Part three. Thoracic cavity

The cavity of the thorax is completely filled laterally by the lungs, each lying in its pleural cavity. The space between the pleural cavities occupying the centre of the thoracic cavity is the mediastinum. It contains the heart and great blood vessels, oesophagus, trachea and its bifurcation, thymus, thoracic duct, lymph nodes, phrenic and vagus nerves. The loose connective tissue between these structures connects freely with that of the neck. Mediastinitis may complicate infections in the neck.

Divisions of the mediastinum

There is a plane of division to which the whole topography of the mediastinum can be related, namely a plane passing horizontally through the sternal angle (of Louis), i.e. the manubriosternal joint (Fig. 4.9). From the second costal cartilages, this plane passes backwards to the lower border of T4 vertebra. Above, between it and the thoracic inlet, lies the superior mediastinum. Below the plane, the inferior mediastinum is divided into three compartments by the fibrous pericardium: a part in front, the anterior mediastinum; a part behind, the posterior mediastinum; and the middle mediastinum in between containing the pericardium and heart together with the adjoining parts of the great vessels and the lung roots. The anterior and posterior mediastina are in direct continuity with the superior mediastinum; their separation from it is purely descriptive, not anatomical. The plane passes through the bifurcation of the trachea, the concavity of the arch of the aorta, and just above the bifurcation of the pulmonary trunk. On the plane the azygos vein enters the superior vena cava, and the thoracic duct reaches the left side of the oesophagus in its passage upwards from the abdomen. Also lying in the plane are the ligamentum arteriosum, with the left recurrent laryngeal nerve recurving below it, tracheobronchial lymph nodes, and the superficial and deep parts of the cardiac plexus.

Figure 4.9 Divisions of the mediastinum, showing the continuity with the tissue spaces of the neck. The superior mediastinum is above the interrupted line passing from the sternal angle to the lower border of T4 vertebra. The anterior mediastinum is continuous through the superior mediastinum with the pretracheal space of the neck, up to the hyoid bone. The posterior mediastinum is continuous with the retropharyngeal and paratracheal space of the neck, up to the base of the skull. The pharynx and oesophagus are not depicted.

The prevertebral and pretracheal fasciae extend from the neck into the superior mediastinum. The former fuses with the anterior longitudinal ligament over T4 vertebra; the latter blends with the pericardium over the front upper part of the heart. Thus, neck infection in front of the pretracheal fascia is directed into the anterior mediastinum, while infection behind the pre-vertebral fascia is

imprisoned in the superior mediastinum in front of the vertebral bodies (Fig. 4.9). From elsewhere in the neck infection may extend through the superior into the posterior mediastinum.

Figure 4.11 CT scan above the level of the aortic arch. Viewed from below, the oesophagus with a rather small round lumen is seen in front of the vertebral column, and the round translucency in front of the oesophagus is the trachea. The four opacities adjacent to the trachea are, from the right side of the body to the left, the superior vena cava, brachiocephalic trunk, left common carotid artery and left subclavian artery.

Figure 4.12 Mediastinum: right aspect.

The veins entering the superior mediastinum are the right and left brachiocephalic veins, each formed by the confluence of the internal jugular with the subclavian vein. They lie in front of the arteries and are asymmetrical. The right brachiocephalic vein passes vertically downwards; the left vein runs across the superior mediastinum, above the arch of the aorta, to join the right (Fig. 4.10). The confluence of the brachiocephalic veins produces the superior vena cava, which passes vertically downwards behind the right edge of the sternum, anterior to the right pulmonary hilum (Fig. 4.12). The right phrenic nerve descends in contact with the lateral aspect of the right brachiocephalic vein and superior vena cava.

