The pleura of the thorax are the serous membranes which enclose the upper chest cavity. The pleura enclose the |lungs| and protect them from friction against the wall of the thorax. It is formed of two layers - the visceral and parietal pleura - between which is lubricated by serous fluid. The parietal pleura is the exterior layer of this pulmonary pleural sac, which connects to the thorax wall, the mediastinal membrane, and the |diaphragm| muscle.