The pericardium is composed of fibrous tissue lined with a serous membrane.

When affected by inflammation the amount of fluid contained in it becomes increased and it becomes distended and may interfere with the functions of the heart and adjacent structures.

If the effusion is serous it is sometimes drawn off by puncture; if it is purulent drainage is instituted.

The pericardium in shape is somewhat conical. Its base rests on the central tendon of the diaphragm and its apex envelops the great vessels, as they emerge from the base of the heart, for a distance of 4 to 5 cm. The attachment to the diaphragm is most firm at the opening of the inferior vena cava. As the fibrous layer of the pericardium proceeds upward it becomes lost in the fibrous tissue (sheath) covering the great vessels. This is continuous above with the deep cervical fascia, especially with its pretracheal layer. Anteriorly the pericardium is attached above and below to the sternum by the so-called sternopericardiac ligaments (Fig. 218).

In front of it above are the remains of the thymus gland and triangularis sterni muscle of the left side from the third to the seventh costal cartilages. The internal mammary arteries, running down behind the costal cartilages about a centimetre from the edge of the sternum above and somewhat more below, are separated from the pericardium by the edges of the lungs and pleurae, these latter reaching nearly or quite to the median line. The triangularis sterni muscle also lies beneath the artery and farther from the surface. As the left pleura slopes more rapidly toward the side than does the right there is a small portion of the pericardium uncovered by the pleura at about the sixth intercostal space close to the sternum. The incisura of the left lung leaves a space where the pericardium is separated from the chest-walls only by the pleura.

On each side the pleura and pericardium are in contact, with the phrenic nerves between them. Posteriorly the pericardium lies on the bronchi, the oesophagus, and the thoracic aorta.

Owing to the fibrous nature of the pericardium it will not expand suddenly. While only about a pint of liquid can be injected into the normal pericardia) cavity after death, if a chronic effusion exists in a living person as much as three pints may be present.

Sudden effusion occurring in the living patient will cause obstruction of the circulation at the base of the heart; it may by pressure on the bronchi at the bifurcation produce suffocative symptoms and by pressure on the oesophagus difficulty in swallowing. The lungs are displaced laterally, and the stomach and liver downward. The largest effusions are slow in their formation.

Pressure on the left recurrent laryngeal nerve as it winds around the aorta sometimes produces alteration or loss of the voice.

In children, according to Osier, the praecordia bulges and the anterolateral region of the left chest becomes enlarged as does also the area of the cardiac dulness.