*The Alveoli.*—The alveoli, or air cells, are the small divisions of the infundibula (Fig. 36). They are each about one one-hundredth of an inch (1/4 mm.) in diameter, being formed by the infolding of the infundibular wall. This wall, which has for its framework a thin layer of elastic connective tissue, supports a dense network of capillaries (Fig. 37), and is lined by a single layer of cells placed edge to edge. By this arrangement the air within the alveoli is brought very near a large surface of blood, and the exchange of gases between the air and the blood is made possible. It is at the alveoli that the oxygen passes from the air into the blood, and the carbon dioxide passes from the blood into the air. At no place in the lungs, however, do the air and the blood come in direct contact. Their exchanges must in all cases take place through the capillary walls and the layer of cells lining the alveoli. [Fig. 37] Fig. 37—*Inner lung surface (magnified)*, the blood vessels injected with coloring matter. The small pits are alveoli, and the vessels in their walls are chiefly capillaries. [Fig. 38] Fig. 38.—*Diagram to show the double movement of air and blood through the lungs.* The blood leaves the heart by the pulmonary artery and returns by the pulmonary veins. The air enters and leaves the lungs by the same system of tubes. [Fig. 39] Fig. 39—*Diagram to show air and blood movements in a terminal air sac.* While the air moves into and from the space within the sac, the blood circulates through the sac walls. *Blood Supply to the Lungs.*—To accomplish the purposes of respiration, not only the air, but the blood also, must be passed into and from the lungs. The chief artery conveying blood to the lungs is the _pulmonary artery_. This starts at the right ventricle and by its branches conveys blood to the capillaries surrounding the alveoli in all parts of the lungs. The branches of the pulmonary artery lie alongside of, and divide similarly to, the bronchial tubes. At the places where the finest divisions of the air tubes enter the infundibula, the little arteries branch into the capillaries that penetrate the infundibular walls (Figs. 38 and 39). From these capillaries the blood is conveyed by the pulmonary veins to the left auricle.