What is the double membrane sac that protects the human heart called?

The heart is a muscular organ located in the chest cavity, right between the lungs. It is positioned slightly towards the left in the thoracic region.

It is enveloped by a thin sac called the pericardium. The pericardium is a membranous structure comprising an inner double layer of serous membrane and an outside fibrous layer. It protects and lubricates the heart and keeps it in place within the chest cavity.