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I quote: "Two layers of thin membrane surround the lungs" this is called the pleura, it lines the chest wall and covers the lungs. The inner layers are stuck to the lungs and the outer; lines the chest wall and diaphragm. Between the layers is a space (mostly empty) called the pleural cavity. The two layers of the pleura are lubricant and smooth so that your lungs can move smoothly against the chest wall as you inhale and exhale.