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Mediastinal Structures Anatomy


Mediastinal Structures Anatomy
Brachiocephalic veins
On each side, the brachiocephalic vein is formed in the root of the neck by the union of the internal jugular and subclavian veins. At its origin, the vein lies behind the sternoclavicular joint and in front of the first part of the subclavian artery.

The right brachiocephalic vein runs a short vertical course in the superior mediastinum to unite with the left brachiocephalic vein (Fig. 2.56) behind the medial end of the first right costal cartilage. It receives the right vertebral and internal thoracic veins, together with the right jugular and subclavian lymph trunks and the right lymph duct. The vessel is accompanied by the right phrenic nerve.
The left brachiocephalic vein enters the thorax and runs obliquely to the right, passing behind the manubrium. The vessel lies in front of the origin from the arch of the aorta of the left common carotid artery and the brachiocephalic trunk. At its commencement, the vein is joined by the termination of the thoracic duct and, along its course, receives the left vertebral, internal thoracic and superior intercostal veins and, usually, the inferior thyroid veins.

Superior vena cava
Formed by the union of the two brachiocephalic veins, this large vessel descends vertically (Fig. 2.56) and terminates in the right atrium of the heart. It lies to the right of the ascending aorta and to the left of the right phrenic nerve and receives the azygos vein before piercing the fibrous pericardium.
Relationships of the brachiocephalic veins to the great arteries arising from the aortic arch.

Fig. 2.56 Relationships of the brachiocephalic veins to the great arteries arising from the aortic arch.

Arch of aorta and branches
The arch of the aorta lies within the superior mediastinum, in continuity with the ascending aorta. The vessel curves backwards and to the left to reach the left side of the fourth thoracic vertebral body, where it becomes the descending aorta. The arch possesses a concavity inferiorly, left and right sides and a superior convexity.
The concavity is related to the bifurcation of the pulmonary trunk and the left main bronchus. The ligamentum arteriosum attaches the pulmonary trunk (or left pulmonary artery) to the concavity of the aortic arch and is closely related to the left recurrent laryngeal nerve (Figs 2.46 & 2.57).
The left side of the aortic arch is crossed by the left phrenic and vagus nerves (Fig. 2.57) and covered by mediastinal pleura. The phrenic nerve lies in front of the vagus and passes onto the fibrous pericardium in front of the lung root. The vagus nerve inclines backwards to pass behind the lung root, having given off the left recurrent laryngeal nerve. The left superior intercostal vein passes forwards across the arch and usually terminates in the left brachiocephalic vein (Fig. 2.57).
The right side of the arch is related, from in front backwards, to the superior vena cava, trachea, left recurrent laryngeal nerve, oesophagus and thoracic duct. These structures lie between the aorta and the right mediastinal pleura.
The convexity of the arch gives rise to the brachiocephalic trunk, left common carotid and left subclavian arteries (Fig. 2.58), which ascend into the root of the neck. The brachiocephalic trunk is the first branch of the arch of the aorta and arises behind the left brachiocephalic vein. The trunk slopes upwards and to the right across the anterior surface of the trachea, leaving the thorax to the right of the trachea to divide in the root of the neck into the right subclavian and right common carotid arteries.
The left common carotid artery arises behind the brachiocephalic trunk and ascends, in company with the left phrenic and vagus nerves, through the superior mediastinum on the left of the trachea into the root of the neck (Fig. 2.58).
The left subclavian artery is the most posterior artery arising from the aortic arch and lies immediately behind the left common carotid artery. It runs upwards and laterally, closely related to the pleura covering the apex of the left lung, entering the root of the neck behind the sternoclavicular joint.

Phrenic nerves
The right and left phrenic nerves (C3, C4 & C5) pass through the superior thoracic aperture behind the respective subclavian veins. Owing to the asymmetry of the mediastinal organs, the intrathoracic courses of the two nerves differ. The right phrenic nerve, covered by mediastinal pleura, accompanies the right brachiocephalic vein and the superior vena cava in front of the root of the right lung (Fig. 2.59).
It descends vertically across the fibrous pericardium covering the right atrium and pierces the diaphragm alongside the infe- rior vena cava.
The left phrenic nerve, also covered by mediastinal pleura, lies lateral to the left common carotid artery and crosses the left side of the aortic arch to gain the fibrous pericardium in front of the left lung root (Fig. 2.57). The nerve then descends across the pericardium as far as the apex of the heart, where it pierces the diaphragm (Fig. 2.60).
The phrenic nerves supply the muscle of the diaphragm, excluding the crura. They give sensory fibres to the fibrous and parietal serous pericardium and the mediastinal and diaphragmatic pleura, and sensory branches to the peritoneum covering the inferior surface of the diaphragm (pp 36, 205).
Oblique view showing the course of the right phrenic nerve.

