It involves the adhesion of the two pleurae. The lungs are surrounded by the pulmonary pleurae. Pleurisy is a condition of inflamed pleurae. Pleural fluid is a serous fluid produced by the serous membrane covering normal pleurae. Only the parietal pleurae are sensitive to pain. Another type, mesothelium, forms the walls of the pericardium, pleurae, and peritoneum. The side lobes of the thorax (or pleurae, singular pleura) have rounded ends. The underlying mechanism involves the rubbing together of the pleurae instead of smooth gliding. The mediastinal and central portions of the diaphragmatic pleurae are innervated by the phrenic nerves. Pleurisy and other disorders of the pleurae can be serious, depending on what caused them. The pleurae are clearly wider than the axis. The pleurae of each lung are not connected in any way. The pleural cavity, with its associated pleurae, aids optimal functioning of the lungs during breathing. The tips of the pleurae are free, which resembles an old-fashion central heating radiator. The pleurae are sometimes abbreviated or extended to form long spines.