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. Sections from those areas of the lung where the involvement is more marked may show a histological picture not unlike that described for the aplastic stage, but, in addition, there are groups of lobules where the exudate is typically purulent and pus cells not only form the greater part of the exudate in the lumen, but are prominent in the distended vessels of the alveolar wall (Fig. XXIX). Often these leucocytes are multilobed and frequently their protoplasm is granulated with phagocytized bacteria. The bacteria are also encountered free in the alveolus along with other elements; namely, red blood cells, strands of fibrin, or precipitated albumin (Fig. XXI). The bacteria, however, are not particularly conspicuous, for generally they are either single, in pairs, or in chains; and it is only when they become clumped to form large masses, often larger than any normal tissue cell, that they attract attention. When this appearance is encountered, the alveolar wall is no longer distinct and well preserved. Although the wall may still be made out, it often stains rather homogeneously and much of the finer architecture is lost in the thrombo-necrotizing process that has been instituted (Fig. XVII).