The superior mediastinum is cranially delimited by a plane passing on the superior margin of the first rib and the superior margin of the first dorsal vertebra, while caudally it is delimited by a plane between the inferior margin of the sternal manubrium and the fourth thoracic vertebra. Anteriorly, it is delimited by the sternal manubrium, posteriorly by the first fourth dorsal vertebrae and laterally by the parietal layer of the mediastinal pleura that covers the apical part of the lungs. The superior mediastinum is therefore a transition area between the cervical and the thoracic regions and includes the origin of the sternohyoid and sternothyroid muscles, the distal part of the long neck muscle, the aortic arch, the innominate artery (or brachiocephalic artery), left common carotid and subclavian arteries, left and right supreme intercostal veins, the superior vena cava, left and right brachiocephalic veins and part of the azygos and hemiazygos veins. We also find some nerve structures, the proximal portion of the vagus nerve, the left recurrent laryngeal nerve and the superior portion of the phrenic and cardiac nerves. The last structures of the superior mediastinum are the trachea, the proximal portion of the oesophagus and the superior part of the thymus and lymph nodes.

The anterior mediastinum is delimited by the sternum, anteriorly, and, posteriorly, by the anterior surface of the parietal layer of the pericardium and the big mediastinal vessels; laterally, it is delimited by the anterolateral portion of the mediastinal parietal pleurae. It contains lymph vessels and lymph nodes and the internal mammary veins and arteries. In children and teenagers, the thymus occupies most of the anterosuperior part of the mediastinum and a small portion of the anteroinferior one; such an organ has two lobes and its size and activity depend on the age: during puberty it reaches its maximum size and progressively reduces; however, also in adults there is a remaining portion of active parenchyma.

The middle mediastinum is delimited, anteriorly, by the anterior parietal pericardial layer and, posteriorly, by the posterior parietal layer. It includes the heart, pericardium, heart vessels and nerves, common arterial trunk, main pulmonary arteries, bronchial arteries and veins, distal portion of the pulmonary veins, ascending aorta and inferior tract of the aortic arch, superior vena cava and part of the inferior vena cava and azygos veins. Other structures of the middle mediastinum are the carina of the trachea, main bronchi, lymph nodes and lymph vessels and superior phrenic arteries and phrenic nerves.

The posterior mediastinum is delimited by the posterosuperior mediastinal pleura. In the posterior mediastinum we find the descending aorta; the thoracic portion of the oesophagus; posterior intercostal arteries and veins; middle oesophageal arteries and veins; and azygos, hemiazygos and accessory hemiazygos veins. The left and right vagus nerves are, respectively, located in the front and behind the oesophagus; the ganglia of the thoracic sympathetic system are other nerve structures.

The thoracic duct and some other lymph nodes are the most important lymphatic elements. In physiological conditions the lymph nodes of the mediastinum have a diameter between few and of 20 mm (or less); they can increase for infectious or oncologic diseases. Table 6.1 shows the anatomical classification of the mediastinal lymph nodes currently used by the IASLC (International Association for the Study of Lung Cancer).

In the following paragraph, you will find a description of the most important vascular structures of the mediastinum.

The heart, central organ of the circulatory system, is located in the mediastinum, between the lungs (Fig. 6.3). It looks like a conic trunk, flattened on the anterior-posterior line, with the base upward and the apex downward on the left. The major axis is oblique from the back to the front, from top to bottom and from right to left; the heart, therefore, is not in the centre of the symmetrical plane; 2/3 of its volume, in fact, peer out from the left. Some anatomical variations are considered normal, such as mesocardia, when the cardiac axis is vertical, and dextrocardia, when the cardiac apex is on the right; this last condition is often associated with the situs viscerum inversus.

The heart volume may vary and depends on sex and age; it is proportional to the size of the person and related to the weight and the body surface, rather than the height. In adults the heart weighs 280–340 g in men and 230–280 g in women.

It is protected by the sternum, anteriorly, and the costal cartilages (from the third to the sixth one), ensuring a safe protection from the outside. The posteroinferior face leans against the diaphragmatic vault, while above it continues into the great vessels of the cardiac pedicle, consisting of, from right to left: superior vena cava, ascending aorta and pulmonary arterial trunk.

The heart is contained in a fibroserous sac, the pericardium, that attaches it to the diaphragm and isolates it from the surrounding organs. The pericardium consists of two layers, the parietal and visceral layers; normally, the space between them is a merely virtual space and therefore not visible with the available imaging techniques; it is rather visible in case of interposition of a third element (such as pericardial effusion, either haematic or not) between the layers, which may occupy the virtual lumen. In view of the above, it is important to be reminded that the pericardium also covers the initial part of the great vessels of the cardiac pedicle and the pulmonary veins: it is therefore easy to understand how a traumatic event, fissurations or breakage of such structures, may lead to an acute intrapericardial haematic effusion with consequent cardiac tamponade.

