THE LUNGS. 719 



A little above the middle of the inner surface of each lung, and nearer its pos- 

 terior 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 inclosed 

 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, thus: 



On the right side, their position is, bronchus, pulmonary artery, pulmonary 

 veins; on the left side, their position is, pulmonary artery, bronchus, pulmonary 

 veins : this 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 1 to 37, in the latter as 1 to 43. The specific gravity of the lung tissue varies 

 from 345 to 746, water being 1000. 



The color of the lungs at birth is of a pinkish white ; in adult life, mottled in 

 patches, of a dark slate color ; and, as age advances, this mottling assumes a 

 dark black color. The coloring 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 poly- 

 hedral 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, 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 bronchris, 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 



