Pleura

The lung is not loose inside the chest: it comes wrapped. That wrapping is called the pleura, and it is a serous membrane, the kind that lines the chest cavity. Each lung is enclosed in its own cavity, surrounded by the pleura.

The pleura is not one sheet but two. The visceral one clings to the lung and dips into its fissures, which are the clefts on its surface; the parietal one is the outer sheet, the one attached to the chest wall, to the mediastinum, which is the central space where the heart sits, and to the diaphragm, the muscle that separates the chest from the abdomen. Between the two sheets there is a space, the pleural cavity, and the two are continuous at the hilum, where one sheet turns into the other. To picture it, think of a deflated balloon that you push your fist into: one layer ends up against your fist, another on the outside and a space in between. And the pleura is not alone in the chest: there are three serous membranes in all, the two pleurae covering the lungs and the pericardium, which does the same for the heart.