must have many of the yellow elastic fibers of connective tissue in their walls. They are lined with an exceedingly thin membrane of epithelial cells through which oxygen and carbon dioxid are exchanged. In the walls of the air cells there is a network of capillaries (Fig. 71). The dark red blood comes into these capillaries from the pulmonary arteries, and is changed to a bright red by the time it leaves them to enter the pulmonary veins. The air leaves the lungs warmer, moister, and containing more carbon dioxid than when it entered.
Most of the mucous membrane lining the air passages has a surface layer of ciliated cells. Cilia are tiny thread-like projections (Fig. 72) which continually wave to and fro, the quicker stroke always being outward; for their function is to remove particles of dust and germs that may find entrance to the air passages. When the mucus containing the dust is raised nearly to the larynx, it may be thrown out by coughing. Near the opening of the nostrils are placed many hairs, hundreds of times larger than cilia, through which the air is strained as it enters the nose. Hairs are multicellular; cilia are parts of cells. See Animal Biology, Fig. 14.
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Fig. 72.—Ciliated Cells, lining the air passages.
The Lungs.—The entire chest cavity is occupied by the lungs except the space occupied by the heart, the larger blood vessels, and the gullet. The right lung has three lobes, or divisions, and the left lung has two lobes. The lungs are light pink in early life, but become grayish and darker as age advances. This change is more marked in those who dwell in cities, or wherever the atmosphere is smoky and dusty. The lungs are covered and inclosed by
