pericardium

https://doi.org/10.1351/goldbook.11078

Serous membranous sac that surrounds the heart, consisting of an inner visceral layer (the epicardium), and an outer parietal pericardium that is attached to the sternum and diaphragm.

Source:
PAC, 2016, 88, 713. (Glossary of terms used in developmental and reproductive toxicology (IUPAC Recommendations 2016)) on page 791 [Terms] [Paper]