254 HUMAN ANATOMY. 



The lungs in infancy are of a pale-rose color, but later become of 

 a leaden hue, variegated with dark slate and bluish-black. They 

 are highly elastic, crackle on pressure, and float in water, having 

 a specific gravity of 0.345 to 0.746. In the foetus before birth, 

 and also in certain Diseases, they sink in water. The structure 

 of the lung consists of an external serous -coat, the visceral layer of 

 the pleura, a subserous areolar tissue, highly elastic, and the 

 parenchyma, or proper substance of the lungs. The latter is 

 made up of small polyhedral primary lobules, which unite to 



FIG. 122. 



Heart and lungs: 1, right ventricle; 3, right auricle; 5, pulmonary 

 artery; 9, aorta; 10, superior cava; 20, root of lung; 21, 22, 23, 

 upper, middle, and lower lobes of right lung; 24, 25, upper and lower 

 lobes of left lung. 



form secondary lobules, the latter giving rise to the markings on 

 the surface. Both the primary and secondary lobules are held 

 together by connective tissue. A primary lobule represents the 

 structure of the entire organ, consisting of a bronchiole and in- 

 fundibula, or air-passage, communicating with numerous air- 

 cells. 



The air-cells are minute polyhedral cavities, from one two- 

 hundredths to one-seventieth of an inch in diameter, separated 

 from each other by a thin lamina, and communicating freely with 

 the infundibula. They vary much in size, being largest on the 

 surface, the thin borders, and the apices. Each air-cell is com- 

 posed of the basement membrane, lined . with squamous epi- 