Great vessels

Arch of the aorta

Emerging from the pericardium the ascending aorta approaches the manubrium and then at the level of the manubriosternal joint becomes the arch, which passes backwards over the left bronchus to reach the body of T4 vertebra just to the left of the midline. From its upper convexity, which reaches as high as the midpoint of the manubrium, arise the three great arteries for the head and upper limbs: the brachiocephalic trunk, and the left common carotid and left subclavian arteries (Figs 4.10 and 4.13). The arch is crossed on its left side by the phrenic and vagus nerves as they pass downwards in front of and behind the lung root respectively. Between them lie the sympathetic and vagus branches to the superficial part of the cardiac plexus. The left superior intercostal vein passes forwards across the arch superficial to the vagus, deep to the phrenic, to empty into the left brachiocephalic vein. The left recurrent laryngeal nerve hooks around the ligamentum arteriosum to pass upwards on the right side of the arch of the aorta, in the groove between the trachea and oesophagus. The pulmonary trunk bifurcates into right and left pulmonary arteries in the concavity of the arch. On the right side of the arch lie the trachea and oesophagus.

Figure 4.13 Mediastinum: left aspect.

The adventitial layer of the arch contains baroreceptors (like the carotid sinus in the wall of the internal carotid artery, see p. 343) innervated by vagal nerve fibres, which are concerned with the reflex control of the heart rate. Under the arch in the region of the ligamentum arteriosum there are some very small masses of tissue, the aortic bodies (also supplied by vagal fibres), which like the carotid bodies (see p. 343) are chemoreceptors concerned with respiratory reflexes.

The brachiocephalic trunk (innominate artery) arises in or a little to the left of the midline of the body. It slopes upwards across the trachea to the back of the right sternoclavicular joint, where it divides into the right common carotid and right subclavian arteries. It has no branches apart from the rare thyroidea ima artery, which may arise from it or directly from the arch of the aorta. The termination of the left brachiocephalic vein lies in front of the artery (Fig. 4.10).

T he left common carotid artery arises just behind the brachiocephalic trunk from the upper convexity of the aortic arch. It passes straight up alongside the trachea into the neck (Fig. 4.13). It has no branches in the mediastinum.

The left subclavian artery arises just behind the left common carotid; the two run upwards together. The subclavian artery arches to the left over the pleura and the apex of the lung, which it deeply grooves. It moves away from the left common carotid at a point directly behind the left sternoclavicular joint. It has no branches in the mediastinum.

Ligamentum arteriosum

This is the fibrous remnant of the ductus arteriosus of the fetus, a channel that short-circuited the lungs. It passes from the commencement of the left pulmonary artery to the concavity of the aortic arch (Fig. 4.16), beyond the point where the left subclavian artery branches off. It lies almost horizontally. The left recurrent laryngeal nerve hooks around it. The superficial part of the cardiac plexus lies anterior to it, and the deep part is on its right, between the aortic arch and tracheal bifurcation.

Surgical approach. When the ductus persists after birth (patent ductus arteriosus) and requires surgical interruption the pleura over the aortic arch is incised behind the vagus nerve and upwards towards the origin of the left subclavian artery. The pleural flap is reflected forwards with the vagus and its left recurrent laryngeal branch to give sufficient access to the ductus. Video-assisted thoracoscopy is utilised for patent ductus arteriosus closure. The lumen of the ductus may also be obliterated by an occlusive device inserted by interventional radiology techniques.

Brachiocephalic veins

The brachiocephalic (innominate) veins are formed behind the sternoclavicular joints by confluence of the internal jugular and subclavian veins. In the neck the internal jugular vein lies lateral to the common carotid artery, in front of the upper part of scalenus anterior. The subclavian vein lies lateral to and then in front of the lower part of the muscle. Medial to scalenus anterior, these veins have joined to form the brachiocephalic vein, which lies in front of the first part of the subclavian artery. This part of each brachiocephalic vein thus receives tributaries corresponding to the branches of the first part of the subclavian artery (vertebral, inferior thyroid, internal thoracic and, on the left side only, superior intercostal).

T h e right brachiocephalic vein commences behind the right sternoclavicular joint and runs downwards. At its commencement it receives the right jugular, subclavian and bronchomediastinal lymph trunks separately or their confluent channel, the right lymphatic duct.

