Excerpt from The Cyclopaedia of Anatomy and Physiology
Pleura is the name given to the serous sac of the lung and the cavity containing it. There are two pleural sacs, one for the right lung and right side of the thorax, the other for the left lung and left side of the thorax. These two sacs being apposed and adherent to one another in the middle line, form there a median, antero-posterior, vertical septum, called the mediastinum, which divides the thoracic cavity into two lateral compartments. Each pleura is, like all other serous membranes, with one exception, a shut sac; and there being but one organ contained in each pleural cavity, and that organ being of a tolerably simple form, the well-known comparison of a double nightcap, expressive of the manner in which a serous sac lines the interior of a cavity and invests the exterior of the viscus contained in it, is extremely apt in the case of these sacs lining the chest and covering the lungs. Of the two surfaces of the sacs, the inner one is everywhere free and the outer everywhere adherent; such in fact is universally the case with all serous membranes. Each pleura invests its respective lung, and lines the moiety of the thoracic cavity to which it belongs, in the simplest manner possible, as simply and accurately as though it were a coating of paint, dipping into the fissures of the lungs and into the acute angles formed by the cost? with the arching diaphragm in the most neat and accurate manner. It only remains then, in order to complete our description of the course of these membranes, to examine the manner in which they pass from the parietes to the viscus. It is thus: - the two pleur?, above, below, behind, and in front, meet one another in or near the middle line, and form the mediastinum above-mentioned; between the layers of the mediastinum are situated the heart and great vessels and the termination of the trachea; from these issue on each side a bronchus, pulmonary artery, pulmonary veins, &c. destined to the lung, which, bound loosely together by areolar tissue, have received the appellation of the root of the lung; this root of the lung emerges from the mediastinum at about the middle of its posterior upper quarter, and is covered with a layer of the pleura, which thereby becomes conducted from this point of the mediastinum to the lung.
The term mediastinum is applied by some writers to the antero-posterior vertical septum of the chest, by others to the spaces occupied by the viscera situated between its layers; in the latter sense of the term three mediastina are enumerated - anterior, posterior, and middle; the anterior, which is very large, is the space occupied by the heart in its pericardium, thymus gland, or its remains, and phrenic nerves; the middle contains the bifurcation of the trachea, the arch of the aorta, the pulmonary and other great vessels; the posterior contains the aorta, ?sophagus, &c. All these organs, their position, &c. will be found described in other parts of this work; their right and left aspects are invested by the right or left pleura respectively. It is in their larger interspaces only that the two pleurae come into actual contact and adhesion with one another. The smaller interspaces are not intruded upon by the pleurae, but are occupied with areolar tissue and fat.
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