_A._ The outer surface of one lung. Note the flattened oval shape. It is not divided into lobes. 1, The bronchus. 2, Primary tubules showing openings leading from the primary tubules to the secondary tubules. 3, Openings of two of the large tubes into the diaphragmatic and abdominal air-sacs. _B._ Sectioned surface of lung. 1, Secondary tubules. 2, Tertiary tubules. 3, Interlacing capillaries and air cells. _C._ 1, Cavity of tubule. 2, Its lining membrane supporting blood-vessels with large areolæ. 3, Perforations in the membrane at the orifices of the lobular passage. 4, Interlobular space containing the terminal branches of the pulmonary vessels supplying the capillary plexus, 5, to the meshes of which air gets access by the lobular passage. ] The lungs occupy only about one-seventh of the thoracic space. They are long, flattened, and oval, extending along each side of the spine from the second dorsal vertebra to the anterior end of the kidneys, and laterally to the juncture of the vertebral with the sternal portions of the ribs. They present two faces, a _superior convex_ and an _inferior concave_; two borders, an _external_ and an _internal_; and two extremities, an _anterior_ and a _posterior_ (Fig. 61). The convex surface is also called the dorsal, costal, or superior face. It is moulded on the walls of the thorax and occupies a part of the intercostal space, pushing the intercostal muscles outward. When the surface of the lung is examined it is seen to be furrowed where the ribs pressed during life. These furrows are as deep as the ribs are thick. The sides of the lungs are covered with connective tissue which attaches them to the costal walls. The concave, or inferior, face also called the _diaphragmatic_ or _visceral face_ is directed downward. The diaphragm separates it from the abdominal viscera. The surface is covered by connective tissue which closely attaches it to the diaphragm. It is perforated by the five tubules which bring the posterior air-sac into communication with the lungs. The borders of the lungs extend parallel to the long axis of the body. The internal border is rectilinear, thick, and rounded. The external border is convex, thin, and sharp. The anterior extremity terminates in a sharp point which occupies a space formed by the ribs externally and the inferior spines internally. The posterior extremity is somewhat rounded and extends as far back as the anterior border of the kidneys. As soon as the bronchi enter the lungs they become broadened, the cartilaginous rings disappear, and they continue as membranous channels whose diameters gradually decrease, as they extend backward, to the point where they terminate in the _ostium caudale_, at which point they are surrounded by a cartilaginous ring. The ostium caudale brings the tubules into communication with the ventral air-sacs.