Which membrane lines the thoracic walls and abuts the mediastinum medially?

• A. Parietal peritoneum
• B. Visceral pleura
• C. Parietal pleura
• D. Arachnoid mater

Answer: (C)

The membranes around the lungs form two layers: the parietal pleura and the visceral pleura. The layer that lines the inside of the thoracic walls and extends to cover the mediastinal surface is the parietal pleura, specifically the mediastinal portion. It forms the lining of the chest cavity and stops at the reflection onto the lung at the hilum, where it becomes continuous with the visceral pleura that covers the lungs themselves. The parietal pleura is what you’d expect to be in contact with the chest wall and mediastinum, not the surface of the lungs (visceral pleura). The other options don’t fit: the parietal peritoneum lines the abdominal cavity, the arachnoid mater is a brain/spinal cord covering, and the visceral pleura covers the lungs.