Thickening, cellular infiltration and fibrosis of the bronchial walls with interstitial inflammation and fibrosis of immediately adjacent alveolar septa are found about the ramifications of the bronchial tree and may be followed to the smallest bronchi. When the respiratory bronchioles are reached it will be found that the alveoli which stud their walls are implicated in the change. The fibrin which they contain is infiltrated with lymphoid and plasma cells, and with progress of the lesion is invaded by fibroblasts and capillaries. Infiltration and fibroid thickening extends from the bronchiolar wall to the alveolar septa continuous with it (Fig. 31 with measles). Similar changes occur about the alveolar ducts, and about the orifices of the tributary infundibula (Fig. 32), peribronchiolar foci of acute inflammation having assumed the characters of a chronic inflammatory process. Fibrin within the alveoli contains round cells and fibroblasts. With thickening of alveolar walls the alveolar lumina may be much diminished in size and often persist as spaces lined by cubical cells. Polynuclear leucocytes are usually numerous within the alveolar duct and in a few alveoli immediately adjacent to it, but elsewhere throughout the focus of inflammation round cells are predominant. The changes which have been described correspond with the transformation of ill-defined, gray or reddish gray spots of consolidation grouped about the terminal bronchi into firm sharply defined grayish white nodules having the consistence and appearance of miliary tubercles. One of the most constant characters of pneumonia following influenza is its hemorrhagic character. In the earlier stages of pneumonia phagocytosis of red blood corpuscles by large mononuclear cells is frequently seen. In association with the chronic changes which have been described, large mononuclear cells filled with brown pigment, doubtless formed from red corpuscles, are often found within the alveoli. These pigment containing cells are similar to those commonly associated with chronic passive congestion of the lungs. In one instance (Autopsy 457) hemorrhagic peribronchiolar pneumonia has been found in process of organization. The bronchioles and alveoli adjacent to them contain polynuclear leucocytes, but intervening alveoli almost uniformly contain blood and are the site of new formation of connective tissue. Interalveolar septa are thickened and alveoli which are lined by cubical epithelium are often diminished in size. In many places fibroblasts have penetrated in considerable number into the blood within the alveoli and occasionally newly formed capillaries are found within them.