The fibrous pericardium is a sac of dense connective tissue surrounding the heart. In addition to the heart, it encloses the roots of the great arteries and veins and is covered on its inner surface by serous pericardium (see below). The broad base of the fibrous pericardium is attached to the central tendon of the diaphragm (Fig. 2.29) and is pierced by the inferior vena cava.
Fig. 2.29 The fibrous pericardium and phrenic nerves revealed after removal of the lungs.
Superiorly, the sac fuses with the adventitial layers of the aorta, pulmonary trunk and superior vena cava. On each side, the posterior part of the sac blends with the walls of the pulmonary veins.
The anterior aspect of the fibrous pericardium is related to the anterior parts of the two lungs and the anterior reflections of the pleura. Between the pleural reflections, the pericardium lies close to the body of the sternum and to the medial ends of the adjacent fourth and fifth left costal cartilages and associated intercostal structures.
During infancy and childhood, the thymus (most of which lies in the superior mediastinum) is related to the anterior surface of the pericardium, but after puberty, the thymus regresses and is gradually replaced by fat.
Laterally, the pericardium is covered by mediastinal pleura and is crossed by the right and left phrenic nerves as they descend to the diaphragm. These nerves supply sensory fibres to the fibrous peri- cardium, the parietal serous pericardium and the mediastinal pleura. Most of the blood supply to the fibrous pericardium is provided by the internal thoracic arteries and veins via pericardiacophrenic vessels that accompany the phrenic nerves.
Behind the fibrous pericardium lie the oesophagus, the descending thoracic aorta and the thoracic duct (pp 62, 63).
Fig. 2.30 The fibrous pericardium has been opened to expose the visceral pericardium covering the anterior surface of the heart.
Deep to the fibrous pericardium lies the serous pericardium, consisting of parietal and visceral layers. Between the two layers is the pericardial cavity, a narrow space containing a thin film of serous fluid. The parietal layer lines the inner surface of the fibrous pericardium, to which it is firmly attached. The visceral layer covers the outer surface of the heart and the roots of the great vessels (Fig. 2.30).
These two layers slide freely against each other and are in continuity where the great vessels pierce the fibrous pericardium. The reflections between the parietal and visceral layers form two sleeves. One sleeve surrounds the ascending aorta and pulmonary trunk; the second is more extensive and surrounds the superior and inferior venae cavae and pulmonary veins. The two pericardial sleeves lie adjacent to each other and the narrow intervening channel is called the transverse pericardial sinus (Fig. 2.40). A second sinus lies behind the left atrium of the heart. This is the oblique pericardial sinus, which is limited superiorly by the pericardial reflection around the pulmonary veins and superior vena cava (Fig. 2.35). An accumulation of fluid (e.g. blood) within the pericardial cavity may compromise venous return to the heart and therefore reduce cardiac output (cardiac tamponade).