Microscopic examination demonstrates the presence of acute bronchitis; the lumina of the small bronchi contain polynuclear leucocytes and red blood corpuscles. Accumulation of blood may separate the epithelium from the basement membrane. The mucosa immediately below the epithelium contains polynuclear leucocytes in fair abundance and the blood vessels of the bronchial wall are much engorged. Respiratory bronchioles are distended with polynuclear leucocytes and red blood corpuscles. In a zone about each bronchiole, in areas corresponding to the small gray spots seen upon the cut surface of the lung, the alveoli are filled with polynuclear leucocytes. In the lung tissue intervening between these spots of leucocytic pneumonia the alveoli are distended with red blood corpuscles. [Illustration: Fig. 5.—Bronchopneumonia with hemorrhagic peribronchiolar consolidation. ] In favorable sections it is occasionally possible to follow the bronchiole and alveolar duct, both filled with leucocytes, into an infundibulum. The proximal part of the infundibulum contains polynuclear leucocytes, whereas the distal part and its tributary alveoli are filled with serum and red blood corpuscles. When the lesion has persisted for a short time there is evidence of beginning migration of polynuclear leucocytes from the blood vessels into the alveoli which are filled with blood. The alveolar walls contain numerous polynuclear leucocytes and leucocytes which have entered the intraalveolar blood are numerous in contact with the wall but occur in scant number in the center of the alveolar lumen. Alveolar epithelium in contact with the blood in the lumen is usually swollen and often uniformly nucleated. The inflammatory process is evidently transmitted from the bronchioles and to a less degree from the small bronchi to the adjacent alveoli. Polynuclear leucocytes fill the lumen of the bronchiole and the alveoli immediately adjacent; at the periphery of the focus of pneumonia, the alveoli may contain fibrin. In such instances small bronchi (lined by a continuous layer of columnar epithelial cells) may be surrounded by alveoli containing fibrin. In sections from one part of the lung, the alveoli between the peribronchiolar foci of pneumonia may be uniformly filled with red blood corpuscles, whereas in sections from another part pneumonic foci may be surrounded by a zone of intraalveolar hemorrhage or of hemorrhage and edema outside of which some air-containing tissue occurs. There are transitions between this halo of intraalveolar hemorrhage and edema surrounding each bronchiolar focus and complete hemorrhagic infiltration of all intervening alveoli.