It"s located in the chest, behind the sternum in the mediastinal cavity (or medi- astinum), between the lungs, and in front of the spine. The heart lies tilted in this area like an upside-down triangle. The top ofthe heart, or its base, lies just below the second rib; the bottomof the heart, or its apex, tilts forward and down, toward the left side of the body, and rests on the diaphragm. Is made up of squamous epithelial cells overlying connective tissue. Makes up the largest portion of the heart"s wall. This layer of muscle tissue contracts with each heartbeat. Contains endothelial tissue with small blood ves- sels and bundles of smooth muscle. Surrounds the heart and acts as a tough, protective sac. It consists of the fibrous pericardium and the serous pericardium. The fibrous pericardium, composed of tough, white, fibrous tissue, fits loosely around the heart, protecting it. The fibrous pericardium attaches to the great vessels, diaphragm, and sternum.