The left brachiocephalic vein passes to the right with a downward inclination, across the superior mediastinum, above the arch of the aorta, behind the thymus and the upper half of the manubrium. In the infant the left brachiocephalic vein projects slightly above the jugular notch, and may do so in the

adult if the vein is distended, especially if the head and neck are thrown back. The vein is then vulnerable to suprasternal incisions (e.g. for tracheotomy). The commencement of the vein receives the thoracic duct, which often divides into two or three branches that join the vein separately. In addition to the vertebral and internal thoracic veins the left brachiocephalic vein receives most of the inferior thyroid veins, the left superior intercostal vein, and a large thymic vein (Figs 4.10 and 4.13).

The pretracheal fascia (see p. 331) passes down behind the vein and directs a retrosternal goitre into the space between the vein and the brachiocephalic trunk and trachea.

Superior vena cava

This vessel commences at the lower border of the first right costal cartilage by confluence of the two brachiocephalic veins (Figs 4.10 and 4.12). It passes vertically downwards behind the right border of the sternum and, piercing the pericardium at the level of the second costal cartilage, enters the upper border of the right atrium at the lower border of the third right costal cartilage. Behind the sternal angle it receives the azygos vein, which has arched forwards over the root of the right lung. There are no valves in the superior vena cava, the brachiocephalic veins or the azygos system of veins.

Cardiac plexus

The cardiac plexus consists of sympathetic, parasympathetic and afferent fibres and small ganglia. It is divided into superficial and deep parts, but functionally they are one. Their branches enter the pericardium to accompany the coronary arteries (vasomotor) and to reach the myocardium, in particular the SA and AV nodes (cardioinhibitor and cardioaccelerator).

The superficial part of the cardiac plexus lies in front of the ligamentum arteriosum. The deep part of the cardiac plexus is larger and lies to the right of the ligamentum arteriosum, in front of the bifurcation of the trachea and behind the aortic arch.

The cardiac plexus receives sympathetic fibres from the three cervical and the upper four or five thoracic sympathetic ganglia of both sides, and parasympathetic fibres from both vagi in their cervical course and both recurrent laryngeal nerves. The sympathetic fibres accelerate the heart and dilate the coronary arteries; the parasympathetic fibres slow the heart and constrict the coronary arteries.

The vagi carry afferent fibres concerned with cardiovascular reflexes. Pain fibres run with sympathetic nerves, reaching any of the cervical and upper thoracic sympathetic ganglia. The pain fibres pursue the usual pathway to the central nervous system, passing through the sympathetic ganglia to the spinal nerves via white rami communicantes (see Fig. 1.13C, p. 19). The connection with cervical and thoracic spinal nerves presumably explains the referral of cardiac pain to the arm, chest or neck.

Trachea

The trachea is the continuation of the larynx and commences in the neck below the cricoid cartilage at the level of C6 vertebra, 5 cm above the jugular notch. Entering the thoracic inlet in the midline it passes downwards and backwards behind the manubrium to bifurcate into the two principal or main bronchi a little to the right of the midline, level with the upper border of T5 vertebra (Fig. 4.14). The trachea is about 10 cm long and 2 cm in diameter. In the first year of life the tracheal diameter is only 3 mm and in childhood it is about equal in millimetres to the age in years. In full inspiration the

The pulmonary trunk branches into the right and left pulmonary arteries to the left of the tracheal bifurcation, in front of the left bronchus, and the right pulmonary artery crosses the midline (in front of the oesophagus) just below the tracheal bifurcation (Fig. 4.29).

Blood supply

Branches from the inferior thyroid and bronchial arteries form anastomotic networks in the tracheal wall. Veins drain to the inferior thyroid vein.

Lymph drainage

Lymphatic channels pass to preand paratracheal nodes and to inferior deep cervical nodes.

Nerve supply

The mucous membrane is supplied by afferent (including pain) fibres from the vagi and recurrent laryngeal nerves. Sympathetic fibres from upper ganglia of the sympathetic trunks supply the smooth muscle and blood vessels.

