1196 THE ORGANS OF VOICE AND RESPIRATION 



The chief structures composing the root of each lung are arranged in a similar 

 manner from before backward on both sides viz., the twopulmonary veins in 

 frojit, the pulmonary artery in the middle, and the bronchus, together with the 

 bronchial vessels, behind. From above downward, 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, pulmo- 

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

 the right pulmonary artery, but only its eparterial branch (see p. 1179), 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. 



Divisions of the Bronchi. Just as the lungs differ from each other in the num- 

 ber of their lobes, so the bronchi differ in their mode of subdivision. 



The right bronchus gives off, about an inch from the bifurcation of the trachea, 

 a branch for the upper lobe. This branch arises above the level of the pulmonary 

 artery, and is therefore named the eparterial bronchus. All the other divisions 

 of the main stem come off below the pulmonary artery, and consequently are 

 termed hyparterial bronchi. The first of these is distributed to the middle lobe, 

 and the main tube then passes downward and backward into the lower lobe, 

 giving off in its course a series of large ventral and small dorsal branches. The 

 ventral and dorsal branches arise alternately, and are usually eight in number - 

 four of each kind. The branch to the middle lobe is regarded as the first of the 

 ventral series. 



The left bronchus passes below the level of the pulmonary artery before it 

 divides, and hence all its branches are hyparterial; it may therefore be looked 

 upon as equivalent to that portion of the right bronchus which lies on the distal 

 side of its eparterial branch. The first branch of the left bronchus arises about 

 two inches from the bifurcation of the trachea, and is distributed to the upper lobe. 

 The main stem then enters the lower lobe, where it divides into ventral and dorsal 

 branches similar to those in the right lung. The branch to the upper lobe of the 

 left lung is regarded as the first of the ventral series. 



The true weight of the human lungs as ascertained in the bodies of criminals executed by 

 electricity, in which the mode of death is attended by a nearly bloodless condition of the lungs, 

 is 215 prams (1\ ounces) for the left lung and 240 grams (8? ounces) for the right lung (E. A. 

 Spitzka, Amer. Jour, of Anat., iii, 1, p. v). Ordinarily, with the vascular channels more or less 

 filled with blood and serum, the two lungs together weigh about 42 ounces, the right lung being 

 2 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. The 

 specific gravity of the lung tissue varies from 0.345 to 0.746. 



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 con- 

 sists of granules of 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 surface 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 crepi- 

 tates 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. 



The Fetal Lung. After respiration has been established, the lung fills the pleural cavity. 

 In the fetus, as the lung has never been distended with air and has never received a large amount 

 of blood, it is gathered into a small mass at the back of the thorax. It will sink in water and feels 

 solid to the touch. 



Structure. The structure of the lung is such that the blood brought by the pulmonary 

 artery comes into close relation with the air which enters from the bronchioles. The blood 

 gives materials to the air, and the air gives elements to the blood, and the process of respiration 

 causes the dark blood brought from the heart by the pulmonary artery to return to the heart as 



