The _air cells are elastic_ and enlarge by stretching as the chest expands; hence, the cells 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. [Illustration: 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 a smooth membrane called the _pleura_. This membrane turns back and lines the chest wall, so that when the chest expands, the two sleek membranes glide over each other with far less friction than would be the case if the lungs and chest wall were touching (Exp. 2). [Illustration: FIG. 73.--VERTICAL SECTION OF TRUNK, showing diaphragm, cavities of thorax and abdomen.]