Structure

The patency of the trachea as an airway, its essential function, is maintained by 15–20 horseshoeshaped hyaline cartilages. The gaps in the rings are at the back, where there is smooth muscle, mostly transverse (the trachealis muscle). There is a high content of elastic fibres in the submucosa to facilitate the necessary elastic recoil during respiration. The mucous membrane is of typical respiratory type, with pseudostratified columnar ciliated epithelium and goblet cells, mucous glands and scattered lymphoid nodules.

Function

The cartilaginous rings keep the airway open, mucus traps particles, cilia beat upwards to clear debris, and glandular secretion helps to humidify the passing air. During swallowing the trachea is stretched as the larynx moves upwards (the bifurcation does not move) and elasticity restores the normal position. The trachealis muscle controls the diameter of the tube. During coughing there is a 30% increase in transverse diameter produced by compressed air in the trachea while the vocal cords are shut, but the trachea narrows to 10% less than the resting diameter at the instant the cords open. Like the choke barrel of a shotgun this greatly increases the explosive force of the blast of compressed air.

Phrenic and vagus nerves

Phrenic nerve

Arising principally from C4 in the neck, the nerve passes down over the anterior scalene muscle across the dome of the pleura behind the subclavian vein. It crosses anterior to the vagus and runs through the mediastinum in front of the lung root. Each nerve is in contact laterally with the mediastinal pleura throughout the whole of its course.

The right phrenic nerve is related medially with venous structures throughout its thoracic course (Fig. 4.12). The right brachiocephalic vein, the superior vena cava, the pericardium over the right atrium, and the inferior vena cava, lie to its medial side. It reaches the undersurface of the diaphragm

by passing through the central tendon alongside the inferior vena cava, piercing the tendon fibres that fuse with the caval wall.

The left phrenic nerve is related medially to arterial structures throughout its thoracic course (Fig. 4.13). It has the left common carotid and left subclavian arteries that arise from the arch of the aorta to its medial side. It crosses the arch lateral to the superior intercostal vein and in front of the vagus nerve, and then runs laterally down the pericardium over the left ventricle towards the apex of the heart. It reaches the undersurface of the diaphragm by piercing the muscular part just to the left of the pericardium.

About two-thirds of the phrenic nerve fibres are motor to the diaphragm. The rest are sensory to the diaphragm (except for the most peripheral parts which receive intercostal afferent fibres), and to the mediastinal pleura, the fibrous pericardium, the parietal layer of serous pericardium, and the central parts of the diaphragmatic pleura and peritoneum. Pain referred from the diaphragmatic peritoneum is classically felt in the shoulder tip (C4), but pain from thoracic surfaces supplied by the phrenic nerve (pleura, pericardium) is usually only vaguely located there.

Vagus nerve

In their descent through the thorax, the right vagus (Fig. 4.12) is in contact with the trachea, while the left vagus (Fig. 4.13) is held away from that structure by great arteries that spring from the arch of the aorta. The left nerve crosses the arch medial to the left superior intercostal vein, and the right nerve lies on the trachea medial to the arch of the azygos vein. Each vagus passes down behind the lung root, dividing into branches which contribute to the pulmonary plexuses and pass onwards to form a plexus around the oesophagus. On the arch of the aorta the left vagus nerve flattens out and gives off i ts recurrent laryngeal branch (Figs 4.10 and 4.13). This nerve hooks around the ligamentum arteriosum, and, passing up on the right side of the aortic arch, ascends in the groove between trachea and oesophagus. The right recurrent laryngeal nerve is given off at the root of the neck and hooks around the right subclavian artery (Fig. 4.10). Both recurrent laryngeal nerves give branches to the deep part of the cardiac plexus. Both recurrent laryngeal nerves supply the whole trachea and the adjacent oesophagus (i.e. above the lung roots), and proceed to supply the larynx (see p. 396).

The oesophagus lies against the vertebrae at the back of the superior mediastinum. The thoracic duct lies to its left. Both structures pass through the posterior mediastinum; they are described on pages 208 and 210.