The Pleura is a serous membrane which invests the lungs and lines the thoracic cavity. It is analogous to the peritoneum of the abdomen, and performs the same function- i.e., forms a lubricated covering, so that the organs can move freely in the cavity. The part lining the cavity is called the parietal pleura, and the part covering the lungs the visceral pleura. The latter is very thin and adherent to the lung, and dips into all the fissures.

The parietal pleura is named according to the part over which it passes. The cervical pleura rises up to the level of the neck of the first rib. Owing to the oblique position of the latter, the pleura is about an inch and a half above the level of the clavicle; it is strengthened by Sibson's fascia, a strong membrane attached to the seventh cervical vertebra and the inner margin of the first rib. The costal pleura lines the inner surfaces of the ribs and the intercostal spaces; it reaches to the sternum in front and the bodies of the vertebrae behind. The diaphragmatic pleura covers the surface of the diaphragm except for the middle part, occupied by the pericardium, and a narrow strip at the costal attachment. The mediastinal pleura passes back from the sternum to the vertebral column and lines the space (mediastinum) between the lungs. As it passes back it is reflected by the root on to the lung, and becomes the visceral pleura (see Fig. 53). Below the root of the lung the two layers hang down in a deep fold called the ligamentum latum pulmonis. The parietal pleura is bigger than the lungs require at rest, and is not quite so big as the entire cavity, the line of reflection being-

Fig. 53 - The Pleura.

Right pleura: Anterior, down the middle line of the sternum to the back of the ensiform cartilage, along the seventh costal cartilage, and across the bony extremities of the eighth and ninth ribs. In the mid-axillary line the pleura reaches its-lowest limit- that of the bony tip of the tenth rib. It then ascends slightly, cutting across the eleventh rib to the middle of the twelfth rib and on to the first lumbar vertebra. The posterior line of reflection is to the left of the mid-line of the vertebral bodies, the aorta intervening between them.

Left pleura: This only differs from the right in the anterior line of reflection. Opposite the fourth costal cartilage the left pleura deviates to the left, leaving a small area of pericardium uncovered. It cuts across the fifth, sixth, and seventh costal cartilages to the bony tip of the eighth rib, after which the line-of reflection resembles that of the right pleura.