MEDIASTINUM2
On the right side, their position is, bronchus, pulmonary artery, pulmonary veins; but on the left side, their position is, pulmonary artery, bronchus, pulmonary veins; which is accounted for by the 'bronchus being placed on a lower level on the left than on the right side.
The weight of both lungs together 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, their proportion to the body being, in the former, as i to 37, in the latter, as I to 43. The specific gravity of the lung tissue varies from 345 to 746, water being 1000.
The colour of the lungs at birth is of a pinkish white; in adult life, mottled in patches, of a dark slate colour; and, as age advances, this mottling assumes a dark black colour. The colouring matter consists of granules of a carbonaceous substance, deposited in the areolar tissue near the surface of the organ. It increases in quantity as age advances, and is more abundant in males than in females. The posterior border of the lung is usually darker than the anterior. The surface of the lung is smooth, shining, and marked out into numerous polyhedral spaces, which represent the lobules of the organ, and the area of each of these spaces is crossed by numerous lighter lines.
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 also highly elastic; hence the collapsed state of these organs when they are removed from the closed cavity of the thorax.
Structure. The lungs are composed of an external serous coat, a subserous areolar tissue, containing a large proportion of elastic fibres, and the pulmonary substance or parenchyma.
The serous coat is derived from the pleura; it is thin, transparent, and invests the entire organ as far as the root.
The subserous areolar tissue contains a large proportion of elastic fibres; it invests the entire surface of the lung, and extends inwards between the lobules.
The parenchyma is composed of lobules, which, although closely connected together by an interlobular areolar tissue, are quite distinct from one another, being easily separable in the foetus. The lobules vary in size; those on the surface are large, of a pyramidal form, the base turned towards the surface; those in the interior are smaller, and of various forms. Each lobule is composed of one of the ramifications of the bronchial tube and its terminal air-cells, of the ramifications of the pulmonary and bronchial vessels, lymphatics, and nerves: all of these structures being connected together by areolar fibrous tissue.
The bronchus upon entering the substance of the lung, divides and subdivides dichotomously throughout the entire organ. Sometimes three branches arise together, and occasionally small lateral branches are given off from the sides of a main trunk. Each of the smaller subdivisions of the bronchi enters a pulmonary lobule (lobular bronchial tube), and again subdividing, ultimately terminates in the intercellular passages and air-cells of which the lobule is composed. Within the lungs the bronchial tubes are circular, not flattened, and their constituent elements present the following peculiarities of structure.
The Cartilages are not imperfect rings, but consist of laminated plates, of varied form and size, scattered irregularly along the sides of the tube, being most distinct at the points of division of the bronchi. They may be traced into tubes the diameter of which is only one-fourth of a line. Beyond this point, the tubes are wholly membranous. The fibrous coat, and longitudinal elastic fibres, are continued into the smallest ramifications of the bronchi. The muscular coat is disposed in the form of a continuous layer of annular fibres, which may be traced upon the smallest bronchial tubes: they consist of the unstriped variety of muscular fibre. The mucous membrane lines the bronchi and its ramifications throughout, and is covered with columnar ciliated epithelium.
According to the observations of Mr. Rainey,* the lobular bronchial tubes, on entering the substance of the lobules, divide and subdivide from four to nine times, according to the size of the lobule, continuing to diminish in size until they attain a diameter of 1/50th to 1/30th of an inch. They then become changed in structure, losing their cylindric.il form, and are continued onwards as irregular passages (intercellular passages), through the substance of the lobule, their sides and extremities being closely covered by numerous saccular dilatations, the air-cells. This arrangement resembles most closely the naked eye appearances observed in the reticulated structure of the lung of the tortoise, and other reptilia.
The air-cells are small, polyhedral, alveolar recesses, separated from each other by thin septa, and communicating freely with the intercellular passages. They are well seen on the surface of the lung, and vary from 1/200th to 1/70 th of an inch in diameter; being largest on the surface, at the thin borders, and at the apex; and smallest in the interior.
At the termination of the bronchial tubes, in the intercellular passages, their constituent elements become changed: their walls are formed by an interlacing of the longitudinal elastic bundles with fibrous tissue; the muscular fibres disappear, and the mucous membrane becomes thin and delicate, and lined with a layer of squamous epithelium. This membrane lines the air-cells, and forms by its reduplications the septa intervening between them.
The Pulmonary Artery, conveys the dark, impure 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-cells. From this network, the radicles of the pulmonary veins arise, coalescing into large branches, they accompany the arteries, and return the blood, purified by its passage through the capillaries, to the left auricle of the heart. In the lung, the branches of the pulmonary artery are usually above and in front of a bronchial tube, the vein below.
The Pulmonary Capillaries form plexuses which lie immediately beneath the mucous membrane, on the walls and septa of the air-cells, and upon the walls of the intercellular passages. In the septa between the cells, the capillary network forms a single layer. The capillaries are very minute, the meshes being only slightly wider than the vessels: their walls are also exceedingly thin.
The Bronchial Arteries supply blood for the nutrition of the lung: they are derived from the thoracic aorta, and, accompany the bronchial tubes, are distributed to the bronchial glands, and upon the walls of the larger bronchial tubes and pulmonary vessels, and terminate in the deep bronchial veins. Others are distributed in the interlobular areolar tissue, and terminate partly in the deep, partly in the superficial, bronchial veins. Lastly, some ramify upon the walls of the smallest bronchial tubes, and terminate in the pulmonary veins.
The Superficial and Deep Bronchial Veins unite at the root of the lung, and terminate on the right side in the vena azygos; on the left side, in the superior intercostal vein.
The Lymphatics consist of a superficial and deep set: they terminate at the root of the lung, in the bronchial glands.
Nerves. The lungs are supplied from the anterior and posterior pulmonary plexuses, 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 have been found by Bemak upon the smaller branches of these nerves.
