Final answer:
The outermost of the pericardial membranes is the fibrous pericardium, a protective connective tissue layer surrounding the heart.
Step-by-step explanation:
The membrane that directly surrounds the heart is called the pericardium, which consists of two primary layers. The outermost layer of the pericardial membranes is the fibrous pericardium. The fibrous pericardium is a tough, connective tissue layer that protects the heart and maintains its position in the thorax. Below this layer is the serous pericardium, which has two sublayers: the parietal pericardium, which is fused to the fibrous pericardium, and the inner visceral pericardium, also known as the epicardium, which is fused to the heart itself.