Is it ensure, insure, or assure?
early 15c., medical Latin, from Greek pleura "side of the body, rib," also "flank of an army, page of a book," of unknown origin.
pleura pleu·ra (plur'ə)
n. pl. pleu·rae (plur'ē)
The thin serous membrane that envelops each lung and folds back to make a lining for the chest cavity.
membrane lining the thoracic cavity (parietal pleura) and covering the lungs (visceral pleura). The parietal pleura folds back on itself at the root of the lung to become the visceral pleura. In health the two pleurae are in contact. When the lung collapses, however, or when air or liquid collects between the two membranes, the pleural cavity or sac becomes apparent (see pleurisy). There are actually two pleural cavities, the right and the left; each constitutes a closed unit not connected to the other. The glistening surface of the pleura is made up of a sheet of flat cells, the mesothelium, which covers an underlying layer of loose elastic tissue. The pleura exudes a thin fluid that keeps it moist and lubricated.