Fig. 2.59 Oblique view showing the course of the right phrenic nerve.

Trachea
The trachea descends through the neck, where normally it is palpable above the jugular notch, and enters the thorax in the midline, immediately behind the upper border of the manubrium. It runs vertically through the superior mediastinum and, at the level of the aortic arch, divides into right and left main bronchi (Fig. 2.61).
The right main bronchus is wider than the left and inclines steeply downwards to enter the right lung root. The right upper lobar bronchus often arises outside the hilum of the lung. The left main bronchus runs obliquely to the left within the concavity of the arch of the aorta, passing behind the left pulmonary artery to gain the left lung root.
The thoracic part of the trachea is crossed anteriorly by the brachiocephalic trunk and the left brachiocephalic vein (Fig. 2.59). In addition, the trachea is overlapped by the anterior margins of the pleura and lungs and the thymus (or its remnants). The trachea is related on the left to the arch of the aorta and left common carotid and subclavian arteries, on the right to the superior vena cava, the termination of the azygos vein, the right vagus nerve and the mediastinal pleura, and posteriorly to the oesophagus and the left recurrent laryngeal nerve. (The right recurrent laryngeal nerve does not enter the thorax but passes around the right subclavian artery in the root of the neck; p. 331.)
The vascular supply of the trachea is from the inferior thyroid arteries and veins. The recurrent laryngeal nerves supply sensory and parasympathetic secretomotor fibres to the mucous membrane and motor fibres to the smooth muscle (trachealis).
Trachea and left and right main bronchi, exposed after removal of the anterior part of the aortic arch.

Fig. 2.61 Trachea and left and right main bronchi, exposed after removal of the anterior part of the aortic arch.

Oesophagus
The oesophagus descends through the root of the neck and traverses the superior thoracic aperture behind the trachea. In the superior mediastinum the oesophagus lies in front of the upper four thoracic vertebral bodies and behind the trachea, the left main bronchus and left recurrent laryngeal nerve. The aortic arch and the thoracic duct are on its left while the azygos vein arches forwards on its right (Fig. 2.62).
The oesophagus continues into the posterior mediastinum in front of the fifth thoracic vertebra accompanied by the right and left vagus nerves. It descends behind the fibrous pericardium and inclines to the left to cross in front of the descending aorta. On its right side, the oesophagus is covered by mediastinal pleura. On the left, once anterior to the descending aorta, it is related to pleura as far as the diaphragm. Accompanied by branches of the vagus nerves (see below), the oesophagus passes through the diaphragm at the level of the tenth thoracic vertebra.
The oesophagus is supplied by branches from the inferior thyroid arteries and from the descending thoracic aorta. Its lower part receives branches from the left gastric artery that ascends through the oesophageal opening in the diaphragm. Radicles of the left gastric vein (a tributary of the portal vein) anastomose with veins that drain venous blood from the oesophagus into the azygos system (see Portacaval anastomoses, p. 185). The upper part of the oesophagus is drained by the brachiocephalic veins. Sensory and parasympathetic motor fibres to the oesophagus are provided by the vagi and their recurrent laryngeal branches.

Fig. 2.62 Intrathoracic part of the oesophagus and accompanying vagus nerves after removal of the main bronchi and the lower part of the trachea.

Vagus (X) nerves
In the superior mediastinum, the relationships of the right and left vagi differ. The right vagus nerve (Fig. 2.62) enters the thorax behind the bifurcation of the brachiocephalic trunk and on the right of the trachea. The nerve, covered by mediastinal pleura, inclines backwards and passes behind the right lung root to gain the oesophagus. The left vagus nerve descends behind the left common carotid artery to cross the left side of the aortic arch, gives off the left recurrent laryngeal nerve and continues behind the left lung root to reach the oesophagus.
The left recurrent laryngeal nerve (Fig. 2.62) passes around the arch of the aorta adjacent to the ligamentum arteriosum and ascends in the interval between the trachea and oesophagus. In the posterior mediastinum, the right and left vagus nerves divide on the surface of the oesophagus to form a network, the oesophageal plexus. The terminal branches of the plexus (the anterior and posterior vagal trunks) enter the abdomen with the oesophagus (p. 197).

