What structure lies within the pericardium in the mediastinum?

The structure that lies within the pericardium in the mediastinum is the heart. The pericardium is a double-walled sac that surrounds the heart, providing protection and support. It consists of an outer fibrous layer and an inner serous layer, which is further divided into the parietal and visceral pericardium. The heart is positioned centrally within the thoracic cavity, specifically in the mediastinum, which is the space between the lungs. This anatomical arrangement is crucial for normal heart function, as the pericardium helps to anchor the heart in place and prevent over-distension during contractions.