Histologic examination of lungs with interstitial suppuration shows that the interlobular septa are distended by serum and contain a conspicuous network of fibrin. Polynuclear leucocytes are present in varying number, and at times densely infiltrate the distended tissue; it is not uncommon to find a zone of densely crowded polynuclear leucocytes along each edge of the septum, whereas the central part contains comparatively few. Occasionally, there is hemorrhage into the distended connective tissue. Within the distended septa occur greatly dilated lymphatics filled with polynuclear leucocytes (Figs. 12 and 13). Thrombosis of the distended lymphatics has usually occurred, and a conspicuous network of fibrin in which are polynuclear leucocytes plugs the lumen. Streptococci in chains of variable length are found in the inflamed interstitial tissue, but are present in far greater number within the distended lymphatics. [Illustration: Fig. 13.—Suppurative interstitial pneumonia showing a dilated lymphatic. Autopsy 428. ] Necrosis of the cells which fill the lymphatics occurs in spots, usually in the center of the thrombus, and occasionally affects the entire contents of the lymphatic; polynuclear leucocytes have lost their nuclei or in some the nucleus has undergone fragmentation. In these spots the network of fibrin has disappeared. Not infrequently the wall of the lymphatic in a small sector or throughout the circumference has undergone necrosis, and spots of necrosis may occur in the interlobular septa distended by inflammatory exudate. Wherever necrosis has occurred, chains of streptococci are present in immense number. Accumulation of polynuclear leucocytes, necrosis of these cells, solution of fibrin at first in the centers of the lymphatic thrombus and later throughout, occasionally with necrosis of the wall of the vessel, result in the formation of an abscess at the site of the distended lymphatic. These lymphatics, dilated by purulent fluid, may have a diameter from 2 to 3 mm. and may cause considerable compression and collapse of immediately adjacent alveoli. Lymphangitis, distention of lymphatics, thrombosis and finally suppuration may occur in the lymphatic vessels encircling the blood vessels and in those situated in the adventitia of the bronchi of medium size. The alveoli adjacent to the distended septa are filled by inflammatory products; edema is almost invariably present and the alveoli may contain serum and desquamated epithelial cells; fibrin is often present, but more frequently polynuclear leucocytes are predominant. Not infrequently, abscess formation, recognized microscopically, has occurred in contact with septa most often immediately below the pleura. Polynuclear leucocytes are present in immense number and alveolar septa have disappeared; occasionally, with abscess formation there is more or less widespread necrosis of tissue, cells both of the exudate and of the alveolar walls having lost their nuclei.