Descending thoracic aorta and branches
The descending aorta (Fig. 2.63) is continuous with the aortic arch and initially lies to the left of the fifth thoracic vertebral body. As it traverses the posterior mediastinum, it inclines forwards and to the right, gaining the midline anterior to the twelfth thoracic vertebra. On the right, the upper part of the descending aorta is related to the thoracic vertebral bodies and the oesophagus. The lower part and all of its left side are covered by mediastinal pleura. The thoracic duct and the azygos vein lie to the right of the aorta, and anteriorly it is crossed by the oesophagus sloping obliquely from the midline to the left. The descending aorta leaves the thorax in front of the twelfth thoracic vertebra and behind the median arcuate ligament of the diaphragm with the thoracic duct and azygos vein (Figs 2.64 & 4.104).
Posterior intercostal arteries from the descending aorta supply the third to the eleventh intercostal spaces on both sides. They anastomose with the anterior intercostal arteries derived from either the internal thoracic or the musculophrenic arteries. Other branches from the aorta supply the right and left bronchi and the oesophagus.

Thoracic duct
Arising from the upper part of the cisterna chyli (p. 196), the thoracic duct passes into the thorax, lying between the azygos vein and descending aorta, and with these structures (Figs 2.62 & 2.63) ascends through the posterior mediastinum to gain the superior mediastinum on the left of the oesophagus. The duct then curves forwards and to the left, crossing the apex of the left lung to enter the root of the neck where it terminates in the confluence of the left internal jugular and subclavian veins.

Azygos venous system
This system of veins drains blood from most of the posterior thoracic wall and from the bronchi, the pericardium and part of the intrathoracic oesophagus. The azygos vein enters the thorax through the aortic opening and receives posterior intercostal veins from the lower eight spaces on the right (Fig. 2.64). Veins from the second and third spaces drain into the right superior intercostal vein, which terminates in the azygos vein as it arches over the right lung root to join the superior vena cava. The venous return from the first space drains into the right brachiocephalic vein. The azygos vein also receives the hemiazygos veins.
The hemiazygos and accessory hemiazygos veins drain the lower eight posterior intercostal spaces on the left side. The lowermost four spaces usually empty into the hemiazygos vein, which crosses the midline to terminate in the azygos vein (Fig. 2.65). Veins from the next four intercostal spaces usually join to form the accessory hemiazygos vein, which also crosses the midline to end in the azygos. Sometimes, the hemiazygos and accessory hemiazygos veins drain into the azygos vein by a single vessel. The second and third spaces on the left are drained by the left superior intercostal vein (Fig. 2.57), which crosses the aortic arch to end in the left brachiocephalic vein. The first left intercostal space drains into the corresponding brachiocephalic vein.

Thoracic sympathetic trunk
The thoracic part of the sympathetic trunk (chain) runs along the lateral aspects of the thoracic vertebral bodies (Figs 2.65 & 2.66). In continuity with the cervical and abdominal parts, the thoracic sympathetic trunk consists of a series of interconnected enlargements (ganglia)  occurring  at  intervals  along  its  length. Usually, each thoracic spinal nerve is connected to its own ganglion by two branches, a white (preganglionic) and a grey (post-ganglionic) ramus communicans. Not infrequently, adjacent ganglia fuse together and, most often, the inferior cervical and first thoracic ganglia fuse to form the stellate ganglion.
Branches
Fine nerve filaments running from the sympathetic trunk contribute to the autonomic prevertebral plexuses supplying the thoracic organs, including the heart (cardiac plexuses), lungs (pulmonary plexuses) and the oesophagus (oesophageal plexus). The lower thoracic ganglia give rise to a collection of autonomic fibres that form the greater (Fig. 2.66), lesser and least splanchnic nerves, destined to supply intra abdominal structures, which are gained by piercing the crura of the diaphragm. All thoracic spinal nerves receive from the grey rami communicantes, sympathetic postganglionic fibres, which are distributed to various structures of the body wall (e.g. blood vessels, hair follicles and sweat glands) by the segmental spinal nerves.