Anatomy of the mediastinum

The mediastinum (mediastinum) is the central area of the thoracic cavity, in which a number of organs are located. The mediastinum is delimited posteriorly by the thoracic vertebrae, anteriorly by the sternum, and laterally by the pulmonary pleura, or more precisely by its mediastinal part.

The mediastinum is divided into several parts. This conditional division may differ depending on the classifications.

According to the so-called clinical classification, there are two parts of the mediastinum, namely the anterior and posterior mediastinum.

Anterior mediastinum (mediastinum anterius)

Posterior mediastinum (mediastinum posterius)

The imaginary line drawn along the roots of the lungs in the frontal plane is the border between these parts.

According to the so-called anatomical classification, which is the most common, the two parts of the mediastinum are the superior and inferior mediastinum.

Superior mediastinum (mediastinum superius)

Inferior mediastinum (mediastinum inferius)

These parts are divided by the imaginary line drawn from the angle of the sternum to the 4th and 5th thoracic vertebrae.

The inferior mediastinum, in turn, is divided into anterior, middle, and posterior mediastina. This classification eventually became recognized worldwide.

The superior mediastinum (mediastinum superior)

Superiorly, it is delimited by the superior thoracic aperture, and inferiorly, it is delimited by the already mentioned imaginary line.

The superior mediastinum contains the arch of the aorta

Three major branches arise from it:

1. The brachiocephalic trunk (truncus brachiocephalicus), which arises at the level of the superior border of the cartilage of the 2nd rib. Then it goes superiorly to the right sternoclavicular joint, where it divides into the right common carotid and subclavian arteries.

2. The left common carotid artery (a. carotis communis sinistra), which arises from the brachiocephalic trunk and goes superiorly to the neck region. It runs along the left sternoclavicular joint.

3. The left subclavian artery (a. subclavia sinistra), which goes from the aortic arch to the neck region through the superior aperture of the thoracic cage.

There are several structures located anteriorly and to the right of the aortic arch:

The thymus (thymus), which is developed the most in children and significantly decreases in size in adults due to the process of reverse development called involution.

The thymus consists of two lobes that border, in addition to the aorta, with the posterior fascia. Due to the change in its size, its superior border may reach the neck region, and the inferior one may pass into the anterior mediastinum.

2. Brachiocephalic veins (vv. brachiocephalicae), which are formed by the fusion of the internal jugular and subclavian veins.

Left brachiocephalic vein (v. brachiocephalica sinistra)

Right brachiocephalic vein (v. brachiocephalica dextra)

They are located posteriorly to the thymus. It is worth noting that the left brachiocephalic vein is three times longer than the right one and crosses the superior mediastinum in the superoinferior direction and from left to right. The brachiocephalic veins, in turn, merge and form the superior vena cava.

3. The superior vena cava (v. cava superior), which runs along the right border of the sternum to the 2nd intercostal space. This is where it penetrates into the pericardial cavity.

4. The right phrenic nerve (n. phrenicus dexter), which is located between the right subclavian artery and the eponymous vein, passes inferiorly and then goes along the brachiocephalic vein and the superior vena cava. And then passes anteriorly to the root of the lung.

5. The brachiocephalic lymph nodes (nodi lymphatici brachiocephalici), which lie anteriorly to the brachiocephalic veins. They collect lymph from the pericardium, thymus, and thyroid gland.

The following structures are located anteriorly and to the left of the aortic arch:

1. The left superior intercostal vein (v. intercostalis superior sinistra), which drains venous blood from the three superior intercostal spaces and flows into the left brachiocephalic vein.

2. The left phrenic nerve (n. phrenicus sinister), which is located between the left common carotid and subclavian arteries. Then it passes behind the left brachiocephalic vein and goes anteriorly to the root of the lung.

Left phrenic nerve<br />
(n. phrenicus sinister) — Left phrenic nerve
(n. phrenicus sinister)

3. The left vagus nerve (n. vagus sinister), which passes posteriorly to the diaphragmatic nerve.

The following structures are located posteriorly to the arch of the aorta:

1. The trachea runs slightly to the right of the median line. At the level of the 4th thoracic vertebra, it is divided into two main bronchi.

