Pericardium

pericardium; 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 a serous liquid is released into the sliding gap, which allows the two skins to glide effortlessly against each other.

https://de.wikipedia.org/wiki/Herzbeutel _{(Wikipedia CC-by-sa-3.0)}