The pericardium is a cone-shaped, fibro-serous sac which surrounds the heart and contains a small amount of fluid [liquor pericardii]. Its apex is above at the root of the great vessels, and its base below, adherent to the diaphragm. Its connection with the diaphragm is in part to the central tendon and in part to the muscle, especially on the left side. It consists of an outer fibrous layer and an inner serous layer. The virtual space between the serous pericardium and the epicardium is commonly called the pericardial cavity.
The fibrous layer is strong and inelastic, made of interlacing fibers. Its connection with the central tendon of the diaphragm is intimate, particularly in the region of the caval opening, but elsewhere it is attached loosely by means of areolar tissue. Above, it is lost on the sheaths of the great vessels, all of which receive distinct investments, with the single exception of the inferior vena cava, which pierces it from below. The aorta, superior vena cava, the pulmonary artery, and the four pulmonary veins, are all ensheathed in this manner. The pericardium is connected above with the deep cervical fascia. Two variable bands of fibrous tissue, the sterno-pericardial ligaments [ligg. sterno-pericardiaca], connect the front of the pericardium, above and below, with the posterior surface of the sternum.
The serous layer is smooth and glistening and consists of connective tissue, rich in elastic fibers, covered by endothelium. It lines the interior of the fibrous layer and is continuous with the epicardium or serous covering of the heart. The reflection of the serous layer from the heart to the fibrous layer of the pericardium occurs at both the arterial and venous attachments of the heart. At the arterial attachment, a simple tube of epicardium is reflected along the pulmonary artery and aorta. At the venous attachment, the serous layer is reflected from the front of the pulmonary veins on the left, and from the front of these and from the roots of the venae cavae on the right. This reflection is separated above from that around the aorta and pulmonary artery. Around the lower margin of the left lower pulmonary vein and the root of the inferior vena cava, this reflection is continuous with an arched reflection from the back of the atria. The latter reflection forms a pocket posterior to the atria which is sometimes called the oblique sinus of the pericardium.
Between the reflections of the epicardium at the arterial and venous attachments of the heart there is a dorsal communication between the right and left sides of the pericardial cavity. This is the transverse sinus of the pericardium [s. transversus pericardii]; it passes behind the aorta and pulmonary artery and in front of the superior cava and left atrium.
During early embryonic life, the sinus transversus is closed by the dorsal mesocardium. The primitive ventral mesocardium, which divides the right and left sides of the pericardial cavity ventrally, is lost very early.
The ligament of the left superior cava [lig. venae cavae sinistrae] is a doubling of the serous layer which passes between the left pulmonary artery above and the left superior pulmonary vein below. It contains, besides some fatty and areolar tissue, the shrunken remains of the left superior vena cava. It is usually connected above with the left superior intercostal vein by means of a small tributary of the latter. Passing from its lower end to the left end of the coronary sinus is the small vena obliqua atrii sinistri (oblique vein of Marshall). The root of the left superior intercostal (and the adjacent part of the left innominate) vein; the vein passing from the super or intercostal to the lig. venae cavae sinistrae; the oblique vein of the left atrium, and the coronary sinus all represent parts of the embryonic left vena cava superior.
Relations. In front of the pericardium are found the thymus gland or its remains, areolar tissue, the sterno-pericardial ligaments, the left transversus thoracic muscle, the internal mammary vessels, the anterior margins of the pleural sac and' lungs, and the sternum. Later- ally, it is overlapped by the lungs with their pleural sacs, and it is in contact with the phrenic nerves and their accompanying vessels. Posteriorly, it is in relation with the esophagus and vagus nerves, the descending aorta, the thoracic duct and vena azygos, and the roots of the lungs. Below it is separated by the diaphragm from the stomach and the left lobe of the fiver.
Vessels. The arteries of the pericardium are derived from the pericardiac, esophageal, and bronchial branches of the thoracic aorta and from the internal mammary and phrenic arteries.