Boundaries and general topographical anatomy
The central space of the thoracic cavity is clearly demarcated by adjacent anatomical structures, forming a repository for vital organs. The spatial orientation of this complex is defined by the following boundaries:
- Anterior wall: formed by the internal surface of the sternum, as well as the retrosternal fascia lining it posteriorly.
- Posterior wall: represented by the thoracic vertebral column, rib necks, and the prevertebral fascia covering them.
- Lateral walls: formed by the right and left mediastinal pleurae, which are closely apposed by the corresponding leaflets of the endothoracic fascia.
- Inferior wall: bounded by the tendinous center and muscle bundles of the diaphragm, covered by the diaphragmatic fascia.
- Superior boundary: is an entirely conventional line running at the level of the superior thoracic aperture. It is here that the mediastinal fatty tissue directly merges with the fascial spaces of the neck.
The transverse diameter of this complex is variable and depends on the level of the thoracic cavity and the arrangement of the pleural sacs. For operational surgery, it is critically vital to know the features of pleural topography: anterior to the spine (at the level of Th VII–VIII), the pleural leaves approach each other as closely as possible (this must be taken into account when performing posterior mediastinotomy). However, slightly lower, in the area where the esophagus crosses the aorta, the pleural leaves diverge, allowing the esophagus and aorta to be in close contact with each other.
Compartments of the mediastinum (PNA)
According to the Paris Nomina Anatomica, the entire space is conventionally divided into four main compartments. The dividing boundary is a horizontal plane passing precisely through the upper edge of the roots of the lungs.
- Superior mediastinum. Located above the aforementioned conventional plane. This is where the thymus gland is localized, or the adipose tissue that replaces it in adults. This compartment contains the largest venous trunks: the brachiocephalic veins (vv. brachiocephalicae) and the upper part of the superior vena cava (v. cava superior). Also passing here is the aortic arch (arcus aortae) with its major branching vessels (brachiocephalic trunk, left common carotid, and left subclavian arteries). In addition, the superior compartment contains the trachea, esophagus, thoracic duct, and major nerve trunks (vagus, phrenic, sympathetic), surrounded by fascia and adipose tissue.
- Anterior mediastinum. Localized below the conventional plane, situated in the narrow space between the body of the sternum and the anterior wall of the pericardium. It contains predominantly adipose tissue, internal thoracic vessels, extensions of the endothoracic fascia, and groups of lymph nodes (parasternal, prepericardial, and anterior mediastinal).
- Middle mediastinum. This is the most voluminous central mass. It contains the heart enclosed within the pericardium, as well as the intrapericardial segments of major great vessels. This same compartment houses the tracheal bifurcation, main bronchi, pulmonary arteries and veins, and phrenic nerves with accompanying pericardiacophrenic vessels.
- Posterior mediastinum. Located posterior to the tracheal bifurcation and the posterior wall of the pericardium, extending down to the bodies of vertebrae Th IV—XII. Its massive contents include the descending aorta, esophagus, azygos (v. azygos) and hemiazygos (v. hemiazygos) veins, splanchnic nerves (nn. splanchnici), vagus nerves, and thoracic duct.
Fascial and cellular spaces
Within the mediastinum, between the fascial sheaths of organs and vessels, specific spaces are formed. They are densely packed with adipose tissue, which conceals lymph nodes and nerve plexuses. Several key spaces are distinguished:
- Prepericardial space. Localized posterior to the retrosternal fascia. Posteriorly, it is bounded by the fascial sheath of the thymus and the pericardium. Superiorly, it is separated from the neck by fascial extensions (extending from the periosteum of the clavicle and the manubrium of the sternum), and inferiorly by the diaphragmatic fascia.
- Pretracheal space. Bounded on the left by the aortic arch and the origins of its branches, and on the right by the mediastinal pleura and the arch of the azygos vein. Anteriorly, it is enclosed by the thymus sheath and the posterior wall of the pericardium, and posteriorly by the interbronchial fascia and the anterior layer of the esophagotracheal fascia.
- Periesophageal space. In the superior compartment, it is bounded by the endothoracic and prevertebral fasciae, while anteriorly the esophagus is apposed to the trachea (they are enclosed in a common sheath). In the posterior mediastinum, it lies between the posterior wall of the pericardium and the anterior leaflet of the aortic fascia, where dense fascial bands divide it into anterior and posterior parts.
