Final answer:
The double wall sac around the heart is known as the pericardium, comprising a tough outer fibrous pericardium and an inner serous pericardium with parietal and visceral (epicardium) layers.
Step-by-step explanation:
The outer covering layer, a double-wall sac around the heart, is known as the pericardium or pericardial sac. This protective membrane consists of two main parts: the fibrous pericardium and the serous pericardium. The fibrous pericardium is the sturdy outer layer made of dense connective tissue that safeguards the heart and maintains its position in the thorax. Beneath this is the more delicate serous pericardium, which includes two layers, the parietal pericardium, attached to the fibrous layer, and the visceral pericardium, also known as the epicardium, which lies closer to the heart muscle. Between these serous layers is the pericardial cavity, which contains lubricating fluid that reduces friction as the heart pumps.