Sections from the least involved areas of the lung show a subsidence of the alveolar exudate and the walls are no longer intensely engorged. Perhaps the most prominent feature within the alveoli is the desquamated cells, presumably alveolar cells with broken or pyknotic nuclei. Despite the fact that so many cells of this type occupy the lumen of the alveolus, its wall has a prominent lining of cubical epithelium. Often mitotic figures abound in this new alveolar epithelial lining (Fig. XLVII), an evidence of rapid regeneration in that portion of the lung where the initial irritative process has subsided and where the destruction has not been as deep as elsewhere (79). This picture may be taken as positive evidence of an initial, diffuse, and general pulmonary involvement, which, with the subsidence of the primary reaction, is followed by localization resulting in the different types of consolidation now encountered.
FIG. XXI. AUTOPSY NO. 95. A TYPICAL APLASTIC ALVEOLAR EXUDATE COMPOSED Of RED BLOOD CELLS, FIBRIN, AND BACTERIA. COMPARE FIGURES XX, XXII, AND XXIII.
FIG. XXIII. AUTOPSY NO. 175. NOTE THE ABSENCE OF ALVEOLAR EPITHELIUM, THE ENGORGEMENT OF THE VESSELS OF THE ALVEOLAR WALLS, AND THE SEROUS EXUDATE. ]
FIG. XXV. AUTOPSY NO. 92. THE EXUDATE CONSISTS ALMOST ENTIRELY OF A MASS OF RED BLOOD CELLS. THE DESQUAMATED ALVEOLAR EPITHELIUM IS SCATTERED THROUGH THE HEMORRHAGIC EXUDATE. COMPARE FIGURES XXIV AND XXVI. ]
However, in such areas of slight change, the bronchi and bronchioles may be distended and filled with polymorphonuclear leucocytes, exfoliated epithelium, and bacteria (Fig. X). It is, of course, possible that infection of the parenchyma may recur from these sources. Here the extent of involvement of the bronchiolar wall is variable and analogous to those described previously. Occasionally, too, where the alveolar change has subsided, the interstitial tissue, particularly that which divides groups of neighboring lobules, may retain its increased size with fibrous tissue framework spread apart by exudate and punctuated with an occasional circumscribed purulent mass (92, 95, 110, 156) (Fig. XXXVI). Such a miliary abscess within the lymphatics of the interstitial tissue may compress the neighboring air sacs, themselves entirely free of inflammatory involvement. That these strands of interstitial tissue, the conduits for the lymphatics, are important barriers against the spread of an inflammatory process from lobule to lobule by direct extension, is evidenced by the extreme variation in the amount and type of involvement in neighboring lobules (Fig. XXX). This variation is repeatedly encountered and the band of interstitial tissue, often prominent on account of its edema, separates these lobules the more clearly. In all probability, the sharp demarcation of the lobular consolidation as described in the gross picture depends upon the change in the interstitial tissue which tends to localize the infection (93). This fact suggests that the process within the pulmonary parenchyma spreads along the bronchial tree rather than from lobule to lobule.