The parietal layer is a term used in anatomy to describe a specific membrane that lines the walls of body cavities. It is part of the serous membrane system, which includes the visceral layer that covers organs. The parietal layer helps reduce friction between organs and surrounding structures, allowing for smooth movement. In the context of the pleura, which surrounds the lungs, the parietal layer lines the thoracic cavity. Similarly, in the pericardium, it forms the outer layer surrounding the heart. These layers play a crucial role in protecting organs and facilitating their function within the body.