2. The esophagus (oesophageus), which is located posteriorly to the trachea and anteriorly to the bodies of the vertebrae, or rather to the prevertebral fiber and the endothoracic fascia. The mediastinal pleura adheres to it on the right.

The right vagus nerve (n. vagus dexter), which passes anteriorly to the subclavian artery and laterally to the lateral wall of the trachea, along which it passes to the root of the lung. The right recurrent laryngeal nerve, which arises from the vagus nerve in the superior mediastinum.

4. The left recurrent laryngeal nerve (n. laryngeus recarrens sinister), which arises from the vagus nerve and lies below the arch of the aorta, rising superiorly.

5. The thoracic duct (ductus thoracius), which lies to the left of the esophagus and inserts into the junction of the internal jugular and subclavian veins called the left venous angle in the neck region.

6. Paratracheal lymph nodes (nodi lymphatici paratracheales), which lie around the trachea. They collect lymph from the superior and inferior tracheobronchial lymph nodes.

The inferior mediastinum (mediastinum inferior) is divided into anterior, middle, and posterior mediastina.

The anterior mediastinum (mediastinum anterior) is located anteriorly to the pericardium.

The superior border is an imaginary line drawn from the corner of the sternum to the inferior border of the body of the 4th thoracic vertebra. It borders the diaphragm inferiorly, and the sternum anteriorly. In the cavity of the anterior mediastinum, there is cellular tissue, as well as some major structures:

1. Parasternal lymph nodes (nodi lymphatici parasternales), which adhere to the internal thoracic artery. They collect lymph from the internal inferior quadrant of the mammary gland, the superior third of the anterolateral abdominal wall, deep structures of the anterior thoracic wall, and the superior surface of the liver.

2. Superior diaphragmatic lymph nodes (nodi lymphatici superiores), which lie at the base of the xiphoid process. They collect lymph from the anterior diaphragm and the superior surface of the liver.

The middle mediastinum (mediastinum medium) has the same superior and inferior borders as the anterior one. It is located posteriorly to it.

It consists of:

1. The pericardium (pericardium), or pericardial sac.

It consists of two sheaths, namely the external fibrous (pericardium fibrosum) and internal serous (pericardium serosum) sheaths

Fibrous pericardium (pericardium fibrosum)

Serous pericardium (pericardium serosum)

The serous pericardium has two layers, namely the parietal layer, which lines the fibrous pericardium from the inside, and the visceral layer (or epicardium), which covers the vessels and the heart.

Between these two layers, there is the pericardial cavity (cavitas pericardialis), in which there is a small amount of serous fluid that softens the friction of the sheaths against each other.

2. The heart (cor) is located inside the pericardium.

Its borders are projected onto the anterior thoracic wall at several points. The first point is located at the level of the cartilage of the right 3rd rib 1-1.5 centimeters away from the edge of the sternum; the second one is located at the level of the cartilage of the left 3rd rib 2-2.5 centimeters away from the border of the sternum; the third one is located at the level of the right 6th sternocostal joint and the fourth one is located in the 5th intercostal space at 1-1.5 centimeters internally to the left the midclavicular line.

3. The ascending aorta (pars ascendens aortae), which arises at the level of the cartilage of the 3rd rib to the left of the sternum. At this level, it exits the left ventricle. At the level of the cartilage of the 2nd rib, the ascending aorta passes into an arch.

4. The superior vena cava (v. cava superior), which partially passes in the middle mediastinum. At the level of the 2nd intercostal space, it inserts into the right atrium.

5. The pulmonary trunk (truncus pulmonalis), which arises from the right ventricle of the heart and lies to the left of the ascending aorta.

The bifurcation of the pulmonary trunk (bifurcatio trunci pulmonalis) is located inferiorly to the aorta. At this point, the pulmonary arteries arise from it and go to the hilum of the lungs. It should be noted that the bifurcation of the pulmonary trunk is connected to the inferior surface of the arch of the aorta by the ligamentum arteriosum.

This ligament is formed from the ductus arteriosus /Botalli (ductus arteriosus), which connects the blood flow of the aorta and the pulmonary trunk in fetuses, and then closes after birth.

6. Pulmonary veins (vv. pulmonales)

Exit the hilum of the lungs and penetrate into the pericardium. The right pulmonary veins run posteriorly to the superior vena cava, and the left ones run inferiorly to the aorta.

