topic
- Topography and branches of the aortic arch in the superior mediastinum
- Syntopy of the esophagus and thoracic aorta in the posterior mediastinum
- Features of the formation of the endothoracic fascia
- Lymphatic system and lymph nodes of the mediastinum
- Age-related involution of the thymus
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