The pericardium, properly speaking, is a specimen of what Bichat calls a fibro-serous membrane, consisting of two layers of membrane, an external or fibrous, and an internal or serous layer. It is the immediate envelope of the heart, and of certain portions of the great vessels entering into and issuing from it. Its form is somewhat conoid; the apex corresponds to the large vessels in immediate connection with the heart, in which situation the fibrous layer of the sac may be seen extended over them, and identified with their external tunic: the base may be seen resting on the cordiform tendon of the diaphragm, to which it adheres so firmly in the adult as to be with great difficulty separated from it; it also rests on a small triangular portion of the fleshy fibres of the diaphragm, to the left of the tendon, from which it may very easily be separated. In the foetus the pericardium is but loosely connected with the tendon and fleshy fibres of the diaphragm.

The anterior surface of the pericardium is covered by the thymus gland in the foetus, and in the adult by a considerable quantity of loose areolar tissue, which occupies the situation of the thymus gland; by the internal and anterior portion of each lung and pleura, and by the sternum: and inclining towards the left side infe-riorly, we find lying in front of it also the cartilages of the fourth, fifth, sixth, and seventh ribs. The sides of the pericardium are over-lapped by the lungs, and are covered by the pleurae, the phrenic nerve being interposed at the left, and thrown more anteriorly, so as to bend over the pericardium at a point corresponding to the apex of the heart. Its posterior surface lies in front of the posterior mediastinum and the parts contained within this region, more particularly the oesophagus and descending aorta. An incision may now be made through the anterior part of this envelope, when its internal or serous layer will be exposed: this consists of two portions,—the one lining the inner surface of the fibrous layer, and the other, with which the former is perfectly continuous, surrounding the heart. The continuity of these two portions of the serous membrane may be demonstrated, in the first place, by tracing that lining the inner surface of the fibrous layer from off that structure, to form a cylindrical sheath which encloses both the aorta and pulmonary artery; and secondly, by following the course which that membrane takes in forming partial investments for the two venae cavae and the four pulmonary veins. These two portions of the serous layer, viz., that lining the fibrous layer of the pericardium, and that lining the exterior of the heart itself, are perfectly continuous with each other, thus constituting a completely shut sac, so that the vessels going to, or issuing from, the heart, do not perforate the serous membrane, but receive coverings more or less perfect from it.

Nine openings have been enumerated in the fibrous layer of the pericardium, viz., one for the aorta, two for the right and left branches of the pulmonary artery, four for the four pulmonary veins, and two for the superior and inferior venae cavae. In the foetus there is another for the ductus arteriosus. Strictly speaking, these are not openings in the fibrous layer of the pericardium, for this structure becomes incorporated with the external tunic of the vessels where they come in contact with it.

When the pericardium has been opened, the following parts will be exposed:—the anterior superior surface of the heart, the two venae cavae, the aorta, the pulmonary artery, the right auricular appendix and a portion of the auricle, and the tip of the left auricular appendix. The left auricle is concealed chiefly by the aorta and pulmonary artery.