What is the pleuræ?

What it is

• Each lung is invested by an exceedingly delicate serous membrane, the pleura, which is arranged in the form of a closed invaginated sac. A portion of the serous membrane covers the surface of the lung and dips into the fissures between its lobes; it is called the pulmonary pleura. The rest of the membrane lines the inner surface of the chest wall, covers the diaphragm, and is reflected over the structures occupying the middle of the thorax; this portion is termed the parietal pleura.
• The two layers are continuous with one another around and below the root of the lung; in health they are in actual contact with one another, but the potential space between them is known as the pleural cavity. When the lung collapses or when air or fluid collects between the two layers the cavity becomes apparent.
• The right and left pleural sacs are entirely separate from one another; between them are all the thoracic viscera except the lungs, and they only touch each other for a short distance in front; opposite the second and third pieces of the sternum the interval between the two sacs is termed the mediastinum.
• Different portions of the parietal pleura have received special names which indicate their position: thus, that portion which lines the inner surfaces of the ribs and Intercostales is the costal pleura; that clothing the convex surface of the diaphragm is the diaphragmatic pleura; that which rises into the neck, over the summit of the lung, is the cupula of the pleura ( cervical pleura ); and that which is applied to the other thoracic viscera is the mediastinal pleura.
• Reflections of the Pleura . —Commencing at the sternum, the pleura passes lateralward, lines the inner surfaces of the costal cartilages, ribs, and Intercostales, and at the back part of the thorax passes over the sympathetic trunk and its branches, and is reflected upon the sides of the bodies of the vertebræ, where it is separated by a narrow interval, the posterior mediastinum, from the opposite pleura.
• From the vertebral column the pleura passes to the side of the pericardium, which it covers to a slight extent; it then covers the back part of the root of the lung, from the lower border of which a triangular sheet descends vertically toward the diaphragm.