The double membranes forming a sac enclosing the lungs are the:

• A. Meninges
• B. Pleura
• C. Diaphragm
• D. Pericardium

Answer: (B)

The lungs are enclosed by a double-layered serous membrane called the pleura. This membrane has two parts: the visceral pleura, which adheres directly to the surface of the lungs, and the parietal pleura, which lines the inside of the chest wall and the diaphragm. Between these layers is the pleural cavity, a small space that contains pleural fluid to reduce friction as the lungs move during breathing. The other options refer to different structures: meninges surround the brain and spinal cord, the diaphragm is a muscular barrier between the thoracic and abdominal cavities, and the pericardium surrounds the heart. So the double membranes forming a sac around the lungs are the pleura.