The fibrous pericardium is the tough, outer layer of the pericardial sac that encloses the heart, providing mechanical protection and anchoring it within the mediastinum. Its rigidity limits acute cardiac distension, which is crucial in conditions like pericardial effusion where fluid accumulation can cause cardiac tamponade, a life-threatening compression of the heart. Understanding its structure and function aids in diagnosing and managing pericardial diseases and guiding pericardiocentesis procedures.