7. The diaphragmatic nerves (nn. diaphragmales) are located respectively between the right and left mediastinal pleura and pericardium. They accompany the pericardiacophrenic arteries, which are branches of the internal thoracic arteries, as well as venous tributaries of the internal thoracic veins (vv. thoracicae internae).

Internal thoracic veins (vv. thoracicae internae)

The pericardial cavity has sinuses:

1. The transverse sinus (sinus transversus) borders the aorta and pulmonary trunk anteriorly, and the left atrium, right pulmonary artery, and superior vena cava posteriorly.

2. And the oblique sinus (sinus obliquus) has the following borders: the anterior border is the left atrium, the posterior border is the parietal layer of the serous pericardium, and the superior left border consists of the left pulmonary veins.

The inferior vena cava adheres to it posteriorly on the right.

3. The third sinus is not always distinguished in the nomenclature. It has no name and is located in the area where the anterior wall of the pericardium passes into the inferior one.

The posterior mediastinum (mediastinum posterius) lies behind the bodies of the 5th-12th thoracic vertebrae.

It borders the pericardium anteriorly, the diaphragm inferiorly, and the mediastinal pleura bilaterally. The superior border coincides with the previous two parts of the inferior mediastinum.

There are also many structures in the posterior mediastinum:

1. The largest structure of the posterior mediastinum is the descending aorta (pars descendens aortae). It is located to the left of the bodies of the vertebrae, passes inferiorly, and shifts towards the median line.

The following major branches arise from the descending aorta:

1.1 Pericardial branches (rr. pericardiaci), which supply the posterior region of the pericardium with blood

Pericardial branches<br />
(rr. pericardiaci) — Pericardial branches
(rr. pericardiaci)

1.2 Bronchial arteries (aa. bronchiales), which supply the lungs and the walls of the bronchi with blood

1.3 Esophageal arteries (aa. oesophageales), which supply the wall of the thoracic esophagus with blood

1.4 Mediastinal branches (rr. mediastinales), which supply the lymph nodes and connective tissue of the mediastinum with blood

1.5 Posterior intercostal arteries (aa. intercostales posteriores), which supply the skin and muscles of the back, as well as the spinal cord with blood. They anastomose with the anterior intercostal arteries

Posterior intercostal arteries (аа. intercostales posteriores)

1.6 The superior phrenic artery (a. phrenica superior), which supplies the superior surface of the diaphragm with blood

The following structures pass around the descending aorta:

2. The right and left main bronchi (bronchus principalis dexter et sinister), which arise from the tracheal bifurcation at the level of the inferior border of the 4th thoracic vertebra. The left main bronchus is located posteriorly to the arch of the aorta, and the right main bronchus is located laterally to it.

Right main bronchus (bronchus principalis dexter)

Left main bronchus (bronchus principalis sinister)

3. The esophagus (oesophagus), which passes posteriorly to the left atrium and to the right of the descending aorta.

In the inferior third of the mediastinum, the esophagus passes anteriorly to the aorta and enters the esophageal triangle. This triangle is formed by the pericardium anteriorly, by the diaphragm inferiorly, and by the descending aorta posteriorly. There is an esophageal plexus (plexus oesophagealis) on the anterior and posterior surfaces of the esophagus. It is formed by two vagus nerves and branches of the pectoral nodes of the sympathetic trunk.

4. The azygos vein (v. azygos), which arises from the abdominal cavity to the right of the vertebral column.

In the mediastinum, it circumflexes the right main bronchus and inserts into the superior vena cava. All the posterior intercostal veins of the right side, bronchial, esophageal, and mediastinal veins insert into the azygos vein.

5. The hemi-azygos vein (v. hemiazygos), which passes from the retroperitoneal space behind the descending aorta and insert into the azygos vein. The tributaries of the hemi-azygos vein are the five inferior left intercostal veins, esophageal, and mediastinal veins, as well as the accessory hemi-azygos veins.

6. The accessory hemi-azygos vein (V hemiazygos accessoria), which passes to the left of the vertebral column. The first 5-6 posterior left intercostal veins insert into it.

7. The thoracic duct (ductus thoracicus), which passes between the azygos vein and the descending aorta from the retroperitoneal space. In the mediastinum, it passes posteriorly to the esophagus and continues in the superior mediastinum.

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