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 appokd and adherent to one another in the middle line, form there a median, antero-posterior, vertical septum, called the me diastinum, 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 ex tremely apt in the case of these sacs lining the chest and covering the lungs. Of the two sur faces 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 mariner pos sible, 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 costw with the arching diaphragm in the most neat and accurate manner. It only remains then, in order to complete our descrip tion of the course of these membranes, to ex amine the manner in which they pass from the parietes to the viscus. It is thus r—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, pul monary 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 media stinum 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 re mains, 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, cesophagus, &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 pleurw come into actual contact and adhesion with one another. The srnaller interspaces are not intruded upon by the pleurw, but are occupied with areolar tissue and fat. In most of the lower (mammalian) animals, where the chest is deep and narrow, and in the human fcetus, the two pleurw come into adhesion with one ano ther in frnn4 brIbe':h6aft',;: !mit iiilf.the *adult human siiltie-ce thisis ncrt the ease, lb° Wricar dium coming into immediate contact with the anterior thoracic parietes. Also the pleurm are prevented by the adhesion of the pericardium to the diaphragm from adhering to one another below the heart. It is almost superfluous to state that the heart and pericardium encroach more upon the left pleural cavity than upon the right. The median thoracic septum of the hu man subject is able, partly on account of its small antero-posterior extent, 'to resist any con siderable lateral displacement, such as might result from accumulation of effusion into one pleural cavity ; but in deep-chested animals it admits of displacement to such an extent that the whole of the chest may be filled vvith an effusion into one pleural cavity. In a Chetah, which died of pleurisy at the gardens of the Zoological Society, dissected by the author, the immediate cause of death was suffocation occa sioned by effusion into the right pleura, which occupied the whole chest, and compressed the left lung, the left pleura being unaffected. The mediastinum may be regarded as a kind of mesentery to the heart, and in some reptiles it is very obviously seen to be a part of the great median mesentery wherein all the viscera are suspended. This great median mesentery of reptiles is attached to parietes in front as well as behind as far down as the falciform liga ment of the liver; as though a fold of serous membrane had been pulled down by the umbi lical vein. In some reptiles, as the chame leon, the anterior parietal attachment is conti nued even down to the small intestines, so that the stomach and part of the small intestine are enclosed betvveen the layers of the mediasti nurn. The serial homology of this septurn is ob scured in Mammalia by the diaphragm being interposed between it and the other mesenteries.