THE •PERICARDIUM.

The serOU8 layer invests the heart, and is then reflected on the inner surface of the pericardium. It consists, therefore, of a visceral and parietal portion. The former invests the surface of the heart, and the commencement of the great vessels,
. to the extent of an inch and a half from their origin; from these it is reflected upon the inner surface of the fibrous layer, lining, below, the upper surface of the central tendon of the Diaphragm. The serous membrane encloses the aorta and pulmonary artery in a single tube, so that a passage, termed the transverse sinus of the pericardium, exists between these vessels in front and the auricles behind.

The membrane only partially covers the superior vena cava and the four pulmonary veins, and scarcely covers the inferior cava, as this vessel enters the heart almost directly after it has passed through the Diaphragm. Its inner surface is smooth and glistening, and secretes a serous fluid, which serves to facilitate the movements
of the heart.
Arteries of the Pericardium.-These are derived from the internal mammary
and its musculo-phrenic branch, and from the descending thoracic aorta.
Nerves of the Perieardium.-These are branches from the vagus, the phrenic,
and the sympathetic.