The anterior mediastinum is posterior to the body of the sternum and anterior to the pericardial sac (see Fig. 3.57).
Its superior boundary is a transverse plane passing from the sternal angle to the intervertebral disc between vertebra TIV and TV, separating it from the superior mediastinum.
Its inferior boundary is the diaphragm.
Laterally, it is bordered by the mediastinal part of parietal pleura on either side.
The major structure in the anterior mediastinum is an inferior extension of the thymus gland (Fig. 3.58). Also present are fat, connective tissue, lymph nodes, mediastinal branches of the internal thoracic vessels, and sternopericardial ligaments, which pass from the posterior surface of the body of the sternum to the fibrous pericardium.
The middle mediastinum is centrally located in the thoracic cavity. It contains the pericardium, heart, origins of the great vessels, various nerves, and smaller vessels.
The pericardium is a fibroserous sac surrounding the heart and the roots of the great vessels. It consists of two components, the fibrous pericardium and the serous pericardium (Fig. 3.59).
The fibrous pericardium is a tough connective tissue outer layer that defines the boundaries of the middle mediastinum. The serous pericardium is thin and consists of two parts:
The parietal layer of serous pericardium lines the inner surface of the fibrous pericardium.
The visceral layer (epicardium) of serous pericardium adheres to the heart and forms its outer covering.
The parietal and visceral layers of serous pericardium are continuous at the roots of the great vessels. The narrow space created between the two layers of serous pericardium, containing a small amount of fluid, is the pericardial cavity. This potential space allows for the relatively uninhibited movement of the heart.
The fibrous pericardium is a cone-shaped bag with its base on the diaphragm and its apex continuous with the adventitia of the great vessels (Fig. 3.59). The base is attached to the central tendon of the diaphragm and to a small muscular area of the diaphragm on the left side. Anteriorly, it is attached to the posterior surface of the sternum by sternopericardial ligaments. These attachments help to retain the heart in its position in the thoracic cavity. The sac also limits cardiac distention.
The phrenic nerves, which innervate the diaphragm and originate from spinal cord levels C3 to C5, pass through the fibrous pericardium and innervate the fibrous pericardium as they travel from their point of origin to their final destination (Fig. 3.60). Their location, within the fibrous pericardium, is directly related to the embryological origin of the diaphragm and the changes that occur during the formation of the pericardial cavity. Similarly, the pericardiacophrenic vessels are also located within and supply the fibrous pericardium as they pass through the thoracic cavity.
The parietal layer of serous pericardium is continuous with the visceral layer of serous pericardium around the roots of the great vessels. These reflections of serous pericardium (Fig. 3.61) occur in two locations: one superiorly, surrounding the arteries—the aorta and the pulmonary trunk; the second more posteriorly, surrounding the veins—the superior and inferior vena cava and the pulmonary veins.
The zone of reflection surrounding the veins is J-shaped, and the cul-de-sac formed within the J, posterior to the left atrium, is the oblique pericardial sinus.
A passage between the two sites of reflected serous pericardium is the transverse pericardial sinus. This sinus lies posterior to the ascending aorta and the pulmonary trunk, anterior to the superior vena cava, and superior to the left atrium.