THE LUNGS. 785 



monary 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 

 yena^ 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 chief 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; but on the left side their position is, pulmonary artery, bronchus, pul- 

 monary 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 accord- 

 ing 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 a pinkish white ; in adult life, a dark slate 

 color, mottled in patches ; and, as age advances, this mottling assumes a 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 polyhedral spaces, 

 indicating the lobules of the organ : 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, 

 and are 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 smaller, and of various forms. Each lobule is composed 

 of one of the ramifications of the bronchial tube and its terminal air-cells, and 

 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 com 

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