Section 40. the Lungs (lg. Figure 1, Sheet 1) Are Moulded to the

shape of the thoracic cavity and heart; they communicate with the pharynx by the trachea (tr. in Figure 1, Sheet 1) or windpipe, and are made up of a tissue of continually branching and diminishing air-tubes, which end at last in small air-sacs, the alveoli. The final branches of the pulmonary arteries, the lung capillaries, lie in the walls of these air-sacs, and are separated from the air by an extremely thin membrane through which the oxygen diffuses into, and the carbon dioxide escapes from, the blood.