What is the most superficial layer of the pericardial sac?

The most superficial layer of the pericardial sac is the fibrous pericardium.

Explanation:

The human heart is a hollow, muscular organ located in the chest between the lungs, posterior and slightly left of the breastbone (sternum) and superior to the diaphragm. The heart together with the blood vessels form the circulatory system. The heart pumps blood to other parts of the body via the blood vessels. The heart is enclosed in a fluid - filled double layered sac known as the pericardium. The layers of the pericardium are the fibrous (outer) and serous (inner) pericardium. This covers the heart and the roots of great blood vessels. The pericardium provides mechanical protection for the heart and great vessels, while the fluid in the sac lubricates the heart to reduce friction between the heart and the surrounding structures.