The parietal layer refers to the outer membrane that lines the walls of a body cavity. In anatomy, it is often associated with the serous membranes, which produce a lubricating fluid to reduce friction between organs. This layer is crucial in protecting and supporting the organs within the cavity. In the context of the pleura, the parietal layer lines the thoracic cavity, while the visceral layer covers the lungs. Similarly, in the pericardium, the parietal layer encases the heart, providing a protective barrier. Understanding the parietal layer helps in comprehending how organs are organized and protected within the body.