Complex sentence: Surrounding the heart is a sac known as the pericardium, which consists of two membranes, the outer layer being the fibrous parietal pericardium and the inner layer being the serous visceral pericardium.
Split into simpler sentences: Surrounding the heart is a sac known as the pericardium, which consists of two membranes. The outer layer being the fibrous parietal pericardium and the inner layer being the serous visceral pericardium.