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1c. The Pleuræ
FIG. 965 - Front view of thorax, showing the relations of the pleuræ and lungs to the chest wall. Pleura in blue; lungs in purple. 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 (Figs. 965, 966).—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. This sheet is the posterior layer of a wide fold, known as the pulmonary ligament. From the back of the lung root, the pleura may be traced over the costal surface of the lung, the apex and base, and also over the sides of the fissures between the lobes, on to its mediastinal surface and the front part of its root. It is continued from the lower margin of the root as the anterior layer of the pulmonary ligament, and from this it is reflected on to the pericardium (pericardial pleura), and from it to the back of the sternum. Above the level of the root of the lung, however, the mediastinal pleura passes uninterruptedly from the vertebral column to the sternum over the structures in the superior mediastinum. Below, it covers the upper surface of the diaphragm and extends, in front, as low as the costal cartilage of the seventh rib; at the side of the chest, to the lower border of the tenth rib on the left side and to the upper border of the same rib on the right side; and behind, it reaches as low as the twelfth rib, and sometimes even to the transverse process of the first lumbar vertebra. Above, its cupula projects through the superior opening of the thorax into the neck, extending from 2.5 to 5 cm. above the sternal end of the first rib; this portion of the sac is strengthened by a dome-like expansion of fascia (Sibson’s fascia), attached in front to the inner border of the first rib, and behind to the anterior border of the transverse process of the seventh cervical vertebra. This is covered and strengthened by a few spreading muscular fibers derived from the Scaleni. 
FIG. 966 - Lateral view of thorax, showing the relations of the pleuræ and lungs to the chest wall. Pleura in blue; lungs in purple. In the front of the chest, where the parietal pleura is reflected backward to the pericardium, the two pleural sacs are in contact for a short distance. At the upper part of the chest, behind the manubrium, they are separated by an angular interval; the line of reflection being represented by a line drawn from the sternoclavicular articulation to the mid-point of the junction of the manubrium with the body of the sternum. From this point the two pleuræ descend in close contact to the level of the fourth costal cartilages, and the line of reflection on the right side is continued downward in nearly a straight line to the xiphoid process, and then turns lateralward, while on the left side the line of reflection diverges lateralward and is continued downward, close to the left border of the sternum, as far as the sixth costal cartilage. The inferior limit of the pleura is on a considerably lower level than the corresponding limit of the lung, but does not extend to the attachment of the diaphragm, so that below the line of reflection of the pleura from the chest wall on to the diaphragm the latter is in direct contact with the rib cartilages and the Intercostales interni. Moreover, in ordinary inspiration the thin inferior margin of the lung does not extend as low as the line of the pleural reflection, with the result that the costal and diaphragmatic pleuræ are here in contact, the intervening narrow slit being termed the phrenicocostal sinus. A similar condition exists behind the sternum and rib cartilages, where the anterior thin margin of the lung falls short of the line of pleural reflection, and where the slit-like cavity between the two layers of pleura forms what is called the costomediastinal sinus. The line along which the right pleura is reflected from the chest-wall to the diaphragm starts in front, immediately below the seventh sternocostal joint, and runs downward and backward behind the seventh costal cartilage so as to cross the tenth rib in the mid-axillary line, from which it is prolonged to the spinous process of the twelfth thoracic vertebra. The reflection of the left pleura follows at first the ascending part of the sixth costal cartilage, and in the rest of its course is slightly lower than that of the right side. The free surface of the pleura is smooth, polished, and moistened by a serous fluid; its attached surface is intimately adherent to the lung, and to the pulmonary vessels as they emerge from the pericardium; it is also adherent to the upper surface of the diaphragm: throughout the rest of its extent it is easily separable from the adjacent parts. The right pleural sac is shorter, wider, and reaches higher in the neck than the left. Pulmonary Ligament (ligamentum pulmonale; ligamentum latum pulmonis).—From the above description it will be seen that the root of the lung is covered in front, above, and behind by pleura, and that at its lower border the investing layers come into contact. Here they form a sort of mesenteric fold, the pulmonary ligament, which extends between the lower part of the mediastinal surface of the lung and the pericardium. Just above the diaphragm the ligament ends in a free falciform border. It serves to retain the lower part of the lung in position. Structure of Pleura.—Like other serous membranes, the pleura is covered by a single layer of flattened, nucleated cells, united at their edges by cement substance. These cells are modified connective-tissue corpuscles, and rest on a basement membrane. Beneath the basement membrane there are net-works of yellow elastic and white fibers, imbedded in ground substance which also contains connective-tissue cells. Bloodvessels, lymphatics, and nerves are distributed in the substance of the pleura. Vessels and Nerves.—The arteries of the pleura are derived from the intercostal, internal mammary, musculophrenic, thymic, pericardiac, and bronchial vessels. The veins correspond to the arteries. The lymphatics are described on page 719. The nerves are derived from the phrenic and sympathetic (Luschka). Kölliker states that nerves accompany the ramifications of the bronchial arteries in the pulmonary pleura.
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