elicate membrane lying within the fibrous sac and lining its walls (Gray, 1918). The serous pericardium is composed of a single layer of flattened cells which rest on loose connective tissue. The heart invaginates the walls of the serous pericardium above and behind, almost obliterating its cavity, leaving only a potential space. The visceral portion of the serous pericardium, the epicardium, covers the heart and the great