The Lungs are the two principal organs of respiration, placed one on each side of the chest, separated from each other by the heart and other contents of the mediastinum. 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, downward and forward to the lower part of the anterior border. This fissure penetrates nearly to the root. In the right lung the upper lobe is partially subdivided by a second and shorter fissure, which extends from the middle of the preceding, forward and slightly upward to the anterior margin, making a small triangular portion, the middle lobe. The right lung is larger and heavier than the left; it is broader, owing to the inclination of the heart to the left side; in consequence of the diaphragm rising higher on the right side to accommodate the liver, it is also shorter by an inch. The weight of the lungs is about forty-two ounces, the right lung being two ounces heavier than the left; but much variation is met with, according to the amount of blood or serous fluid they may contain. The lungs are heavier in the male than in the female. Each lung is conical in shape, and presents an Apex, Base, two Borders, and two Surfaces for examination. Apex. - The apex extends into the root of the neck about an inch or an inch and a half above the level of the first rib, and forms a tapering cone. Base. - The base is concave, broad, 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. Borders. - The anterior border is sharp and thin, overlapping the front of the pericardium. The posterior border is broad and rounded, and is received into the deep concavity on either side of the spinal column. It is much longer than the anterior border and projects between the ribs and the diaphragm. Surfaces. - The surface is smooth, shining, and marked out in numerous polyhedral spaces, which indicate the lobules of the organ. The area of each of these spaces is crossed by numerous lighter lines. The inner surface is concave; in front presenting a depression corresponding to the convex surface of the pericardium; and behind a deep fissure, which gives attachment to the root of the lung. The thoracic or external surface is smooth and convex; corresponding to the form of the cavity of the chest, being deeper behind than in front. Root. - A little above the middle and 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 trachea. The root is formed by the bronchial tube, pulmonary artery, pulmonary vein, the bronchial arteries and veins, the pulmonary plexus of nerves, lymphatics, bronchial glands, and areolar tissue, all of which are enclosed by a reflexion of the pleura. The root of the right lung lies behind the superior vena cava and the ascending portion of the aorta, and below the vena azygos major. The root of the left lung passes beneath the arch of the aorta 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. Substance. - The substance of the lung is of a light, porous, spongy texture; it floats in water and crepitates when handled, owing to the presence of air in the tissue; it is very elastic, hence the collapsed state of these organs when they are removed from the cavity of the thorax. Cells. - The air-cells are small, polyhedral recesses, composed of a fibrillated connective tissue, and surrounded by a few involuntary muscular and elastic fiber. They can be plainly seen on the surface of the lung, and vary from 1-200 to 1-70 of an inch in diameter. Tubes. - The bronchus, upon entering the substance of the lung, divides and subdivides throughout the entire organ; sometimes three branches arise together, and occasionally small lateral branches are given off from the sides of a larger. Each of the smaller subdivisions of the bronchi enters a pulmonary lobule, and is termed a lobular bronchial tube or bronchiole. Its walls now begin to present irregular dilatations, air-cells, at first sparingly and on the one side of the tube only, but as it proceeds onward, these dilatations become more numerous and surround the tube on all sides, so that it loses its cylindrical character. Blood-Vessels. - The pulmonary artery conveys the venous blood to the lungs; it divides into branches, which accompany the bronchial tubes, and terminate in a dense capillary network upon the walls of the intercellular passages and air-cell. In the lung the branches of the pulmonary artery are usually above and behind the bronchial tube, the vein below and in front. The pulmonary capillaries form plexuses, which lie immediately beneath the mucous membranes in the walls and septa of the air-cells and of the infundibula. In the septa between the air-cells the capillary network forms a single layer. The capillaries form a very minute network, the meshes of which are smaller than the air-cells themselves; their walls are also exceedingly thin; the arteries of neighboring lobules are distinct from each other and do not anastomose, whereas the corresponding venous anastomoses are exceedingly fine. The radicles of the pulmonary veins commence in the pulmonary capillaries and coalesce into larger branches, which accompany the arteries and return the oxygenated blood to the left auricle of the heart. The radicles come together in the septa between the infundibula, entirely separate from the small arterial ramifications. Those which are near the surface of the lungs have an undivided course for some distance, and then either unite with some deeper lying vein, or form with their companions a wide-meshed superficial plexus. The bronchial arteries supply blood for the nutrition of the lung, and are derived from the thoracic aorta, accompanying the bronchial tubes, and are distributed to the bronchial glands and upon the walls of the larger bronchial tubes and pulmonary vessels. Those supplying the bronchial tubes form the capillary plexus in the muscular coat, from which branches are given off to form a second plexus in the mucous coat. This plexus in the lobular bronchioles is continued with that of the pulmonary artery, and the blood which the bronchial artery brings is thus carried back by the pulmonary vein. Others are distributed in the interlobular areolar tissue, and terminate partly in the deep and partly in the superficial bronchial vein. Some ramify upon the surface of the lung, beneath the pleura, where they form a capillary network. The bronchial vein is formed at the root of the lung, receiving superficial and deep veins corresponding to the branches of the bronchial artery. It does not, however, receive all the blood supplied by the artery, as some of it passes into the pulmonary vein. It terminates on the right side in the vena azygos major, and on the left side in the superior intercostal or left upper azygos vein. Some authorities state that in other parts of the lung than in the lobular bronchioles, bronchial veins, even those coming from the larger bronchial tubes, join more or less freely with pulmonary veins. The intercostal arteries give small branches to the surface of the lungs by way of the ligamentum latum pulmonis. The lymphatics consist of a superficial and deep set; they terminate at the root of the lung in the bronchial glands. Nerves. - The nerve-supply of the lungs is from the anterior and posterior pulmonary plexus, formed chiefly by branches from the sympathetic and pneumogastric. The filaments from these plexuses accompany the bronchial tubes upon which they are lost. Small ganglia are found upon these nerves. EXPLANATORY As many of our readers are unfamiliar with anatomy and physiology,
we have followed Gray quite closely and have entered rather deeply into the
anatomy of the organs of respiration, for several very important reasons.
The most important of which is to call attention to the immense and complicated
blood-supply of the lungs, and the consequent importance of a perfect and unobstructed
circulation. Another very important point that we discover by referring
to the anatomy is the fact that the lungs are controlled by nerves from the
anterior and posterior pulmonary plexus, formed chiefly by branches from the
sympathetic and pneumogastric nerves. These nerves are affected either
by a pressure or stimulation in the neck, or in the spine at about the fifth
dorsal. |