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 (https://doi.org/10.1515/pac-2015-1202)