THE LUNGS. 975 



behind the superior vena cava and ascending portion of the aorta and below the 

 vena azygos major. 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 backward on both sides viz., the two pulmonary veins in 

 front; the pulmonary artery in the middle; and the bronchus, together with the 

 bronchial vessels, behind. From above downward, on the two sides, their arrange- 

 ment 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, pulmo- 

 nary veins. It should be noted that the entire right bronchus does not lie above 

 the right pulmonary artery, but only its epartcrial branch (see page 966), which 

 passes to the upper lobe of the right lung ; the divisions of the bronchus for the 

 middle and lower lobes lie below the artery. 



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 0.345 to 0.746, 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 inward 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 

 may be teased asunder without much difficulty in the foetus. The lobules vary in 

 size; those on the surface are large, of pyramidal form, the base turned toward 

 the surface ; those in the interior, smaller and of various forms. Each lobule is 

 composed of one of the ramifications of a 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 tissue. 



The bronchus, upon entering the substance of the lung, divides and subdivides 

 bipinnately, throughout the entire organ. Sometimes three branches ar <se 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, 

 and is termed a lobular bronchial tube or bronchiole. Its wall now begins to pre- 

 sent irregular dilatations, air-cells or alveoli, at first sparingly and on one side of 

 tin 1 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. The 



