MEDIASTINUM1
The mediastinum is the space left in the median line of the chest by the non-approximation of the two pleurae. It extends from the sternum in front to the spine behind, and contains all the viscera in the thorax, excepting the lungs. The mediastinum may be subdivided, for convenience of description, into the anterior, middle, and posterior.
The Anterior Mediastinum is bounded in front by the sternum, on each side by the pleura, and behind by the pericardium. Owing to the oblique position of the heart towards the left side, this sparp is not parallel with the sternum, but directed obliquely from above downwards, and to the left of the median line; it is broad below, narrow above, very narrow opposite the second piece of the sternum, the contiguous surfaces of the two pleurae being occasionally united over a small space. The anterior mediastinum contains the origins of the Sterno-hyoid and Sterno-thyroid muscles, the Triangularis sterni, the internal mammary vessels of the left side, the remains of the thymus gland, and a quantity of loose areolar tissue, containing some lymphatic vessels ascending from the convex surface of the liver.
The Middle Mediastinum is the broadest part of the interpleural space. It contains the heart enclosed in the pericardium, the ascending aorta, the superior vena cava, the bifurcation of the trachea, the pulmonary arteries and veins, and the phrenic nerves.
The Posterior Mediastinum is an irregular triangular space, running parallel with the vertebral column; it is bounded in front by the pericardium and roots of the lungs, behind by the vertebral column, and on either side by the pleura. It contains the descending aorta, the greater and lesser azygos veins and superior intercostal vein, the pneumogastric and splanchnic nerves, the oesophagus, thoracic duct, and some lymphatic glands placed one in each of the lateral cavities of the chest, separated from each other by the heart and other contents of the mediastinum. Earh lung is conical in the lungs. The lungs are the essential organs of respiration; they are two in number, placed one in each of the lateral cavities of the chest, separated from each other by the heart and other contents of the mediastinum shape, and presents for examination an apex, a base, two borders, and two surfaces.
The apex forms a tapering cone, which extends into the root of the neck, about an inch to an inch and a half above the level of the first rib.
The base is broad, concave, and rests upon the convex surface of the Diaphragm; its circumference is thin, and fits into the space between the lower ribs and the costal attachment of the Diaphragm, extending lower down externally and behind than in front.
The external or thoracic surface is smooth, convex, of considerable extent, and corresponds to the form of the cavity of the chest, being deeper behind than in front.
The inner surface is concave. It presents, in front, a depression corresponding to the convex surface of the pericardium, and behind, a deep fissure (the hilum pulmonis), which gives attachment to the root of the lung.
The posterior border is rounded and broad, and is received in the deep concavity on either side of the spinal column. It is much longer than the anterior border, and projects below between the ribs and Diaphragm.
The anterior border is thin and sharp, and overlaps the front of the pericardium.
The anterior border of the right lung corresponds to the median line of the sternum, and is in contact with its fellow, the pleurae being interposed, as low as the fourth costal cartilage; below this, the contiguous borders are separated by a V-shaped interval, formed at the expense of the left lung, and corresponding to which the pericardium is exposed.
Each lung is divided into two lobes, an upper and lower, by a long and deep fissure, which extends from the upper part of the posterior border of the organ, about three inches from its apex, downwards and forwards to the lower part of its anterior border. This fissure penetrates nearly to the root. In the right lung the upper lobe is partially divided by a second and shorter fissure, which extends forwards and upwards from the middle of the preceding to the anterior margin of the organ, marking off a small triangular portion, the middle lobe. The left lung presents a deep V-shaped notch at the lower part of its anterior border, into which the apex of the heart is received.
The right lung is the largest; it is broader than the left, owing to the inclination of the heart to the left side; it is also shorter by an inch, in consequence of the Diaphragm rising higher on the right side to accommodate the liver. The right lung has also three lobes.
The left lung is smaller, narrower, and longer than the right, and has only two lobes.
A little above the middle of the inner surface of each lung, and nearer its posterior than its anterior border, is its root, by which the lung is connected to the heart and the trachea. It is formed by the bronchial tube, the pulmonary artery, the pulmonary veins, the bronchial arteries and veins, the pulmonary plexus of nerves, lymphatics, bronchial glands, and areolar tissue, all of which are enclosed by a reflection of the pleura. The root of the right lung lies behind the superior cava and upper part of the right auricle, and below the vena azygos. That of the left lung passes beneath the arch of the aorta, and in front of the descending aorta; the phrenic nerve and the anterior pulmonary plexus lie in front of each, and the pneumogastric and posterior pulmonary plexus behind each.
The structures composing the root of each lung are arranged in a similar manner from before backwards on both sides, viz.: the pulmonary -veins most anterior; the pulmonary artery in the middle; and the bronchus, together with the bronchial vessels, behind. From above downwards, on the two sides, their arrangement differs.