Through the pericardium the heart enters into contact with the right lung, posteriorly, and with the organs adjacent to the posterior mediastinum, in particular with the oesophagus. This organ, along with the azygos vein, the thoracic duct, the vagus nerves and the descending aorta, separates the heart from the thoracic vertebrae, from the fifth to the eighth one, which are also named, for this reason, cardiac vertebrae.

On the external surface of the heart, we find the anterior face, or sternocostal face, the posteroinferior or diaphragmatic face, the base (to which the vessels of the pulmonary and systemic circulation are connected), the apex, which is oriented downward on the left, and the two margins, the right one, acute, and the left one, obtuse (Figs. 6.4 and 6.5).

The diaphragmatic face includes the atrioventricular groove (or coronary sulcus), which is transverse and separates the posterior-superior (or atrial) portion, from the anteroinferior, or ventricular, one; we also find the interatrial sulcus that, along with the posterior interventricular sulcus, delimits the external border of the four cardiac cavities. On the sternocostal and diaphragmatic faces, there are two longitudinal sulci, the anterior and the posterior one (also named interventricular sulci), going from the coronary sulcus to the apex of the heart. The atrioventricular groove and the interventricular sulcus are very important because they contain the coronary arteries.

The heart is a hollow organ, divided into two independent parts, the left and right parts, each of them consisting of two cavities: the superior cubic cavity, the atrium, and the inferior cavity, pyramidal or conical, the ventricle (Fig. 6.6).

On the plane passing between the two atria and the ventricles, there is a fibrous structure, the so-called cardiac skeleton, namely, the insertion point of the bundles of the atrial and ventricular myocardium; this is the place where heart valves are placed, in order to ensure the structural continuity of the ventricles and the big arteries. There are four fibrous rings, on the border of the venous and arterial ostia, two fibrous trigones, interposed between the fibrous rings, the tendon of the conus arteriosus and the interventricular membranous septum.

The right atrium is anterior to the left one. It looks roughly a cube, with six walls. The anterosuperior wall coincides with the sternocostal face; the posterosuperior one with the heart base, where we can find the outlets of the venae cavae; the posteroinferior wall with the small diaphragmatic face receives the end of the coronary sinus; on the anteroinferior one we find right atrioventricular orifice; the medial wall consists of the interatrial septum, where we find the fossa ovalis, the remnant of a thin fibrous sheet that covered the foramen ovale; and lastly, the lateral wall has a hole of connection with the right auricula, and it is characterised by fleshy reliefs due to the presence of the musculi pectinati (they resemble the teeth of the comb, hence the name).

The right ventricle has a capacity of approximately 200 ml, and it looks like a pyramid with a triangular base and three walls: the anterior wall, corresponding to most of the sternocostal face of the heart; the posteroinferior wall, consisting of the right, less-extended portion of the diaphragmatic face of the heart; the medial or septal wall, consisting of the interventricular septum; the base has two orifices or ostia, a venous atrioventricular one, located on the right, in the back; and an arterious or pulmonary one, in the front on the left side, through which the ventricle communicates, respectively, with the right atrium and the pulmonary trunk.

The right atrioventricular orifice (or tricuspid orifice, or ostium atrioventriculare) is supplied by a valve (tricuspid valve) consisting of three leaflets called cusps. For their positions, the cusps are divided into medial, septal, anterior and posterior cusps; they consist of fibrous tissue, covered with endocardium. The free margins of the valvular leaflets, irregularly dentate, allow the insertion of the chordae tendineae (tendinous chords), thin tendons which are detached from the apex of the conical muscles, protruding into the cavity, called papillary muscles. The pulmonary orifice has three semilunar valves (membranous folds) that can be distinguished into anterior, right and left valves.

The right ventricular wall is thicker than the atrial wall but almost 1/3 thinner of the left ventricular wall.

The left atrium is less voluminous than the contralateral one, but its walls are thicker. We can distinguish six walls: anterosuperior wall, corresponding to the sternocostal face, in connection with the descending part of the aorta and the pulmonary trunk; posterosuperior wall, in relation with the oesophagus through the interposition of the pericardium; posteroinferior wall, less extended, corresponding to the diaphragmatic face; anteroinferior wall, with the atrioventricular orifice; medial wall, in correspondence of the interatrial septum; and the lateral one, providing access to the auricle.

The four pulmonary veins, two on each side, open up in the posterosuperior wall of the atrium, through four orifices, devoid of valves; however, they also have a high level of individual variability (Fig. 6.7).