Pericardium

The heart bag; consists of two leaves: the outer fibrous parietal leaf and the inner serous visceral leaf, the epicardium. Protects the heart against overstretching and inflammation. Between pericardium and epicardium glands release a serous liquid into the sliding gap, which allows the two skins to glide effortlessly against each other.

https://en.wikipedia.org/wiki/Pericardium _([1])