Microscopic examination shows that the patches of dense fibrous tissue seen at autopsy almost invariably surround dilated bronchi with no cartilage in their walls (Fig. 28) and with a diameter of from 1 to 2 or more millimeters. These bronchi have lost their epithelial lining; they contain polynuclear leucocytes, and their wall in contact with the lumen is infiltrated to a varying distance with the same cells. Their inner surface is very irregular, and superficial necrosis occurs. The limits of the preexisting bronchial wall is no longer recognizable in the dense surrounding fibrous tissue richly infiltrated with lymphoid and plasma cells. In contact with the bronchus, often in a wide zone, all traces of alveoli have been destroyed, but further outward alveoli are represented by spaces lined by cubical epithelium. At the periphery of the zone of fibroid induration alveolar walls are much thickened and richly infiltrated with mononuclear wandering cells; the lumina of the alveoli contain plugs of organized fibrous tissue often covered by flat or cubical epithelium. In the surrounding tissue a few small bronchi are lined by columnar epithelium; there is scant new formation of fibrous tissue but the alveolar walls are thickened and infiltrated with cells. Epithelium of the larger bronchi with cartilage in their walls is usually intact and there is about them little peribronchial inflammation. Advanced induration about the bronchioles represents a late stage of chronic peribronchiolar pneumonia. A bronchiole cut transversely is found in the center of a focus of induration situated within relatively normal air containing lung tissue. Next the bronchiole which in some instances has wholly or partly lost its epithelium there is very cellular fibrous tissue; further from the bronchiole alveoli are much diminished in size, lined by flat or cubical epithelium and separated by thick cellular walls. Plugs of cellular fibrous tissue sometimes fill the alveolar duct. In favorable sections, cut in a plane which shows the alveolar duct opening out into infundibula, it is found that newly formed fibrous tissue surrounds the alveolar duct and extends into the walls of its tributary alveoli; alveoli may be obliterated by this fibrous tissue. Induration of alveolar walls is evident along the proximal part of the infundibula which are readily demonstrable because they are much dilated. (See Fig. 32.) The distal parts of the infundibula are surrounded by alveoli with delicate walls. One bronchus retains along one side part of its epithelium which has assumed a squamous form. In other places the wall has undergone necrosis which at one spot extends deeply into the surrounding tissue. Necrotic tissue in another part of the circumference is infiltrated with polynuclear leucocytes and separated from the surrounding tissue by a space filled with leucocytes. An abscess communicating with the bronchus is thus formed.