Spread of infectious processes
Purulent-inflammatory processes in the mediastinum (mediastinitis) are extremely dangerous due to the specific features of their anatomical structure.
Anatomical factor: weak and uneven development of fasciae, along with an abundance of loose adipose tissue, leads to the absence of reliable boundaries between various compartments. Dynamic factor: continuous movement of the pleura and diaphragm during respiration, as well as volumetric pulsations of organs, literally "push" the infection into adjacent zones.
Infection can enter the cellular spaces through direct injuries to the chest wall or diseases of internal organs. However, most commonly, pus descends from the cervical region, tracking along the prevertebral fascia, along vascular sheaths, or via the pretracheal adipose tissue directly into the mediastinal cavity.
Briefly: answers to topical questions
Which nerves and nerve plexuses are located in the middle mediastinum?
The phrenic nerves are the neural structures located in the middle mediastinum.
- Phrenic nerves (nn. phrenici) — are formed from the cervical plexus and pass through the superior and middle mediastinum. In this compartment, they lie between the pericardium and the mediastinal pleura, accompanied by pericardiophrenic vessels. These nerves provide innervation to the diaphragm, pericardium, and pleura.
There are no direct indications in sources regarding the location of specific nerve plexuses in the middle mediastinum.
Which groups of lymph nodes are localized in the posterior mediastinum?
Visceral posterior mediastinal lymph nodes are localized in the posterior mediastinum. This group includes:
- Bronchopulmonary (hilar) nodes (nodi lymphoidei bronchopulmonales/hilares) — located at the hilum of the lung.
- Superior and inferior tracheobronchial nodes (nodi lymphoidei tracheobronchiales superiores et inferiores) — situated around the bifurcation of the trachea.
- Juxtaesophageal pulmonary nodes (nodi lymphoidei juxtaeosophageales pulmonales) — localized along the esophagus.
In addition, in the lower part of the posterior mediastinum along the course of the thoracic duct, there are nodes that collect lymph from the organs of this region.
What branches does the descending aorta give off in the posterior mediastinum?
In the posterior mediastinum, the descending (thoracic) aorta gives off visceral and parietal branches.
Visceral branches:
- Bronchial branches (rr. bronchiales) — run to the bronchi, pleura, and esophagus.
- Esophageal branches (rr. esophagei / aa. esophageales) — supply blood to the wall of the esophagus.
- Mediastinal branches (rr. mediastinales) — run to the adipose tissue and lymph nodes of the posterior mediastinum.
- Pericardial branches (rr. pericardiaci) — go to the posterior surface of the pericardium.
Parietal branches:
- Posterior intercostal arteries (aa. intercostales posteriores) — 10 pairs.
- Subcostal artery — passes below the 12th rib.
- Superior phrenic arteries (aa. phrenicae superiores).
Rule of the 4 PNA compartments: Superior — above the lung roots. Below the roots: Anterior — to the heart, Middle — the heart itself, Posterior — behind the heart.
Class and exam questions
How do the fascial spaces of the neck communicate with the mediastinum?
They are separated only conventionally at the level of the superior thoracic aperture. The neck spaces communicate with the mediastinum through fascial cords and layers along the major vessels, trachea, and esophagus.
Where do the pleural membranes come closest to each other?
The maximum approximation of the right and left mediastinal pleura occurs anterior to the spine, at the level of the VII–VIII thoracic vertebrae.
What serves as the boundary between the superior and inferior mediastinum according to the PNA?
The boundary is an imaginary horizontal plane drawn along the upper margin of the roots of the lungs.
Why do purulent processes in the mediastinum spread rapidly?
Due to the weak development of fasciae, there are no clear barriers between the compartments, while continuous respiratory movements of the diaphragm and vascular pulsation facilitate the rapid spread of infection.
What else to study on this topic
- Topographic Anatomy and Boundaries of the Pleural Sacs
- Surgical Approaches: Technique of Posterior Mediastinotomy
- Topography of the Aortic Arch and Its Branches in the Superior Mediastinum
- Structure and Functions of the Esophagotracheal Fascia
- Topography of the Thoracic Duct and Azygos Veins
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